# Gamma-ray Transient Network Science Analysis Group Report

Eric Burns Department of Physics & Astronomy, Louisiana State University,
Baton Rouge, LA 70803, USA  Michael Coughlin School of Physics and Astronomy,
University of Minnesota, Minneapolis, Minnesota 55455, USA  Kendall Ackley
Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4
7AL, UK  Igor Andreoni Joint Space-Science Institute, University of Maryland,
College Park, MD 20742, USA  Department of Astronomy, University of Maryland,
College Park College Park, MD 20742, USA  Astrophysics Science Division, NASA
Goddard Space Flight Center, Greenbelt, MD 20771, USA  Marie-Anne Bizouard
Artemis, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Nice
06300, France  Floor Broekgaarden Center for Astrophysics $|$ Harvard $\&$
Smithsonian, 60 Garden St, Cambridge, MA 02138  Nelson L. Christensen Artemis,
Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Nice 06300,
France  Filippo D’Ammando Italian National Institute for Astrophysics,
Institute for Radioastronomy, Bologna, 40129, Italy  James DeLaunay Department
of Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA
Henrike Fleischhack Catholic University of America, Washington, DC, USA
Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD
20771, USA  Center for Research and Exploration in Space Science and
Technology, NASA/GSFC, Greenbelt, MD 20771, USA  Physikalisch-Technische
Bundesanstalt (National Metrology Institute), Braunschweig, Germany  Raymond
Frey Department of Physics, University of Oregon, Eugene, OR 97403, USA  Chris
L. Fryer Center for Non Linear Studies, Los Alamos National Laboratory, Los
Alamos, NM, 87545, USA  Adam Goldstein Science and Technology Institute,
Universities Space and Research Association, 320 Sparkman Drive, Huntsville,
AL 35805, USA.  Bruce Grossan Space Sciences Laboratory, University of
California, Berkeley, Berkeley, CA 94720, USA  R. Hamburg Université Paris-
Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France  Dieter H. Hartmann Clemson
University, Department of Physics & Astronomy, Clemson, SC 29634-0978, USA
Anna Y. Q. Ho Department of Astronomy, Cornell University, Ithaca, NY 14853,
USA  Eric J. Howell OzGrav-UWA, School of Physics & Astrophysics, University
of Western Australia, Crawley WA 6009, Australia  C. Michelle Hui ST12
Astrophysics Branch, NASA Marshall Space Flight Center, Huntsville, AL 35812,
USA  Leah Jenks Kavli Institute for Cosmological Physics, University of
Chicago, Chicago, IL 60637, USA  Alyson Joens Space Sciences Laboratory,
University of California, Berkeley, Berkeley, CA 94720, USA  Stephen Lesage
Department of Space Science, University of Alabama in Huntsville, 320 Sparkman
Drive, Huntsville, AL 35899, USA  Center for Space Plasma and Aeronomic
Research, University of Alabama in Huntsville, Huntsville, AL 35899, USA
Andrew J. Levan  Department of Astrophysics/IMAPP, Radboud University, P.O.
Box 9010, NL-6500 GL Nĳmegen, The Netherlands  Amy Lien University of Tampa,
Department of Chemistry, Biochemistry, and Physics, 401 W. Kennedy Blvd,
Tampa, FL 33606, USA  Athina Meli College of Science and Technology,
Department of Physics, North Carolina A&T State University, Greensboro, NC
27411, USA  STAR Institute, Institut d’Astrophysique et de Geophysique,
University of Liege, Liege 4000, BE  Michela Negro University of Maryland,
Baltimore County, Baltimore, MD 21250, USA  Astrophysics Science Division,
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  Center for
Research and Exploration in Space Science and Technology, NASA/GSFC,
Greenbelt, MD 20771, USA  Tyler Parsotan Astrophysics Science Division, NASA
Goddard Space Flight Center, Greenbelt, MD 20771, USA  Oliver J. Roberts
Science and Technology Institute, Universities Space and Research Association,
320 Sparkman Drive, Huntsville, AL 35805, USA.  Marcos Santander Department of
Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA  Jacob
R. Smith George Mason University, Department of Physics & Astronomy, 4400
University Drive, Fairfax, VA 22030, USA  Naval Research Laboratory, 4555
Overlook Avenue SW, Washington, DC 20375, USA  Aaron Tohuvavohu Department of
Astronomy & Astrophysics, University of Toronto, Toronto, ON M5S 3H4  John A.
Tomsick Space Sciences Laboratory, University of California, Berkeley,
Berkeley, CA 94720, USA  Zorawar Wadiasingh Department of Astronomy,
University of Maryland, College Park College Park, MD 20742, USA  Astrophysics
Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
Center for Research and Exploration in Space Science and Technology,
NASA/GSFC, Greenbelt, MD 20771, USA  Péter Veres Department of Space Science,
University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL 35899,
USA  Center for Space Plasma and Aeronomic Research, University of Alabama in
Huntsville, Huntsville, AL 35899, USA  Ashley V. Villar Department of
Astronomy and Astrophysics, Pennsylvania State University, 525 Davey
Laboratory, University Park, PA 16802, USA  Institute for Computational & Data
Sciences, The Pennsylvania State University, University Park, PA, USA
Institute for Gravitation and the Cosmos, The Pennsylvania State University,
University Park, PA 16802, USA  Haocheng Zhang Astrophysics Science Division,
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  Sylvia J. Zhu
Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany

=999

## Executive Summary

The Interplanetary Network (IPN) is a detection, localization and alert system
that utilizes the arrival time of transient signals in gamma-ray detectors on
spacecraft separated by planetary baselines to geometrically locate the origin
of these transients. Due to the changing astrophysical landscape and the new
emphasis on time domain and multi-messenger astrophysics (TDAMM) from the
Pathways to Discovery in Astronomy and Astrophysics for the 2020s, this Gamma-
ray Transient Network Science Analysis Group was tasked to understand the role
of the IPN and high-energy monitors in this new era. The charge includes
describing the science made possible with these facilities, tracing the
corresponding requirements and capabilities, and highlighting where improved
operations of existing instruments and the IPN would enhance TDAMM science.
While this study considers the full multiwavelength and multimessenger
context, the findings are specific to space-based high-energy monitors. These
facilities are important both for full characterization of these transients as
well as facilitating follow-up observations through discovery and
localization. The full document reports the history of this field in Section
1, followed by our detailed analyses and findings in some 68 pages, providing
a holistic overview of the role of the IPN and high-energy monitors in the
coming decades.

We here summarize our core conclusions. Scientific breakthroughs which require
capable high-energy monitors are fully detailed in Section 2. Specific
questions of interest include:

  * •

What are the sources of gamma-ray bursts, fast radio bursts, gravitational
waves, neutrinos, magnetars, relativistic supernovae, and exotic transients?

  * •

What sub-classes exist within these sources, and what causes their
differences?

  * •

What produces a gamma-ray burst?

  * •

What powers fast radio bursts?

  * •

How were the heaviest elements produced?

  * •

What governs the cosmological evolution of the universe? Is the universe flat?
Is dark energy a cosmological constant?

  * •

What processes underlie ultra-relativistic jets?

  * •

What is the equation of state of dense matter?

  * •

Is General Relativity the correct theory of gravity?

  * •

What caused the matter excess over antimatter near the beginning of time?

In Section 3 we explore the requisite observational capabilities necessary to
answer these questions. We find that the key instrument capability drivers are

  * •

Precise localizations of gamma-ray transients – prompt precision of at least
$\sim$arcminute scale, and prompt or successful follow-up precision to at
least $\sim$1 arcsecond

  * •

Rapid alerts of triggers – ideally within 10 s of trigger time

  * •

Total coverage of the transient sky – approaching instantaneous 4$\pi$ sr with
$\sim$weeks long contiguous observing intervals

  * •

Precise timing – absolute timing precision to $\sim$1 ms accuracy and relative
timing precision to $\lesssim$100 $\mu$s

  * •

Unbiased, complete, and sensitive observations of sources – kilosecond scale
background stability, temporal coverage with no observing gaps, and fairly
uniform sensitivity down to $\sim$1 photon s-1 cm-2, though improved
sensitivity would greatly enhance scientific return

Without continuous all-sky coverage of gamma-ray transients, we will be blind
to key diagnostic information. This would result in a total loss of scientific
discovery that cannot be recovered through any other means. The IPN is capable
of fulfilling the majority of these requirements and thus has a foundational
role to play in this new era of TDAMM. We envision an enhancement to the
current operations of the IPN to produce products needed by the wider TDAMM
community. This is described in Section 5. Key community requests include:

  * •

The automatic collation of gamma-ray transient data and alert dissemination: A
critical IPN product is the localizations of gamma-ray transients. The
automation of these localizations and inclusion of auxiliary information is
key to enhancing the return of the current network. The automatic association
of signals and calculation of localization information would allow for
dedicated GCN alert streams that utilize all available information for
classifying gamma-ray bursts, solar flares, magnetar flares, and other
transients detected by these instruments. This further facilitates joint
searches such as gamma-ray bursts in concert with gravitational waves and
neutrinos or magnetar X-ray flares with fast radio bursts. Such a system will
enable dozens of prompt and afterglow detections of gamma-ray bursts each year
with observations from Rubin, Neo Surveyor, and other wide-field telescopes in
their normal survey operations.

  * •

Signal-based gamma-ray transient catalogs: Most current gamma-ray burst and
magnetar flare catalogs are instrument-specific, requiring a great deal of
effort by external groups to check observations against all gamma-ray
transients of interest. The IPN should take on the responsibility of building
signal-based catalogs, which could automatically update via the above
described alert streams.

  * •

A shared TDAMM database and archive: A necessary step to enable the previous
two products is construction of a shared database and archive to gather the
relevant data in one place. Given the IPN use of both private and public data
this requires dedicated infrastructure.

  * •

Development of multi-mission coherent analysis: The shared database/archive
would enable coherent analysis of all active gamma-ray burst monitors,
facilitating vastly improved sub-threshold searches and deeper upper limits
than is possible with individual instruments. This work would require careful
inter-calibration studies. Development of new searches for long-duration
transients and coherent temporal monitoring of point sources would extend the
usefulness of these instruments to new sources.

  * •

Preservation of historic data: Ensuring preservation of archival data,
enabling ease of access, and applying modern analysis to old observations are
in the scientific zeitgeist. A concerted effort to preserve historic gamma-ray
burst monitor data including from Vela, Pioneer Venus Orbit, and the Venera
series of satellites should be undertaken as soon as possible, while the
necessary knowledge is still in living memory.

  * •

Additional improvements to the IPN: A variety of other products will rely upon
critical IPN work such as the characterization of systematic errors these
instruments. These products include information on prioritization of prompt
detections for follow-up, advanced statistical multi-instrument analysis
techniques, and shared open-source analysis software. Development of modern
corner plots reporting from prompt gamma-ray burst analysis would follow the
advancements in related fields. Such results require enhanced data sharing
within the IPN and generation of instrument responses of planetary instruments
for gamma-ray burst analysis.

These products are necessary to meet community needs with current and
forthcoming missions and are well matched to the priorities of three Decadal
Surveys. There are major roadblocks that have prevented support within the US
for development of these products. The IPN is necessarily community-led, which
precludes consideration by any sufficient funding mechanism. Additionally, the
use of assets from multiple NASA Science Divisions is also a necessity, but
the separation prevents the IPN from accomplishing many goals. These findings
on enhancing the IPN echo those in the Multimessenger Astrophysics Science
Analysis Group final report three years ago. We find that elevation of the IPN
to a strategic NASA Astrophysics TDAMM role is necessary to ensure the
requisite sustained support for the necessary IPN enhancements and improvement
operations.

The benefits are not only scientific. The proposed IPN products will enable
ease of access, facilitating use by a growing community. The IPN, as a
strategic initiative and with funding to support education and public
outreach, can help these transients and TDAMM enter the zeitgeist, perhaps
becoming as well known as supernovae. Within the astrophysics community, the
IPN could foster collaboration between individuals on instrument teams and in
follow-up groups, producing more valuable scientific results and visibility
for early-career scientists, analogous to the standard in the Exoplanet
community. The United States has played a leading role in astrophysics for
more than a century, sometimes alone, and sometimes through international
collaboration such as the IPN partnership with Russia. The release of
international scientific results is a key form of soft power. TDAMM discovery
facilities play a special role in American soft power as they enable positive
press worldwide, including the Global South and, perhaps uniquely, non-aligned
countries. Within the US, the IPN utilizes public and private data together
for public alert, a key theme of Pathways to Discovery in Astronomy and
Astrophysics for the 2020s, and is thus a unique example of NASA’s Transform
to Open Science initiative. Adding the numerous gamma-ray smallsats to the
collaboration would allow for greater inclusion of the international
community, ensure maximum gain of their data, act to facilitate inclusion of
private data towards productive TDAMM results, and allow the United States to
support the developing space capabilities in other countries.

We identify a number of specific actions that could be taken by NASA to
increase the TDAMM return of current, forthcoming, and future facilities,
discussed in Section 4. These include:

  * •

Enhanced space-based communications: Providing additional communication
contacts for gamma-ray burst monitors will lead to faster retrieval of data
and data of higher quality (e.g. high resolution time-tagged event data). The
immediate allocation of additional contacts for the Neil Gehrels Swift
Observatory is important to discovery in the on-going gravitational-wave
observing run (the capability was requested in the latest Senior Review round
but not mentioned in the report). Similar improvements can be made to
forthcoming Astrophysics missions like StarBurst and the Compton Spectrometer
and Imager (COSI). This finding also applies to assets in NASA Planetary and
Heliophysics, as well as missions led by other space agencies, where
additional or more frequent contacts would boost the scientific return of the
IPN and other TDAMM facilities. This provides an avenue for a TDAMM program
built with international partners.

  * •

A return to the intentional launch of distant, dedicated gamma-ray burst
monitors: The United States has not launched a dedicated gamma-ray burst
monitor on a distant spacecraft since Ulysses in 1990. The IPN has operated by
adapting gamma-ray spectrometers of use in mapping element distributions on
distant planetary bodies and in studies of solar flares, but the science
supported by the IPN could be vastly enhanced if the gamma-ray transient
capabilities were intentional rather than a happenstance patchwork. Origins,
Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology
2023-2032 identify the forthcoming capabilities of megarockets, such as the
SpaceX Starship, and optical communications, which will soon be demonstrated
on Psyche, as capability leaps in Planetary research. These advancements would
also improve the operation of the IPN, enabling arcsecond precision
localizations, if the IPN is sufficiently supported within NASA. The IPN would
also benefit from the multi-decade development of high-precision optical
clocks in space for use in fundamental physics studies prioritized in the
Thriving in Space: Ensuring the Future of Biological and Physical Sciences
Research: A Decadal Survey for 2023-2032.

  * •

The Decadal-recommended TDAMM High-Energy Monitor and Program: For the IPN
localization technique to achieve the necessary precision, interplanetary
baselines are required. The finite speed of light is inherent to this
technique but precludes immediate dissemination of precise localizations.
Thus, while this report focuses on the IPN, we consider what would be
necessary to meet the needs of the full high-energy transient TDAMM community.
We note that despite the enormous advancements in observing gravitational
waves and neutrinos, and development of survey instruments across the
electromagnetic spectrum, the current 15 year gap is the longest span of time
without a new, dedicated NASA gamma-ray burst mission, at the explorer scale
or larger, since these events were first discovered 56 years ago. This report
provides quantitative evidence in support of the $500-$800M recommended scale
investment by NASA in a strategic TDAMM program for a worthy successor to
Swift and Fermi, a fundamental necessity for the multiwavelength and
multimessenger community.

  * •

Facilitating greater support for the IPN, both across and within NASA science
divisions: The IPN necessarily relies on data from multiple NASA Science
Divisions. The stovepipes, while necessary, act against successful cross-
divisional work. General enhancement examples include contributions of on-
board GRB trigger algorithms, support for improved absolute timing capability,
prioritization of GRB data at the start of scheduled downlinks, additional
downlinks, and development of instrument responses for the study of gamma-ray
transients. We find that linkages across divisions at the level of instrument
teams, between program officers, and support for interdisciplinary work in the
reviews of missions are important to maximizing the return of the IPN.
Dedicated support at headquarters is required; this is especially important
with the need to continue the IPN beyond the life of Fermi and Swift. One
option could be assignment of the IPN to a program officer within the NASA
Astrophysics Division. Specific cross-division opportunities include dedicated
GRB monitors on Gateway and the recommended Uranus Orbiter and Probe mission,
where minimal investment could enable sub-arcsecond timing annuli for more
than a decade. An example of an Astrophysics Division-specific opportunity
would be launching StarBurst on the dedicated COSI launch. Lastly, ensuring
long-term support for the IPN via dedicated funding mechanisms is critical to
successful enhancement of the IPN products.

###### Contents

  1. 1 Introduction
  2. 2 Sources
    1. 2.1 Magnetars
      1. 2.1.1 An Origin of Short Gamma-ray Bursts
      2. 2.1.2 The Neutron Star Equation of State
      3. 2.1.3 A Potential Source of Gravitational Waves
      4. 2.1.4 The Origin and Physical Mechanism of Fast Radio Bursts
      5. 2.1.5 Discovery of New Magnetars
      6. 2.1.6 Magnetar Formation Channels
      7. 2.1.7 Magnetar Properties
      8. 2.1.8 Physics of Magnetar Bursts
      9. 2.1.9 Summary of Capability Requirements
      10. 2.1.10 Requested Deliverables from GRB Monitors
    2. 2.2 Compact Mergers
      1. 2.2.1 Plateau Emission
      2. 2.2.2 Prompt Radio Signatures
      3. 2.2.3 Early Ultraviolet Emission
      4. 2.2.4 Binary Neutron Star Merger Classes
      5. 2.2.5 Neutron Star-Black Hole Classes
      6. 2.2.6 Progenitors of Short Gamma-Ray Bursts
      7. 2.2.7 Progenitors of Long Gamma-Ray Bursts
      8. 2.2.8 Origin and Classification of Gravitational Waves
      9. 2.2.9 White Dwarf-Neutron Star and White Dwarf-Black Hole Mergers
      10. 2.2.10 Stellar Formation and Evolution
      11. 2.2.11 Origin of the Elements
      12. 2.2.12 Dense Matter
      13. 2.2.13 Cosmology
      14. 2.2.14 Ultrarelativistic Jets
      15. 2.2.15 Fundamental Physics
      16. 2.2.16 Summary of Capability Requirements
      17. 2.2.17 Requested Deliverables from GRB Monitors
    3. 2.3 Collapsars
      1. 2.3.1 Collapsar and Collapsar-Like Progenitors
      2. 2.3.2 Dirty Fireballs and Optically-identified Transients
      3. 2.3.3 Origin of Short GRBs
      4. 2.3.4 Ultra-long GRBs
      5. 2.3.5 High Redshift GRBs
      6. 2.3.6 Very-High-Energy Emission
      7. 2.3.7 Origin of Neutrinos and Ultra-High Energy Cosmic Rays
      8. 2.3.8 Gravitational Waves
      9. 2.3.9 Polarization and the Blandford-Znajek Mechanism
      10. 2.3.10 Prompt Emission Mechanism
      11. 2.3.11 Lorentz Invariance Violation
      12. 2.3.12 Summary of Capability Requirements
      13. 2.3.13 Requested Deliverables from GRB Monitors
    4. 2.4 Miscellany
      1. 2.4.1 Jetted Tidal Disruption Events
      2. 2.4.2 Non-Astrophysical Sources
      3. 2.4.3 Unexpected Discoveries
  3. 3 Capability Requirements
    1. 3.1 Localization Precision
    2. 3.2 Alert Latency
    3. 3.3 Field of View, Livetime, and Contiguous Viewing Intervals
    4. 3.4 Background Stability
    5. 3.5 Timing Capabilities
    6. 3.6 Energy Range
    7. 3.7 Sensitivity
    8. 3.8 Maximum Photon Rate
    9. 3.9 Spectral Resolution
    10. 3.10 Polarization
  4. 4 Actionable Items for Missions and Instruments
    1. 4.1 Active Missions
    2. 4.2 Forthcoming Missions
    3. 4.3 The Decadal-Recommended High Energy Monitor
    4. 4.4 The Future of the InterPlanetary Network
  5. 5 Actionable Items for the Gamma-ray Transient Network
    1. 5.1 Support for Community-led Strategic Initiatives
    2. 5.2 Products
      1. 5.2.1 Event-based Real-time Alert Stream
      2. 5.2.2 Event-based Prompt GRB Catalog
      3. 5.2.3 A Shared Database and Archive
      4. 5.2.4 Additional Automated Analysis and Prioritization of Follow-up Observations
      5. 5.2.5 Event-based SGR Flare Reporting and Catalogs
      6. 5.2.6 Coherent Multi-mission Searches and Upper Limits
      7. 5.2.7 Intercalibration of High Energy Monitors
      8. 5.2.8 Systematic Characterization
      9. 5.2.9 Modernized Results Presentation
      10. 5.2.10 Shared Open Source Analysis Software
    3. 5.3 Examples
      1. 5.3.1 GRB 170817A
      2. 5.3.2 GRB 230307A
      3. 5.3.3 GRB 221009A
      4. 5.3.4 GRB 200612A and GRB 220627A
      5. 5.3.5 GRB 200826A
      6. 5.3.6 GRB 170114A
      7. 5.3.7 GRB 200415A
    4. 5.4 Additional Benefits
  6. 6 Conclusions

##  1 Introduction

The first Gamma-Ray Burst (GRB) was discovered by the Vela series of
satellites in 1967, launched to monitor Earth for atmospheric detonations of
nuclear bombs after the signing of the Partial Nuclear Test Ban Treaty. These
satellite pairs achieved total global coverage with $\sim 10^{5}$ km orbits on
opposing sides of Earth. The separation distance gives a light travel time
difference of up to $\sim 1$ s. As the temporal profiles of GRBs can be
measured to $\sim$tens of ms, this allowed for localization by drawing a
timing annulus for a pair of detectors, or when the network of satellites was
expanded, the intersections of the annuli generated by each pair of
spacecraft. These multiple annuli localized the origin of GRBs as outside our
Solar System (Klebesadel et al., 1973). This process of localizing transients
is formally called pseudo-range multilateration but is commonly referred to as
triangulation, a colloquialism we will use here. A representation of the
approach is shown in in Figure 1.

Figure 1: Representation of the IPN localization technique. The finite speed
of light ($c$) and differential arrival time ($\delta t_{12}$ of a GRB at a
pair of spacecraft (spacecraft 1 and 2) separated by a distance ($d_{12}$)
allow for geometric determination of an annulus with radius $\theta_{12}$.
Analogous variables are shown for the pair comprised of spacecrafts 2 and 3.
Multiple pairs allow for multiple annuli. Additional information can be
utilized to discriminate between the two regions that remain from the
intersection of two annuli. Figure borrowed from Hurley et al. (2013).

Helios-B, the first mission with an instrument intentionally designed for the
study of cosmic GRBs, was launched in 1976 and reached distances of up to 2
AU, which when paired with the Vela satellites formed the beginning of the
InterPlanetary Network (IPN). They were quickly joined by the Pioneer Venus
Orbiter, the International Cometary Explorer (originally named the
International Sun-Earth Explorer-3) and the Konus and SIGNE-2 series of
instruments. Beginning with the discovery of the 1979 magnetar giant flare and
subsequent repeating bursts, the IPN began observing flares from what became
known as soft gamma-ray repeaters (SGRs). Eventually the IPN triangulations
and temporal and spectral studies separated GRBs and SGR short bursts as
distinct phenomena (Epstein & Hurley, 1988; Hurley, 1989).

The early IPN led to evidence of the isotropic distribution of GRBs (Mazets &
Golenetskii, 1981; Hartmann & Epstein, 1989), pointing to either a
particularly local origin within the Milky Way or a cosmological origin of
GRBs. The latter was unambiguously proven with the Burst And Transient Source
Experiment (BATSE) on-board the Compton Gamma-Ray Observatory, NASA’s gamma-
ray Great Observatory, launched in 1991, through the combination of isotropy
and inhomogeneity inferred via the observed cumulative fluence distribution
deviation from a $-$3/2 power-law due to the expansion of the universe (Meegan
et al., 1992). BATSE remains the most sensitive GRB monitor.

The failure of the Compton onboard flight recorder led to continuous data
downlink via the Tracking and Data Relay Satellite system in order to preserve
data and save the mission. This allowed for the automated dissemination of GRB
triggers to the community through a system known as the BATSE COordinates
DIstribution NEtwork (BACODINE) beginning in 1993. The first conception of a
GRB jet emitting synchrotron radiation powered by the relativistic, jetted
outflow developing up a shock with the surrounding material came out the same
year (Paczynski & Rhoads, 1993). The next major change in the field was the
launch of the Italian-Dutch BeppoSAX mission, which was built to find GRB
counterparts at other wavelengths through localization of the prompt gamma-ray
emission at the $3-5^{\prime}$ level, using a coded aperture mask, reported on
the timescale of hours in International Astronomical Union Circulars
(Frontera, 2019). This combination, more precise than BATSE and more rapid
than IPN, led to the recovery of the first GRB afterglow following GRB 970228
(Costa et al., 1997) as well as the first optical spectrum which directly
proved a cosmological origin for GRB 970508 (Metzger et al., 1997).

Around this time, the BACODINE system was renamed as the Gamma-ray burst
Coordinates Network (GCN), which allowed for automated Notices from multiple
missions and human-written Circulars. This system was utilized by the
community for several recoveries including the recovery of the first supernova
following the BeppoSAX observation of GRB 980425, proving the collapsar origin
of long GRBs (Galama et al., 1998). The Robotic Optical Transient Search
Experiment performed automated wide-field follow-up of the BATSE localization
of GRB 990123 reported through GCN to find an optical signal in the prompt
phase of the burst, giving insight into the prompt emission mechanism of GRBs
(Akerlof et al., 1999). The IPN began disseminating alerts to GCN including
marking the active state of SGR 1900+14 and providing a precise localization
(Hurley et al., 1998) shortly before its 1998 giant flare. This was shortly
after the first proof of magnetars (Kouveliotou et al., 1998a), being neutron
stars with extreme magnetic fields (Duncan & Thompson, 1992), made by X-ray
observations of the precise IPN position of SGR 1806-20 (Hurley et al., 1999).
Continuing progress was made with the first recovery of afterglow from a short
GRB and the IPN discovery of the SGR 1806-20 giant flare.

In 2004, the launch of the Neil Gehrels Swift Observatory (Swift) combined a
large coded aperture mask, the Burst Alert Telescope (BAT) with autonomous
repointing of the X-ray Telescope (XRT) and Ultraviolet and Optical Telescope
(UVOT). This enabled rapid and precise localizations which were disseminated
through GCN every few days, on average. This and the corresponding upgrades of
ground-based telescopes led to the modern era of multiwavelength
characterization of explosive transients across the electromagnetic (EM)
spectrum. The launch of the Fermi Gamma-ray Space Telescope in 2008 provided a
sensitive low Earth orbit broadband spectrometer for the prompt phase in the
Gamma-ray Burst Monitor (GBM) and broader coverage of the prompt and afterglow
phase by the Large Area Telescope (LAT). Discoveries in this time period,
enabled by these missions and GCN, include the isolation of short GRBs as
predominantly arising from neutron star (NS) mergers (Fong et al., 2015), the
first convincing claim of a kilonova following GRB 130603B (Tanvir et al.,
2013), the identification of a low-energy excess in the prompt phase whose
origin is still debated (e.g. Guiriec et al., 2010; Ravasio et al., 2019), and
the compilation of a large populations of well-studied events.

In 2015, Laser Interferometer Gravitational-Wave Observatory (LIGO) began
distributing GCN notices for gravitational wave (GW) triggers. GW Notices are
now distributed by the International Gravitational-wave Network (IGWN),
currently comprised of LIGO, Virgo, and KAGRA. Also in 2015, the SuperNova
Early Warning System (SNEWS) set up to alert the community to the detection
and localization of a Galactic supernova through MeV neutrinos, joined by the
dedicated Super-Kamiokande stream in 2020. IceCube began delivering rapid
alerts and localizations of high energy neutrinos through GCN in 2016. Thus,
GCN is now the backbone for multimessenger transient astronomy. In light of
these changes, the GCN was recently renamed to the General Coordinates
Network. GCN was key to the announcement of GRB 170817A, the identification
and association of GW170817, and the follow-up observations which recovered
the off-axis afterglow and the kilonova signature, marked as the beginning of
the GW era of multimessenger astronomy. Other discoveries in this time include
several recoveries of GRB afterglows with very-high energy (VHE) telescopes
enabling study of inverse Compton components in GRBs for the first time (e.g.
MAGIC Collaboration et al., 2019a) and the discovery of a long GRB with a
prompt phase powered by fast cooling synchrotron emission followed by a
subsequent kilonova (Rastinejad et al., 2022; Gompertz et al., 2023). The
identification of GRB 221009A as the brightest of all time (a.k.a the BOAT)
was enabled also by the collaboration tool Slack, which allowed various
instrument teams to compare results more rapidly than is possible through GCN
alone.

Swift became the dominant source of prompt, precise localizations of GRBs and
the dominant discoverer of magnetars following its launch, resulting in a
general perception that the IPN was no longer necessary. As will be described
in the following sections, among the most important figures of merit for GRB
monitors is detection rate and localization precision. To motivate the
continued need for the IPN we point to Figure 2. The IPN provide systematic
improvement over Fermi-GBM localizations for a large number of events,
including all bright events, with localization precision approaching that of
Swift-BAT. As bright and nearby “Rosetta Stone” TDAMM astronomy events provide
unique advances in understanding their broader populations, the IPN is
indispensible.

Figure 2: The GRB localization precision samples of Fermi-GBM, Swift-BAT and
Swift-XRT, and the IPN. On an annualized basis the IPN improves localizations
for roughly half of GBM GRBs, which is comparable to the rate of Swift-
localized events.

For direct scientific examples, since the launch of Swift the IPN has been
instrumental in proving that some short GRBs arise from extragalatic magnetar
giant flares (Frederiks et al., 2007b; Mazets et al., 2008; Svinkin et al.,
2021; Burns et al., 2021). Key to those observations is Konus onboard the Wind
spacecraft at Earth-Sun L1, which has provided near continuous all-sky
coverage of the high-energy sky since 1994. This, coupled with the background
stability in the Wind orbit, makes Konus the dominant identifier of the bursts
which belong to the ultra-long class of GRBs as well as key to searches for
high-energy emission around optically-identified relativistic transients (e.g.
Margutti et al., 2019; Andreoni et al., 2021; Ho et al., 2022). The
development of sensitive wide-field observatories has allowed tiling of large
localization regions, where higher latency IPN localizations of the prompt
phase can be used to unambiguously associate optical counterparts, as was done
to identify the collapsar with the shortest prompt phase identified (Ahumada
et al., 2021). The flagging of active magnetar states by Swift and the IPN was
key to radio and X-ray observations of SGR 1935+2154, leading to the first
joint detection of an SGR short burst with a contemporaneous fast radio burst
(FRB), proving magnetars as an origin of FRBs (Bochenek et al., 2020;
Mereghetti et al., 2020). Long-term monitoring of the SGR 1935+2154 flares
proved this particular X-ray flare as unique (Ridnaia et al., 2021). Most
recently, the rapid IPN localizations of GRB 230307A enabled most direct
measure of heavy element (r-process) creation (Levan et al., 2023).

Figure 3: The 51 past, present, and future missions of the IPN. Color
corresponds to primary spacecraft discipline. Low Earth orbit satellites have
many hashes, decreasing to solid colors for distant spacecraft. Faded regions
correspond to nominal extensions which are assumed to be 5 years for active
missions, unless precise dates are known. INTEGRAL shows both the planned shut
off date and the Earth reentry date.

The brief historical overview of the study of short-duration gamma-ray
transients serves to highlight a number of themes which will be evident
throughout this report. For 50 years there have been a steady supply of major
breakthroughs in high-energy astrophysics. While these discoveries were
initially made by the high-energy monitors alone, for 30 years it has been the
joint advancement in the capabilities of the high-energy monitors, ground-
based facilities, and growth in the adoption of GCN that has allowed for
continued advancement. The decision of the high-energy monitors to release
open alerts fostered collaboration within the community and has led to grander
characterization of these explosions. The importance of rapid, precise, and
accurate localizations are evident. Throughout this time the IPN has
contributed unique capabilities and complemented the larger-scale, dedicated
NASA GRB missions. The numerous list of IPN missions and instruments is shown
in Figure 3. With implemented upgrades to other wavelength and messenger
capabilities, NASA investment in a major upgrade to GCN, and as we are in the
longest gap since field inception for a dedicated NASA GRB monitor at Explorer
scale or larger, it is time to plan for the next phase of gamma-ray transient
monitors.

The emphasis on time-domain and multimessenger astronomy by Pathways to
Discovery in Astronomy and Astrophysics for the 2020s, the Astro2020 Decadal
report, led to this Science Analysis Group tasked with quantifying what could
and should be done with the IPN. Section 2 contains subsections focused on the
study of magnetars, compact mergers, and collapsars by high energy monitors.
Each discusses the various signals, scientific results, instrument
requirements, and high energy monitor products of relevance for these sources
and affiliated communities. In Section 3 we explore the science unlocked with
fiducial values of various capability requirements for high-energy monitors.
These two sections take a holistic view of the field and conveys the science
that can be done with the IPN. Section 4 provides a specific set of actionable
items that NASA can undertake to maximize scientific return with current,
forthcoming, and potential future gamma-ray instruments. Section 5 outlines
the requested data products from the IPN, identifies where specific
operational and programmatic improvements to the IPN can be made, and
discusses the additional benefits which may arise from sustained investment in
the IPN. We close with conclusions in Section 6. The sections contain some
duplication of information but are separated to allow for self-contained
information for the various use cases of this document, including members of
the IPN, instrument teams, the community, and NASA.

##  2 Sources

GRB monitors recover short-duration transients in the keV to MeV regime. Below
are subsections focused on magnetars, compact mergers, collapsars, and other
transients. Each of these subsections discusses the various signals expected
from these sources, the scientific results that can be made through their
study, the instrument requirements and capabilities that enable this progress,
and a discussion on the products from the high-energy teams that would be of
great value for the broader community which studies those objects.

The most critical requirements, which have never been met with a single
observatory, are localization precision, alert latency to follow-up
facilities, uninterrupted all-sky coverage, and sensitivity. Localization
precision and alert latency are often deeply intertwined. Uninterrupted
viewing requires long-term (ideally, continuous) contiguous viewing intervals
as well as a high total observing fraction which is set by the instantaneous
field of view (often more than half the sky) and livetime (generally in excess
of 80%). The sensitivities quoted are characteristic order-of-magnitude scales
and are baselined against the typical observing range for gamma-ray
transients. Temporal requirements include absolute timing precision (offset
from a standardized reference time), relative timing precision (offset between
events observed on-board), and temporal resolution of the data. Energy range
is discussed only when the requirements exceed the fiducial 50-300 keV
detection range. We additionally discuss background stability, maximum photon
rates, spectral resolution, and polarization, when necessary.

###  2.1 Magnetars

A supernova explosion, or more rarely low-mass mergers or accretion-induced
collapse of a white dwarf, can leave behind a NS remnant, a dense stellar
object supported by quantum degeneracy pressure rather than fusion. Only $\sim
25$ kilometers across, NSs are so dense they exceed the mass of the Sun; these
objects sit near the density limit of matter. A significant fraction of these
new NSs are magnetars (Beniamini et al., 2019), hosting the most powerful
magnetic fields in the cosmos with flux densities of order $10^{13}-10^{15}$
Gauss.

Magnetars are classified as soft gamma-ray repeaters (SGRs) or anomalous X-ray
pulsars (AXPs). SGRs are the subclass of interest for high-energy monitors as
they are observed to release X-ray and soft gamma-ray short bursts, magnetar
giant flares (MGFs), and FRBs. Short bursts are brief high-energy flashes with
a duration on the order of $\sim$1-1000 ms, luminosities observed from $\sim
10^{36}-10^{44}$ erg/s, and emission generally between 1 and 150 keV. As the
spectral hardness tends to increase with luminosity, the faintest events are
observed with narrow-field, pointed X-ray telescopes, and the more energetic
events by wide-field GRB monitors. SGR flares are distributed with energy and
waiting time distributions similar to earthquakes (Cheng et al., 1996) tying
the emission trigger to the NS crust. These objects can power SGR storms where
numerous flares are released in rapid succession (e.g. Younes et al., 2020).

The most powerful SGR flares are MGFs. The first signal seen from a magnetar
was an MGF originating in the Large Magellanic Cloud in 1979 (e.g., Cline et
al., 1982), though its origin was unclear at the time. Two more have since
been observed in the Milky Way. These flares have prompt spikes less than 10
milliseconds long with luminosities in the range $\sim 10^{45}-10^{48}$ erg/s
which are followed by an energetically weaker, fainter, periodic tail that
decays exponentially over time. It was these signatures that led to the
theoretical conception of magnetars where some of the plasma ejected in the
initial energy release that powers the spike is entrained to the surface of
the NS by the extreme magnetic fields with the rotation giving rise to the
periodic behavior (Duncan & Thompson, 1992; Thompson & Duncan, 1995).

SGR flares are softer than GRBs, repeat, and always have short durations.
These unique identifiers led to the separation of these transient classes by
the IPN, and the precise triangulation of SGRs (Hurley et al., 1994, 1998) led
to successful X-ray follow-up and proof of the existence of magnetars
(Kouveliotou et al., 1998a, b).

####  2.1.1 An Origin of Short Gamma-ray Bursts

MGFs are sufficiently luminous to be detected from nearby galaxies outside the
Milky Way (Hurley et al., 2005; Svinkin et al., 2021). The short spike is
visible to tens of Mpc, but the periodic tail emission falls below the
sensitivity of existing instruments. The detection then looks very similar to
a typical short GRB from a NS merger. The method to separate MGFs from
cosmological short GRBs is association to nearby host galaxies. There are 5
extragalactic MGF candidates so far which appear as short GRBs, proving the
third known progenitor of these events (Burns et al., 2021, Trigg et al. in
prep).

These objects are of interest for several reasons. To start, they constitute
$\sim$0.3% of all detected GRBs (2% of short GRBs). They are additional giant
flares from magnetars that can be used to probe the science described in the
following subsections. The initial prompt spike of galactic MGFs saturate all
viewing GRB monitors; extragalactic events can thus be studied in more detail
(though these too can saturate large, sensitive detectors). Studies of past
events include identifying multiple pulses in giant flares, measurement of a
$\sim$-5/3 power-law index of the intrinsic energetics distribution, and
confirmation of a peak energy correlation with total energy output (Roberts et
al., 2021; Burns et al., 2021; Chand et al., 2021), behaviors which may
connect with known properties of the more common short bursts and suggest the
origin as a relativistic wind. If sensitivity and time resolution are
sufficient, detection of quasi-periodic oscillations (QPOs) across populations
of extragalactic magnetars could inform on physics of their interiors.

Larger samples of extragalactic MGFs with multiwavelength information of
associated host galaxies (galaxy type, age, star formation rate, metallicity,
potential offsets, etc), would enable deeper understanding of formation
channels of magnetars, discussed further in Section 2.1.6. This would result
in direct comparison with similar measures on other transients such as core-
collapse supernova (CCSN) explosions, superluminous supernovae, collapsars, NS
mergers, and FRBs and inform on their potential relation to magnetars or
magnetar formation. Finally, constraints on the lensing of MGFs (which
requires broadband spectra with high temporal and spectral resolution) may
constrain primordial black holes as dark matter (Katz et al., 2018).

With reasonable assumptions (Burns et al., 2021) four extragalactic MGFs have
been associated to nearby star-forming galaxies with IPN 90% contour region
sizes between $\sim 10-150$ arcminute2 (typical Swift-Burst Alert Telescope
(BAT) localizations at the same confidence are $\sim$30 arcminute2). An
example localizations is shown in Figure 4. When utilizing known properties of
MGFs (sharp rise times, short pulse durations) to reduce the considered
sample, it is possible to associate flares from the likeliest host galaxies
even with $\sim$10 deg2 localizations (A. Trigg, in prep). As the detection
horizon of these events increases, the necessary spatial precision requirement
becomes stringent, with uncertainties on the few arcsecond scale necessary
beyond tens of Mpc. Short GRBs with these properties could be identified as
candidate MGFs with larger localization regions, allowing for observations of
the putative host. The known counterpart is the decaying X-ray tail. Swift-XRT
can recover a typical tail at 3.5 Mpc if it points within $\sim$400 s. Earlier
observations or a more sensitive X-ray telescope which would relax this
stringent timescale would give information on the rotation period of the star,
of relevance for understanding its origin.

Figure 4: IPN localization of GRB 200415A to NGC 253, aka the Sculptor Galaxy.
Figure credit NASA’s Goddard Space Flight Center, DSS, SDSS, and Adam
Block/Mount Lemmon SkyCenter/University of Arizona utilizing data from Svinkin
et al. (2021).

We emphasize the local overdensity of star formation rate is particularly
helpful for our study of extragalactic MGFs. These events are best observed by
all-sky coverage over long timescales. At current sensitivities, $\sim 5\times
10^{-6}$ erg s-1 cm-2, all-sky detection rates are roughly 1 per year within
10 Mpc though most are not separated from the cosmological short GRB sample.
An order of magnitude increase in sensitivity would capture 3 events per year
within the same distance and additional further events, but this requires more
precise localizations given the larger number of galaxies considered. Study of
these events benefit from sub-millisecond temporal resolution and energy range
up to several MeV to constrain spectral curvature and thus intrinsic
energetics.

####  2.1.2 The Neutron Star Equation of State

The tails of the giant flares, and perhaps also the lesser flares, exhibit
QPOs, where the rotational frequency is modulated by higher frequencies. These
frequencies have been attributed to oscillations of the stellar crust or
interior, i.e. global NS eigenmodes (Watts & Strohmayer, 2007; Huppenkothen et
al., 2014). A leading model is that these disturbances are either cause or
effect of quakes associated with the NS crust. These would ring up mechanical
modes which are observed in the X-ray QPO data. It may also be possible to
detect potential NS oscillation mode frequencies in extragalatic MGFs (Roberts
et al., 2021; Castro-Tirado et al., 2021).

Observations of these QPOs may give unique insight into the equation of state
of dense matter, the crust-core interface, and the physical origin of the SGR
flares (Watts & Strohmayer, 2007); however, the current Galactic sample is too
limited for reliable NS core/crust inferences because of confounding factors
such as the influence of the magnetic field on the eigenmodes. Such studies
would clearly benefit from much larger collections of QPO detections enabled
by extragalactic studies. The natural modes of NSs run up to a few kHz. This
sets a limitation on the relative timing precision necessary to perform these
searches, being a factor of a few better than the inverse of these
frequencies, i.e. $\sim$100 $\mu$s resolution. These searches would also be
aided by greater sensitivity than existing instruments.

####  2.1.3 A Potential Source of Gravitational Waves

One specific class of NS natural modes, the $f$-modes, are a plausible source
of GWs. Because of the lack of observational constraints, models for the GW
energy emitted by such events varies widely. LIGO and Virgo have searched for
GW emission associated with magnetar X-ray flares during the observing runs in
which such activity was present. No GWs correlated to magnetar flares have
been detected through the 3rd GW observing run (The LIGO Scientific
Collaboration et al., 2022), known as O3. Giant flares are of particular
interest as they may be energetic enough to activate NS modes (Duncan, 1998).
The last recorded giant flare from a Galactic magnetar (SGR 1806–20) occurred
in 2004 (Palmer et al., 2005), where LIGO reported upper limits (Abbott et
al., 2007) but the sensitivity of the nascent GW detector network was $\sim
100$x worse than the current network. We may expect a similar $\sim 100$x gain
in the IGWN over the next 20 years as planning has begun for the third
generation (3G) GW interferometers. Even before the 3G network is active, we
should enter an era where GW observations of extragalactic MGFs are more
sensitive than the current limits on the Galactic SGR 1806–20 event.

SGR storms may provide yet another particularly promising target. While each
burst may impart only minor energy into the modes, the integration of this
effect from hundreds or thousands of bursts may enter the interesting regime.
Again, the only current observations are from the previous LIGO generation
(Abbott et al., 2009). Stacking of individual bursts during a given observing
run may also prove fruitful.

The ever-increasing sensitivity of the IGWN network, coupled with the
possibility of a more energetic burst, nearby magnetar, or SGR storms,
inspires optimism. Even improved upper limits can provide astrophysically
interesting constraints on these mysterious and observationally elusive
objects. We may perhaps expect other GW signals from magnetars. As one
example, deformation of the magnetar following an MGF would produce GWs at
twice the rotational frequency, though this is generally below the frequency
range accessible to current ground-based interferometers.

Complete cataloguing of the X-ray bursts will be crucial to the success of
this program. This requires continuous monitoring of the high-energy sky
during the observing run, and identification of SGR flares. These bursts are
preferably associated to specific known magnetars which requires at least
$\sim$1 deg scale localizations. While discovery of new magnetars in crowded
fields might demand arcminute-scale or better capability, this precision can
be met by stacking numerous localizations of individual flares (e.g. Hurley et
al., 1998, 1999). These targeted GW searches would be greatly aided if QPOs
could be identified in the high-energy bursts, providing specific frequencies
to target. SGR flares identified and studied in real time, particularly giant
flares, may allow for rapid targeted GW follow-up. If a GW signal is
identified then additional follow-up resources can be marshalled for greater
characterization of the magnetar behavior.

####  2.1.4 The Origin and Physical Mechanism of Fast Radio Bursts

FRBs are $\sim$ ms long radio flashes whose dispersion measures place their
origin at cosmological distances, with the first bursts detected decades ago
(Linscott & Erkes, 1980; Lorimer et al., 2007). Yet, only in the last decade
have FRBs garnered community interest as their genuine astrophysical origin is
now firmly established (Petroff et al., 2019; Cordes & Chatterjee, 2019;
Petroff et al., 2022; Zhang, 2022). In many ways, the quest to identify the
sources of FRBs followed the same path as GRBs: searches for repetition,
counterparts in other wavelengths and messengers, and identification and
characterization of host galaxies. The searches for repetition have led to the
phenomenological classes of non-repeating and repeating FRBs, the latter of
which show similarities and are thought to be related to SGR short bursts
(Scholz et al., 2016; Wadiasingh & Timokhin, 2019; Wadiasingh et al., 2020;
Oostrum et al., 2020; Wei et al., 2021; Zhang et al., 2021; Wang et al., 2023;
Lander, 2023). However, radio observations of the dispersion measure,
polarization, and rotation provide additional and unique diagnostic
information on the origin of these sources (Petroff et al., 2019).

In April 2020, the first Galactic FRB was observed, and was associated with a
frequently flaring magnetar, SGR 1935+2154 (Bochenek et al., 2020; CHIME/FRB
Collaboration et al., 2020). This provided new insight into FRB progenitors,
in particular for repeating FRBs, and provides additional impetus to the need
for excellent X-ray monitoring and localization. Of crucial importance to
models is the time delay between radio and X-rays after de-dispersion (i.e.
infinite frequency-equivalent arrival time of the radio). For some models, the
FRB counterpart can never arrive before the magnetar short burst. For the SGR
1935+2154, the FRB arrived several seconds after its corresponding high-energy
short burst (Tavani et al., 2020; Zhang et al., 2020). However, when corrected
for dispersion the FRB lagged the high-energy spikes by several milliseconds
(Mereghetti et al., 2020; Ge et al., 2023), favoring a magnetospheric origin
for the FRB. Therefore accurate absolute timing precision for the X-ray flares
of $<1$ ms is an important requirement. The FRB–X-ray time delays, and
potential rise time of X-rays, can also inform on how spatially co-located
respective the emission regions are and constrain plasma conditions or
radiative processes processes responsible for the X-ray counterpart.

A crucial question is why there is not a one-to-one correspondence with SGR
X-ray short bursts and FRBs. Hundreds of X-ray bursts were observed during the
recent SGR 1935+2154 active state, but the one seen with the FRB is spectrally
harder than the others (Ridnaia et al., 2021; Younes et al., 2021).
Additionally, other bright FRBs were seen from SGR 1935+2154 without
corresponding X-ray flares (Good & Chime/Frb Collaboration, 2020; Kirsten et
al., 2021), though the X-ray upper limits are insufficient to be interesting.
In 2022 another FRB from SGR 1935+2154 (Dong et al., 2022) was seen in
coincidence with an X-ray short burst (Wang et al., 2022; Frederiks et al.,
2022). Continued monitoring of the high-energy sky is necessary to determine
active and quiescent times of the Galactic magnetars and to provide
contemporaneous observations of FRBs. A key goal is to determine if other
Galactic magnetars also produce FRB emission, or if something specific about
SGRB 1935+2154 makes it unique (e.g., the environment). Construction of
complete (in time and with a well-defined, stable, and uniform flux threshold)
and detailed SGR burst catalogs with spectral properties is likely key to
understanding the origin of SGR activity and FRBs.

Localization precision is necessary to associate SGR flares to specific
magnetars, though $\sim$deg scale localizations are likely sufficient for
flares from magnetars known to be in active states. Improved sensitivities of
the high energy monitors and lowering the low energy threshold to $\sim$1 keV
would result in more complete coverage of the flares and bursts.

We emphasize that the association of FRBs to the SGR short burst X-ray
emission is the only identified counterpart for FRBs. This is despite enormous
observations across the EM, GW, and neutrino spectra. This is, so far, the
only method of studying FRBs with detections with multiple diagnostics.

####  2.1.5 Discovery of New Magnetars

Continued monitoring of the high-energy sky is also necessary to identify the
rest of the Galactic SGRs which is generally achieved by observing their X-ray
flaring and burst activity. The first magnetars discovered were SGRs
identified and localized by the IPN, while Swift now discovers them at a rate
of approximately once per year. As SGR flares can be dormant for decades and
faint bursts can be undetected, sensitive all-sky X-ray monitoring is key to
complete this sample.

Given the decades-long quiescence seen from many SGRs, it is critical to
maintain continuous coverage of the high-energy sky to complete the Galactic
census of magnetars. This matters for understanding stellar formation and
evolution. As one example, the current constraint that $>10$% of NS produced
in CCSN are magnetars is hard to explain against other observations, e.g. the
rates of superluminous supernovae. It may also be possible to identify SGR
flares from previously unidentified magnetars (and also past flares from known
magnetars) in historic data, if a dedicated search were performed. Lastly,
identification of extragalactic MGFs allows the study of magnetars in other
galaxies which enables studies on their origins and formation channels.

####  2.1.6 Magnetar Formation Channels

Observations generally support that magnetars are produced from a subset of
CCSN, as these cataclysmic events produce the majority of NSs. However,
several formation channels exist beyond the base CCSN model, including
superluminous supernovae (Nicholl et al., 2017), low-mass mergers (Price &
Rosswog, 2006; Levan et al., 2006), and some white dwarfs which undergo
accretion-induced collapse (Dessart et al., 2007). Resolving this question can
uncover the fundamental question of how the most powerful magnetic fields are
created, probing whether they arise from the compression of the progenitor
material or are the result of emergence in dynamos of the proto-NS. It is also
unclear what role, if any, stellar binarity plays in CCSN formation of
magnetars, as no magnetars are known to exist in binaries (with a possible
exception, Richardson et al., 2023). However, magnetars may have been born and
kicked from their originating binary system as most massive stars that form NS
are formed in binaries and triples and interact before the supernova occurs
(Sana et al., 2012).

Specific outcomes of these studies can provide insight into the origin of
superluminous supernovae and GRBs, even if magnetars are simply a rare
byproduct of basic CCSN. Modeling the broad behavior of known Galactic
magnetars (next section) favors the fraction of CCSN that produce magnetars to
be $\gtrsim$10% (Beniamini et al., 2019) which is supported by the high
intrinsic rate of MGFs (Burns et al., 2021). If proven, then CCSN modelers
must consider extreme magnetic fields, and formation of NSs over black holes,
in a non-negligible fraction of these events. Studies of MGFs thus offer a
method of studying populations of relatively young NSs beyond our galaxy.

We can determine the relative importance of magnetar formation channels
through several means. Greater characterization of the Galactic magnetar
population and new magnetars as they are identified is critical (Beniamini et
al., 2019). Identifying more MGFs and further constraining their rate to a
high value would require the formation channels to also have high rates,
likely limiting the options to common CCSN or accretion-induced collapse of
white dwarfs. Completion of this picture will involve association of magnetars
to specific hosts. In the case of Galactic sources, this means classifying the
origin of the remnant they are associated to, either to CCSNe or Type Ia SNe,
which may be done with COSI through the study of nuclear lines from their
supernova remnants. For extragalactic MGFs or repeating FRBs (presuming the
magnetar origin) the host galaxy and location within the host galaxy is key,
again repeating the approaches of the GRB field. Events from star-forming
regions favor massive star origins while association to older regions or large
offsets from host galaxies favor the merger origin.

Accomplishing this science thus requires advancements across several aspects
of the field. The requirements specific to the GRB monitors are those listed
in other sections here, i.e., the identification of new magnetars,
identification of more SGR flares and MGFs, providing those locations to the
community, and determining the origin of FRBs.

####  2.1.7 Magnetar Properties

To understand the magnetar formation channels, it is important to understand
the behavior of magnetars themselves. A more complete, well-characterized
sample will permit additional insight into the intrinsic physics of magnetars,
such as understanding the lifetime and activity rate of magnetars and
connections to the physical mechanism of their magnetic field evolution and
decay (Beniamini et al., 2019). These are basic questions on the most powerful
magnets.

One additional question is whether each magnetar can produce at most one MGF.
It is possible that a single giant flare is triggered by a nuclear or
magnetohydrodynamic phase transition that prevents another from occurring from
the same magnetar (Mallick & Sahu, 2014). This involves the behavior of ultra-
dense matter with extreme magnetic fields, particularly stability conditions
associated with the cores of NSs. A direct answer to this question would be
identifying two giant flares from the same source with low false alarm
probability. This requires monitoring the three magnetars in the Milky Way and
Large Magellanic Cloud which have undergone MGFs since observations began in
the 1970s, requiring continued all-sky coverage. An alternative approach is
precise localizations of extragalactic MGFs from nearby star-forming galaxies,
which may have more active magnetars ($\sim 1^{\prime}$ at 3.5 Mpc corresponds
to a physical scale of 1 kpc). A future IPN could achieve $1^{\prime\prime}$
precision, capable of monitoring magnetars in many local galaxies for
repetition, and could provide a decisive answer. Otherwise, an indirect
statistical method, requiring improved identification of extragalactic MGFs,
can be used to infer a giant flare rate that would only be possible with
repetition (Burns et al., 2021).

####  2.1.8 Physics of Magnetar Bursts

The exact mechanism or trigger that produces magnetar bursts (in both SGRs and
AXPs) remains unknown, although we know it ought to involve the crust of the
NS. Diagnostics including time offset from FRB emission, QPOs, GWs, rotational
phase dependence of bursts, and broadband time-resolved spectroscopic
information can all provide useful information to answer this question. X-ray
studies with GRB instruments have shown most magnetar bursts can generally be
described by two blackbodies of similar luminosities (Israel et al., 2008; van
der Horst et al., 2012; Younes et al., 2014), one of low area and high
temperature ($kT\sim 10-20$ keV and the other high area and lower temperature
($kT\sim 3-5$ keV). This is consistent with a confined Comptonized fireball
trapped within a flux tube of the magnetar magnetosphere. Above $\sim 50$ keV,
single photon splitting (a third-order, untested, but standard
quantumelectrodynamic process, $\gamma\rightarrow\gamma\gamma$) is expected to
operate and be relevant for magnetar bursts by imprinting spectral and
polarimetric modifications onto some bursts, depending on the viewing geometry
(Hu et al., 2019). Long term monitoring of SGRs and AXPs is insightful into
understanding what separates this class, especially with the rare SGR-like
flares seen from some AXPs (Savchenko et al., 2010). Continued all-sky
monitoring to determine the waiting time distributions and any identifiable
behavior on the active versus quiescent period durations would also be
constructive.

We additionally emphasize that polarimetry of bright SGR flares, or the
stacking of many bursts, may probe the physical emission region, the geometry
of the magnetic field, and the extrinsic properties of inclination with
respect to magnetic and rotation axes. Similar inferences can be made in the
polarization observations of MGF tails (Yang & Zhang, 2015). Success here from
high-energy monitors requires polarization sensitivity down to tens of keV.

New observations of Galactic MGF tails with more sensitive instruments which
have higher temporal resolution could reveal the presence of a deviation from
a single peak phase-gram, giving information on the geometry of the emitting
region on the surface, which may not be compatible with a simple fireball
model.

Magnetar burst spectra have been found to have a predominately Comptonized or
thermal spectrum. Recent observations of unique events from the outburst of
prolific magnetar SGR 1935+2154 strongly suggest that QPOs sometimes occur
over the peak energy of $\nu F\nu$ spectra (Roberts et al., 2023). These
quasi-periodic spectral oscillations, can be explained by magnetospheric
density and pressure perturbations where burst emitting plasma consisting of
purely e+e- pairs propagate along a highly magnetized flux tube of several
tens to a hundred NS radii. Some of these events occur within days of each
other which provides an interesting constraint on the activation timescales
over which the environment of the magnetosphere of SGR J1935+2154 (and
perhaps, magnetars in general), change. Therefore, detailed time-resolved
analyses of other magnetar bursts are encouraged to evaluate the rarity of
these events.

####  2.1.9 Summary of Capability Requirements

The most stringent localization requirement for high-energy monitors observing
magnetar flares is driven by extragalactic MGFs. For particularly nearby
events ($\lesssim 5$ Mpc) from bright, star forming galaxies, robust
association of individual events to 3$\sigma$ requires localizations at 90%
confidence of order $10$ deg2. For events to $\sim$20 Mpc, or those from less
likely host galaxies, this requires uncertainty at the level of a few
arcminutes. At even greater distances arcsecond scale localizations are
required for robust association. For science from the high-energy observations
alone there is no alert latency requirement.

To recover the known fading tail counterparts with Swift-XRT requires
observations beginning in minutes. Rapid and precise localizations could
enable this. For rapid localizations up to $\sim$100 deg2, MGF candidates and
likely hosts could be identified with fiducial cuts on the lightcurve,
allowing for galaxy-targeted follow-up. Higher latency data could confirm or
reject classification.

For the less energetic short bursts localizations of $\sim 1$ deg2 are
sufficient to associate bursts to catalogued magnetars. As flares often repeat
in active periods an alert latency on the order of a day is sufficient to flag
that magnetar as active for X-ray, radio, or other follow-up efforts. The
discovery of Galactic magnetars requires localizations (either individual or
stacked) to $\sim 0.3$ deg, i.e. the typical field of view of X-ray
telescopes.

Monitoring the Galaxy for potential SGR-FRB joint detections, or sensitive
X-ray upper limits of Galactic FRBs, requires continuous all-sky monitoring as
the timescales are so short the only method of counterpart detection is to
have made contemporaneous observations. Discovery of new magnetars or
extragalactic MGFs can be made with smaller fields of view but benefit from
100% sky coverage.

A substantial increase (factor of several) in sensitivity over the historic
average of the IPN would increase the rate of extragalactic MGF identification
from every several years to $\sim$1 per year, sufficient for major advances in
understanding. MGFs drive the high-energy threshold requirement to $\sim$5
MeV, necessary to constrain spectral curvature. The detection of many normal
SGR short bursts requires sensitivity down to $\sim$5 keV and a high-energy
threshold of $\sim$150 keV. Lowering this threshold further would recover
more, softer events allowing for a more complete sample. Pointed X-ray
telescopes can detect fainter events than high-energy monitors, but they may
not be observing contemporaneously with interesting events such as Galactic
FRBs. Substantial polarization sensitivity down to $\sim$tens of keV increases
is required for polarimetric study of these events given their very short
durations.

Magnetar flares drive timing requirements for high energy monitors. Absolute
timing of millisecond precision or better allows for physical insight into the
SGR and FRB emitting regions and processes. Relative timing precision and
temporal resolution to 50 $\mu$s allows for study of the shortest features in
giant flares.

Galactic MGFs provide the most stringent requirement for maximum photon rates.
The SGR 1806–20 giant flare was the most energetic with $\sim 1.5\times
10^{7}$ photons/s/cm2 (Frederiks et al., 2007a). A closer or more energetic
event could hit $10^{8}$ photons/s/cm2, though the other two Galactic events
were a factor of 10 less energetic. Extragalactic MGFs can exceed 1000
photons/s/cm2 (Svinkin et al., 2021; Roberts et al., 2021), suggesting an
instrumental capability threshold of a few thousand ph/s/cm2.

There are only weak requirements on contiguous viewing with individual
instruments and on background stability given the short emission intervals.
These objects are not known to have sharp spectral features so they do not
drive energy resolution requirements.

Complete SGR studies require extremely long baselines to map the long end of
their waiting time distribution, to study the durations of active and
quiescent periods, to search for SGR flares from AXPs, and to discover new
magnetars.

####  2.1.10 Requested Deliverables from GRB Monitors

The science possible from high-energy monitor observations of magnetars would
benefit from several new or improved community products. A key product is an
SGR flare naming convention to be adopted by the GRB monitor teams. This would
be important to implement for a high-latency SGR flare catalog. This catalog
collates information from multiple legacy and current missions and includes
entries on each signal, its arrival time, and any duration and spectral
information. Given the particularly short durations and finite light travel
time, association of flares observed by distant instruments is non-trivial,
especially when the specific magnetar origin is unknown.

A real-time collated Galactic SGR flare alert stream would also be beneficial.
This would provide a central access point for multiwavelength and
multimessenger services, allowing for identification of joint detections fast
enough for prioritized follow-up that would help characterize the event and
its effect on the source magnetar. Such an alert stream would also allow
automatic flagging of active and quiescent state informative for pointed
observations at other wavelengths, e.g. radio. Of particular interest would be
an automated flagging of Galactic giant flares for incredibly bright bursts in
this stream, which would enable automated repointing of various telescopes
like Swift-XRT.

Lastly, automated searches to promptly flag short GRBs as MGF candidates may
be key to recovering X-ray tails from extragalactic MGFs. This requires
collation and calculation of localization information as rapidly as possible,
within minutes, and convolution with local galaxy catalogs.

###  2.2 Compact Mergers

Binary neutron star (BNS) mergers and neutron star-black hole (NSBH) mergers,
both cases herein defined generally as NS mergers, are canonical
multimessenger sources, producing high frequency GWs (up to a few kHz), light
across the EM spectrum, and must also produce neutrinos. These messengers
arise from a variety of emitting regions and processes. Successful
understanding of the extreme physics occurring in these events and the
corresponding signals is difficult, but is critical to achieve: the detection
and characterization of BNS mergers yield a cornucopia of science cases across
astrophysics, including (i) the nucleosynthesis of heavy elements, i.e. those
heavier than iron (Just et al., 2015; Wu et al., 2016; Roberts et al., 2017;
The LIGO Scientific Collaboration and Virgo Collaboration, 2017a), (ii) the
nature of matter in the densest environments known (Bauswein et al., 2013; The
LIGO Scientific Collaboration and Virgo Collaboration, 2017b; Radice et al.,
2018; Bauswein et al., 2017), (iii) what drives the expansion rate of the
Universe and other cosmological parameters (LIGO Scientific Collaboration and
Virgo Collaboration, 2017; Hotokezaka et al., 2019; Coughlin et al., 2020; Du
et al., 2019), (iv) studies on ultrarelativistic particle acceleration (Abbott
et al., 2017; Burns, 2020; Salafia & Giacomazzo, 2021), and (v) unique tests
of fundamental physics (Abbott et al., 2017; Creminelli & Vernizzi, 2017;
Burns, 2020).

The NS merging event is preceded by a long, slow inspiral caused by loss of
energy to GWs. The inspiral accelerates and emission strengthens until the
objects merge producing a crescendo of GW emission at $\sim$kHz frequencies
that exceeds the EM luminosity of GRBs. At least some NS mergers produce
bipolar ultrarelativistic jets which release the prompt GRB signal. As these
jets propagate outwards they build up an external shock with the circumburst
material, releasing synchrotron radiation across the EM spectrum.

EM-bright mergers shed material quasi-isotropically through various processes.
This expelled material has high density and is neutron rich, and thus combines
into heavy, radioactive elements. As the initial cloud is opaque, this creates
a quasi-thermal signature referred to as a kilonova (Li & Paczyński, 1998;
Metzger et al., 2010; Metzger, 2020). These events emit predominantly in the
ultraviolet (UV) to infrared (IR) wavelengths on timescales from hours to
months.

More than 1000 short GRBs have been observed in the prompt phase, and
$\sim$10% of them have detections from follow-up observations at other
wavelengths. Follow-up detections lead to arcsecond-scale localizations which
help determine the host galaxy and the source offset from the host galaxy.
Distances are generally determined from a redshift measurement of the
associated galaxy. Broadband observations of the afterglow over multiple
epochs gives insight into the jet structure, the circumburst density, and
total kinetic energies. $\sim$ 10 short GRBs have evidence of an associated
kilonova, though few are well characterized. There are still only a few GW
detections of BNS and NSBH mergers. The first GW detection of one of these
events was the BNS merger GW170817, associated to GRB 170817A. These
multimessenger joint detection is shown in Figure 5.

Figure 5: The first multimessenger detection involving a GW source. LIGO and
Virgo observed GW170817 and Fermi-GBM and INTEGRAL SPI-ACS observed GRB
170817A only $\sim$2 s later. Image credit NASA’s Goddard Space Flight Center,
Caltech/MIT/LIGO Lab and ESA.

There are a number of additional signatures known from observations or
predicted from theory, as discussed and referenced in the following
subsubsections. Observations show some short GRBs are followed by plateau
emission with durations $\sim 10-10^{5}$ s that can be seen in the GRB
monitors or in rapid follow-up observations in X–rays. There are also claims
of non-thermal precursor activity to the high–energy emission. The ultraviolet
(UV) emission of AT2017gfo, the name of the optical counterpart to GW170817,
was unexpectedly at its brightest when first observed at 12 hours. The origin
of this signal remains debated with options including cocoon / shock-breakout
of the jet through the previously ejected kilonova material, the decay of free
neutrons, energy ejection from a magnetar central engine, or base kilonova
emission. There are several theoretical predictions for which observations
remain ambiguous. Examples include the kilonova afterglow, radio precursors,
X-rays from late-time fallback, and late-time radio excess from a magnetar
central engine. Those which require early observations are described below,
while the others can likely be recovered in follow-up of GW-detected NS
mergers that do not have prompt GRB counterparts. The numerous counterparts
cause confusion in interpreting observations, but they provide an opportunity
to more deeply understand the physics occurring in these events.

Specific to the study of GW-detected NS mergers, multimessenger studies are
enabled with identification and association of EM counterparts. As the GW
interferometers become more sensitive the detection distances grow linearly
and thus the rates as a cubic factor (until source rates evolution becomes
important, first increasing then decreasing). At any projected era the total
detected sample of GW localizations will remain poor for the majority of
events (Petrov et al., 2022). Even major follow-up facilities like the Vera
Rubin Observatory will begin struggle to identify, isolate, and associate
kilonovae at these distances. Thus, beginning perhaps as quickly as O5, GRB
monitors will be the dominant discovery mechanism for EM counterparts to GWs,
especially at greater distances. Ideally, they provide localizations
sufficient for any telescope to utilize for full characterization across the
EM spectrum.

####  2.2.1 Plateau Emission

BATSE discovered the existence of a class of short GRBs which have extended
emission visible in the prompt detection (Connaughton, 2002; Norris & Bonnell,
2006). This signature has subsequently been observed in bursts detected by
Fermi-GBM and Swift-BAT (Kaneko et al., 2015). Follow-up of Swift bursts
established that short GRBs with and without extended emission have similar
offsets from their host galaxies, pointing to a merger origin in both cases
(Fong et al., 2009). Utilizing the autonomous repointing of Swift’s narrow-
field telescopes to BAT-triggered events, a large sample of early afterglow
observations from X-ray through optical has been constructed. A common
signature is an X-ray plateau at early times that sometimes transitions to a
steep decay most often between $10^{2}$ and $10^{5}$ s (Evans et al., 2009).
Different arguments have been invoked to explain these plateaus, including
magnetar central engines (Gompertz et al., 2014), fallback onto a black hole
central engine (Gompertz et al., 2020), as well as high-latitude emission from
structured jets observed on-axis (Oganesyan et al., 2020) or also off-axis
(Ascenzi et al., 2020). These options will be delineated with GW measures of
masses and spins, and perhaps additional GW observables (e.g. Corsi &
Mészáros, 2009). The answer to the physical origin of these plateaus is far
reaching, as highlighted below.

Observations of extended emission drive requirements for background stability
on the order of a few hundred seconds and sufficient sensitivity to recover
the signal. Extended emission is often, but not always, softer than the
typical prompt spike, favoring a lower energy threshold of 10 keV, with
increasing benefit to $\sim$1 keV. Capturing the signal of X-ray plateaus
requires observations beginning within 100 s after the start of the prompt
GRB. The end of the plateau phase occurs between $10^{2}$ and $10^{5}$ s after
the GRB, giving a decreasing plateau identification probability as response
time increases. With current X-ray facilities, this requires localization
accuracy on the order of $\sim 20^{\prime}$.

####  2.2.2 Prompt Radio Signatures

Of all the follow-up signatures which may drive temporal reporting latency
requirements, radio observations may be the most stringent. This is because
the merging systems may produce signals prior to full merger (Lipunov &
Panchenko, 1996; Hansen & Lyutikov, 2001; Lai, 2012; Piro, 2012; Lyutikov,
2019; Zhang, 2020; Pan et al., 2022; Most & Philippov, 2022; Cooper et al.,
2023; Lyutikov, 2023), which can be detected in the radio via follow-up
observations due to dispersive delays by free electrons in the interstellar
and intergalactic media. That is, the precursor signal arrives in the solar
system after the GW or conventional higher-frequency EM counterpart. Looking
ahead to the near future, accurate and prompt GRB localizations will enable
detections or limits on pre-merger pulsed FRB-like coherent radio emission
from NS mergers (Lipunov & Panchenko, 1996; Lyutikov, 2019; Cooper et al.,
2023) by rapidly improving radio facilities (whose capabilities such as FOV
increase directly with improving computational resources). This radio emission
would be an independent test of the chirping orbital dynamics prior to merger.
The expected detection horizon to such radio emission for advanced facilities
such as DSA-2000 and SKA-mid is generally larger than that of the current IGWN
to BNS mergers, out to several Gpc. Free electrons along the line of sight
lead to dispersive delays of radio emission of order

$t_{\rm delay}^{\rm radio}\approx
40\,\,\rm\left(\frac{DM}{1000\rm\,pc\,cm^{-3}}\right)\left(\frac{\nu}{\rm
300\,MHz}\right)^{-2}\quad\rm seconds$ (1)

where DM is the radio dispersion measure that can vary based on sky location
and host galaxy properties, and $\nu$ is the characteristic observing
frequency. This delay is manifestly several tens of seconds to minutes at
CHIME, LOFAR, CHORD, SKA-mid, MWA and SKA1-Low frequencies. Facilities such as
LOFAR can form many independent software beams on the sky to tile
localizations and wait for the signal after the GRB alert. Beamformed LOFAR,
and SKA1-Low attain ${\cal O}(10^{\circ})$ fields of view (see Table 2 and
refs in Cooper et al., 2023), yet their horizon for detection (and thus
expected event rate) is somewhat limited. On the other hand, more sensitive
and higher frequency facilities such as CHORD, DSA-2000 and SKA-AAmid whose
horizon extends to several Gpc would require sub-minute timescale alerts to
repoint.

####  2.2.3 Early Ultraviolet Emission

Another driver for latency requirements is understanding the early ultraviolet
emission seen in AT2017gfo (Arcavi, 2018; Metzger, 2020). There are multiple
possible options and much additional science relies upon understanding which
causes of early UV emission are important in what events. If the ejecta
achieves sufficient velocity, it may be possible to see an early UV signal
powered by the heating from the decay of free neutrons, which have a half life
of $\sim$10 minutes elongated by the relativistic Doppler factor. If this
occurs, it may give the most direct insight into the velocity gradient /
distribution of kilonova ejecta, necessary to properly utilize observations to
understand the underlying event (e.g., the nuclear yield). Alternatively, if
the jet is launched after the initial ejection of material into the polar
regions, it may be possible that an early UV signal from shock break-out /
cocoon heating is produced. This would have monotonic decay and cooling over
time; distinguishing between these two options requires observations beginning
no later than $\sim$30 minutes, with 10 minutes preferable.

For the cocoon and the free neutron cases, the prompt monitors play a role
both in localizing the event for follow-up and in providing information on
inclination, as these may be strongest for face-on events. Thus, full
understanding requires early UV observations of a population of events
discovered both through GW-only, GRB, and GW-GRB detections of NS mergers. UV-
identified events will generally lack inclination information; optical and IR
identified events will likely lack early UV observations.

Bright and early UV emission may also occur from a magnetar remnant powered by
the neutrino-heated, magnetically accelerated wind (Metzger et al., 2018).
This could be disentangled with UV observations in the first few hours.
Lastly, advances in modeling since 2017 have shown that highly ionized ejecta
can limit opacity issues at early times, perhaps powering early UV emission
from base radioactive contributions (Banerjee et al., 2020).

The key requirements for GRB monitors are localization precision and alert
latency. There are a limited number of UV telescopes, and currently only one
that can repoint within 30 minutes of an alert. Swift-UVOT has a field of view
of $17^{\prime}$x$17^{\prime}$ requiring strict alert latency and precise
localizations. ULTRASAT will have a $\sim$200 deg2 field of view in a
geostationary orbit, permitting the ability to immediately repoint. The GRB
monitor requirements are thus an alert within $<10$ minutes and a localization
precision of $\sim$200 deg2. However, we note the extended annuli that may
come from rapid IPN localizations may require tiling. Given the limited depth
to which ULTRASAT can recover these events ($\sim 200$ Mpc), an all-sky field
of view would be most beneficial. Future facilities will likely have fields of
view between UVOT and ULTRASAT, but may be significantly more sensitive. For
the cocoon case, the UV observations can be matched to the immediate prompt
GRB signal at earlier times, if the observing monitor achieves a sufficiently
low energy threshold of 10 keV as a requirement with additional gains to lower
values.

####  2.2.4 Binary Neutron Star Merger Classes

BNS mergers can form four different objects after merger, largely determined
by the masses of the progenitor objects. If the merging NSs are particularly
light, they may be able to form an indefinitely stable NS, supported against
gravitational collapse by degeneracy pressure and nuclear forces. The maximum
mass of a non-rotating NS is referred to as MTOV (Tolman, 1939; Oppenheimer &
Volkoff, 1939), beyond which a black hole is formed. However, metastable NSs
can exist past this threshold if supported against collapse by adding rotation
to aid the balance against gravity. If uniform rotation is sufficient, then
the NS is referred to as “supramassive” and if internal differential rotation
is also required then it is “hypermassive.” Heavier BNS mergers can, in
principle, form these objects. The heaviest BNS mergers will promptly collapse
to a black hole.

Determining when these different cases occur as a function of the masses of
the merging NSs must be a key goal in astronomy this decade, given the broad
scientific results that will follow. Direct determination can be made with GW
interferometers with sufficient sensitivity at a few kHz for particularly
nearby events, or perhaps out to a few hundred Mpc when 3G interferometers
exist (likely 2035+ Radice et al., 2017). The EM signatures provide a viable
path to these results (Margalit & Metzger, 2019; Burns, 2020).

A key question to be answered is whether magnetar central engines can power
GRBs. Either we observe GRBs from low-mass mergers, possibly with a
significant delay due to an engine cleaning time (Zhang, 2019), or we observe
GRBs from short-lived hypermassive and longer-lived remnant cases. The non-
thermal plateau emission signatures have been modeled as originating from
magnetar-driven winds. Longer-lived remnants are thought to create brighter
and bluer kilonova, and may be the case capable of releasing free neutrons.

Mapping out the various remnant cases requires calibration on a few
particularly well-studied events with information from across the EM spectrum,
with follow-up observations needing to begin within $\sim 10-30$ minutes. The
key wavelengths for rapid follow-up are X-ray and UV, where the smallest field
of view drives the localization precision required, on the order of $\sim$0.3
deg. This work also benefits from a sample of NS mergers with total, sensitive
prompt GRB coverage, driving both sensitivity and an all-sky field of view.
The prompt GRB monitors need absolute timing precision of $\sim$0.1 s to
properly determine the GW-GRB timing offset.

####  2.2.5 Neutron Star-Black Hole Classes

We can similarly separate NSBH mergers into those that are EM-bright and those
that are EM-dark. Larger black holes will swallow a merging NS whole, while
small ones will disrupt the NS outside the innermost stable circular orbit
(Foucart et al., 2014), releasing nuclear material which will power a kilonova
and perhaps GRB. Obtaining early panchromatic observations of EM-bright events
is necessary to understand the origin of the various observed signatures and
provide a complete picture of NSBH and BNS mergers. Total, sensitive GRB
coverage of NSBH mergers is necessary to fully exclude bright prompt emission
from these sources. The requirements are similar to those necessary to
understand BNS merger cases.

Establishing the existence and rate of EM-bright NSBH mergers, and
characterization of their multimessenger emission is required for much of the
broader compact merger science. These include unique and precise tests on the
NS equation of state, insight into the spins of BHs and BH fallback, the
formation of ultrarelativistic jets, an origin of short GRBs, and may
contribute a sizable fraction of actinides given the potential for
substantially larger elemental yields with lower electron fractions (e.g.
Metzger, 2020). EM observations with GW detections will help determine the
maximum mass of NSs, the minimum mass of BHs, and probe whether the these two
values are identical or whether some overlap or a mass gap exists. These are
key questions to understand dense matter, black holes, supernova explosions,
and stellar evolution.

####  2.2.6 Progenitors of Short Gamma-Ray Bursts

NS mergers have long been discussed as progenitors of short GRBs (Eichler et
al., 1989) and have been the only convincing candidate for the dominant
progenitor for nearly a decade (Fong et al., 2015). The association of
GW170817 to GRB 170817A directly proved BNS mergers as an origin of short GRBs
(Abbott et al., 2017). It is thought that some NSBH mergers produce short
GRBs, but direct proof remains elusive. With IGWN now detecting compact binary
coalescences with masses in the ranges expected for NSBH mergers, we should
either observe joint GW-GRB detections of these events or never observe a GRB
associated with these mergers. Given that the elongated morphology of GW
localizations are common, and are likely to continue to common in the future,
sensitive all-sky prompt GRB coverage is necessary.

####  2.2.7 Progenitors of Long Gamma-Ray Bursts

Shortly after the launch of Swift, a few long GRBs were identified that had no
associated supernova to constraining intrinsic limits (e.g. Gehrels et al.,
2006; Della Valle et al., 2006; Gal-Yam et al., 2006). One explanation for
this is direct core collapse preventing a strong supernovae but still allowing
for accretion and successful jet launch. However, as the theory and
observations of kilonovae has matured, there are now several long GRBs with
apparent observed kilonova signatures (e.g. Yang et al., 2015). Of particular
note are GRB 211211A (Rastinejad et al., 2022) and GRB 230307A. GRB 211211A
also showed a late-time GeV signature (Mei et al., 2022; Zhang et al., 2022).
A possible explanation is external inverse Compton in the jet with seed
photons originating from a kilonova. If correct, it provides another
diagnostic of both the kilonova and the jet and only deepens the debate on GRB
classification. GRB 230307A localized by the IPN and resulting in the first
late-time NIR spectrum of a kilonova, taken by JWST, being the most direct
observation of r-process nucleosynthesis (Levan et al., 2023). Both of these
events show evidence for the prompt signal arising from fast-cooling
synchrotron, providing clear insight into the prompt emission mechanism
(Gompertz et al., 2023; Levan et al., 2023).

GRBs 211211A and 230307A are unusually nearby with inferred distances based on
the redshift of their putative host galaxies of 350 Mpc (z $\sim$ 0.08) and
300 Mpc (z $\sim$ 0.065), respectively. Such events are likely detectable
through joint GW-GRB searches if observed by IGWN if they have a BNS origin,
and are almost certainly detectable if they have an NSBH origin.

The identification of a kilonova signature, without an accompanying supernova,
points to a merger origin for some long GRBs. This motivates continued
broadband follow-up observations of prompt GRB detections, particularly in
concert with GW observations for incontrovertible proof. The unusual
brightness of GRB 230307A led to rapid dissemination of IPN localizations and
tiling of the region, demonstrating that the community can successfully
follow-up events localized in this manner. However, the lack of early
broadband coverage is affecting on-going analysis. Beyond the localization and
alert latency requirements the current understanding of the prompt emission of
the events favors broad spectral coverage to probe the apparent synchrotron
origin and $\sim$ms scale relative timing precision as a potential method to
identify these events in real-time (Veres et al., 2023).

####  2.2.8 Origin and Classification of Gravitational Waves

Multimessenger observations are also key to identifying and classifying
sources of GWs. When IGWN detects a compact binary coalescence, they provide
estimates on whether the event is from a BNS, NSBH, or binary black hole
merger based on predetermined mass thresholds. The detection of an associated
prompt GRB signal immediately confirms the event as EM-bright, and thus either
a BNS or NSBH merger. This may be of particular interest for higher mass
compact binary coalescences where an NSBH or binary black hole merger origin
is unambiguous, giving guidance on when to execute expensive follow-up
observations and which telescopes to use. In the future, it may be possible to
identify the presence of at least one NS from tidal effects in the high
frequency range of the waveform, but this is well beyond the capability of
current instruments.

Again, the elongated morphology of many of the GW localizations drives the
need for sensitive all-sky coverage. Determining the sub-classes of BNS or
NSBH mergers, discussed above, is enabled with rapid follow-up observations,
driving the need for early and accurate localizations, aided by combining GW
and GRB localizations.

####  2.2.9 White Dwarf-Neutron Star and White Dwarf-Black Hole Mergers

BNS and NSBH mergers have been the dominant merger scenarios considered for
GRBs for several years, but alternatives have existed for decades. White
Dwarf-Black Hole (WDBH) mergers were invoked as possible sources of long GRBs
(Fryer et al., 1999). The White Dwarf-Neutron Star (WDNS) scenario was
considered for the long GRBs from mergers, i.e. GRB 211211A and GRB 230307A
(Zhong et al., 2023; Levan et al., 2023). There is on-going advances in theory
and simulation in this area (e.g. Kaltenborn et al., 2022).

It is difficult to probe such an origin with existing or funded GW detectors
because the merger frequency, driven by the size of the largest object (white
dwarf), occurs at $\sim$Hz frequencies. Thus, it is the responsibility of
prompt discovery and localization of GRBs with appropriate follow-up to
determine if these are a viable progenitor of these events. Key information
for such a determination includes the host galaxy type, offset from the host
galaxy, and broadband characterization to look for any radioactively-powered
signatures. The need to acquire this information drives requirements on
precise localizations fast enough after merger to reliably model the afterglow
to search for later time signals on top of it. The requirement is typically
within an hour, but faster follow-up will only increase the probability of a
successful detection. We note the identification of a merger GRB through EM
follow-up observations and a local redshift that could also be excluded as a
BNS or NSBH merger from GWs would strongly point to a WDNS or WDBH merger
origin.

####  2.2.10 Stellar Formation and Evolution

Compact object mergers are a rare outcome of stellar systems. These events
track the star formation rate with the inspiral time distribution adding a
delay. Most are expected to arise from field binary evolution, with a subset
arising from dynamical capture. One of the latter may have been recently
identified (Levan et al., 2023). Field binaries require an unusual sequence of
evolution to create the compact objects with initial separations small enough
to merger in a Hubble time (i.e., to be observed by us). Thus, an
understanding of these systems provides insight into the formation and
evolution of stellar systems and their binary interactions such as the
stability and physics of mass transfer and common-envelope phases, angular
momentum transfer, stellar winds, and rotation of massive stars – phenomena
that are challenging to observe or study in large populations due to the
scarcity of massive stars. GRBs provide a unique opportunity to study the
lives of massive stars, particularly in environments beyond our own Milky Way.

The requisite observations include the local rates of the different merger
classes, their rate evolution over time, host galaxy types, and offset
distributions of the mergers from the host galaxy, as well as the mass, spin,
and mass ratio distributions of the merging population (Broekgaarden et al.,
2021; Mandel & Broekgaarden, 2022). The mass and spin parameters are only
measured from GW observations. The local rates can be directly measured by GW
detectors, but measurement of rate evolution of BNS mergers is not possible
via GWs until the 3G era. Until then, an understanding of the origin of GRBs
over cosmic history is key to understanding their source evolution. This
requires deep sensitivity over a wide enough field of view for a high rate of
events, with rapid and precise localizations of sufficient accuracy to
associate the sources to their host galaxy and measure their respective
redshifts.

####  2.2.11 Origin of the Elements

A fundamental research area of astrophysics is mapping the origin of the
elements. A key remaining goal in this area is determining the production
sites of the heaviest elements, i.e. the sites of r-process nucleosynthesis
(Burbidge et al., 1957; Cameron, 1957). The ingredients are neutron-rich
material with particularly high densities, pointing to the moments of creation
or destruction of NSs. The constraints include the current amount of heavy
elements that exist, which corresponds to the time-integrated r-process
production, and the abundance pattern of the elements corresponding to the
averaged relative yields. As a key example of multimessenger astronomy,
observations of the rate of dust deposition of 244Pu on Earth (Wallner et al.,
2015) and the identification of r-process element in only one of many dwarf
galaxies (Ji et al., 2016) point to the dominant site of r-process elements as
rare, high-yield events. The viable options are then NS mergers or a rare
subset of supernovae (e.g. collapsars).

NS mergers are almost certainly contributors (Metzger, 2020). Observations of
AT2017gfo matched theoretical expectations for kilonovae which include the
formation of heavy elements, though direct evidence of third peak elements was
not found. Mapping out the relative contribution of NS mergers to the total
r-process produced in the universe requires a determination of the yield of
individual events, the rate of local mergers, and the source evolution through
cosmic time. This is complicated as it must be done separately for BNS and
NSBH mergers, as well as their sub-classes. For example, low-mass BNS mergers
may have a higher overall yield, but they may not be able to produce the
heaviest elements.

Understanding the various types of NS mergers and their integrated and
differential yields is critical. This requires a set of well-sampled local
events, driving greater sensitivity, all-sky coverage, and rapid
$\sim$arcminute-scale localizations in $\sim$10–30 minutes. This also requires
a set of well-studied, off-axis events recovered in GWs to map the inclination
dependence of various signals. Together, and with substantial improvements in
kilonova modeling, individual yields can be measured. Late-time spectra by
JWST and the upcoming extremely large ground-based optical telescopes will
also be key. Then, mapping out the source rate evolution of BNS and NSBH
mergers (and their sub-classes) are necessary to determine the time-integrated
history. This requires recovery of redshifts of distant NS mergers detected
through GRBs, or beginning in the late 2030s, through GWs.

####  2.2.12 Dense Matter

The behavior of matter with densities near nuclear saturation are poorly
understood because the approaches to quantum chromodynamics predictions on
large scales are invalid or infeasible in this regime. This regime can be
probed through more than 15 orders of magnitude in size by studying heavy
element collisions on Earth (Reed et al., 2021) or through astrophysical
observations of NSs (Özel & Psaltis, 2009). In the latter case, equations of
state can be used to make predictions on observable quantities such as the NS
mass, radius, and tidal deformability. Determining the equation of state of
dense matter is key to understanding one of the three fundamental forces.

NS mergers, particularly involving hot NSs without an enormous star blocking
the direct view, are critical tests of dense matter. Perhaps the most
important test is the determine MTOV, the maximum mass of a non-rotating NS,
as this is an asymptotic limit (for NS, though not for strange stars). With 10
confidently classified events and GW measurements the measurement can approach
1% precision (Margalit & Metzger, 2019). Although this relies on an assumption
of a mass ratio of $\sim$1, even a mildly weaker result is still significantly
better than any existing or expected NS equation of state constraint. The
tidal disruption radius for NSBH mergers is a strong function of the radius
and we may expect some NSBH merger detections to more precisely measure the NS
masses, since the higher mass ratio allows for the detection of higher order
modes and breaking of correlations between mass ratio and spin in GW
observations, allowing for particularly precise mass-radius tests of any NS
equation of state. GW observations directly measure tidal deformability and
the kilonova yields are intimately tied to the compactness of NSs and thus the
NS equation of state (Pang et al., 2021).

Observations of the plateau emission may constrain the lifetime of
supramassive NSs, or observations of the delay time may constrain the lifetime
of hypermassive NSs, though only one of these tests is possible (Burns, 2020).
Constraining the hypermassive NS lifetime, either through this method or
through 3G interferometers, can help determine whether QCD phase transitions
exist inside of NS mergers (Bauswein et al., 2019; Most et al., 2019). No
other known proposal to accomplish this test exists.

Determining the lifetime of hypermassive NSs requires that magnetars cannot
power merger GRBs and requires the observation of a particularly fast GW-GRB
event. Should one occur, the GRB monitor needs $\sim$ ms scale absolute
timing. For the supramassive NS case, the GRB monitor needs to meet the
requirements to identify extended emission. To be capable of precise NSBH
mass-radius tests, the GRB monitor needs to meet the requirements to determine
if NSBH mergers are EM-bright. Otherwise the science is achieved through the
requirements for characterizing and classifying the sub-classes of BNS
mergers, e.g. total sky coverage.

####  2.2.13 Cosmology

A quirk of General Relativity allows the measurement of luminosity distance
through GW observations of compact binary coalescences (Schutz, 1986). NS
mergers are the compact binary coalescences which can be detected by both IGWN
and EM observations, the latter of which enable precise localization and
redshift measurement. The GW-EM Hubble diagram is, in principle, preferable to
standard candle measures as the intrinsic zero point energy is determined by
General Relativity, rather than empirically determined such as the approaches
utilizing Type Ia SNe. The current disagreement in the Hubble Constant (e.g.
Riess et al., 2022) would greatly benefit from a third, fully independent
determination. While this tension may be resolved before the precision of the
standard siren measurement is sufficient to provide an answer, standard siren
cosmology is certain to become a fundamental tool of cosmology in the next two
decades.

The standard siren measurement precision scales roughly as $N^{-1/2}$, where
$N$ is the number of NS merger detections in both GW and with EM determination
of redshift. With a single event precision of $\sim$15% (Abbott et al., 2017)
and a useful precision of $\sim$2% for contributing to the Hubble Constant
disagreement, this requires redshift measurements for tens of GW-detected NS
mergers. This is unlikely to occur until at least O5 in the late 2020s.
Additionally, EM counterparts have brightness that varies as a function of
inclination, introducing a bias in the redshift recovery of these events.
Since inclination is correlated to distance in GW measurements, this can set a
systematic error limitation in standard siren cosmology. Additionally, varied
peculiar velocities of nearby host galaxies deviating from the Hubble flow can
also introduce additional systematics. For kilonova counterparts (Chen, 2020)
and because of the unknown GRB jet structure, we are unlikely to reach 2%
total precision with our current understanding of the inclination dependence
of these signals.

In the current observing run, more kilonovae may be recovered than short GRB
counterparts of GW-detected events. However, as the GW network becomes more
sensitive, GW-GRBs will be the dominant multimessenger detections of these
events (Chen et al., 2021). Their restriction on the range of inclination may
also be beneficial, predicated on using close events to gather understanding
of the real structure of these jets. This will allow sub-percent precision
cosmology throughout the universe, complementing existing investments and
resulting in breakthroughs in measuring the equation of state of dark energy,
testing both the current models of cosmology and gravity.

Reduction of the systematic floor requires greater understanding of all the
various signals from NS mergers. This favors immediate identification and
localization of all nearby merger events, promoting detailed study of all
signals. The future GW-GRB program is well served by arcminute localizations
with a monitor of great sensitivity, to recover NS merger GRBs as deep into
the universe as they occur. This will be an absolutely critical piece of the
TDAMM ecosystem during the 3G era.

####  2.2.14 Ultrarelativistic Jets

One surprise with GRB 170817A was that the core of the jet was oriented away
from Earth (e.g. Nynka et al., 2018) leading to lively debate on whether the
prompt signature was from a successful structured jet viewed off-axis or a
“cocoon” breakout, resulting from a delayed jet interacting with the
surrounding ejecta to create a pressurized cocoon that expands outward at
mildly relativistic speeds (Nagakura et al., 2014; Lazzati et al., 2017).
Mapping out the jet structure provides insight into the jet launching
mechanism, what shapes the jet, as well as what and where the primary emitting
region is. This can be probed with complete gamma-ray coverage of GW events
where both high-energy detections and sensitive upper limits can compliment GW
information on the source distance and orientation (Biscoveanu et al., 2020).
Precise localizations of events, particularly nearby ones, enables follow-up
characterization of the afterglow of off-axis events from radio to X-rays
(e.g. Hajela et al., 2022), whose evolution is deeply tied to jet structure
(e.g. Ryan et al., 2020). For the closest events, radio interferometry can
observe proper motion of the jets, giving a direct diagnostic on their
velocity and potential extension (e.g. Ghirlanda et al., 2019).

Observations of these events also provide insight into how jets form and
launch. GRB monitors meeting the requirements to determine the behavior of the
BNS merger remnant will determine if magnetars can be central engines of
merger GRBs, a question currently unsettled between theory and observation.
For particularly well characterized events, measurements of the efficiencies
can be used to probe how the jet is powered, i.e. Blandford-Znajek or
neutrino-antineutrino annihilation (Blandford & Znajek, 1977; Salafia &
Giacomazzo, 2021). In the case of a magnetar central engine or Blandford-
Znajek, the jets can be Poynting-flux dominated where ordered magnetic fields
would create prompt polarization signatures. Measurement of the efficiencies
requires measurement of the prompt spectral turnover, and an energy range that
extends to several MeV.

####  2.2.15 Fundamental Physics

One of the major results from the joint detection of GW170817 and GRB 170817A
was the measurement of the speed of gravity, which is responsible for roughly
a third of all citations to the joint detection publication (Abbott et al.,
2017). Note that this result was so important for the field of gravity that it
rivals the number of citations of the kilonova discovery papers. This
measurement requires sensitive coverage of the GRB sky to detect these events
and absolute timing precision to $\sim$0.01 s accuracy to ensure that the
timing precision is not a limiting factor in improved measurements
($\sim$0.001 s accuracy to ensure it is never a limiting factor).

One additional, unresolved fundamental physics question is whether gravity
violates parity (Alexander & Yunes, 2009) which can be explored most precisely
with mergers jointly detected in GWs and with GRBs (Yunes et al., 2010). There
is evidence for parity violation on large scales (Philcox, 2022; Hou et al.,
2022) which is most naturally explained as arising from gravitational parity
violation. Extensions of general relativity which violate parity are generally
motivated from high energy completions of gravity, such as string theory. If
gravity violates parity, then it may explain the excess of matter over
antimatter near the beginning of time (Alexander et al., 2006) and may
contribute to the existence of dark matter. This requires a GW and prompt GRB
detection of a face-on NS merger, as well as successful follow-up to recover
the redshift, which is necessary to seek the disagreement in the luminosity
distance as measured by GWs against the value from EM observations. This test
is more sensitive for more distant events, being aided by increased prompt GRB
sensitivity. Successful follow-up characterization of the afterglow to confirm
the on-axis nature would also be beneficial, but may not be required in all
cases (i.e. a particularly powerful prompt emission requires an on-axis
event).

The tests of fundamental physics possible with NS mergers are numerous (Burns,
2020). GW-GRB observations can determine the value of the neutrino mass
eigenstates through standard siren cosmology. Tests of extra large dimensions
require comparison of the GW measurement of luminosity distance with an EM
measurement of the same parameter, requiring successful recovery of redshift.
These tests generally require successful follow-up observations, favoring
rapid dissemination of precise localizations.

####  2.2.16 Summary of Capability Requirements

The advent of GW astronomy enables the majority of new scientific results
possible with EM observations of merger GRBs. The IGWN is expected to detect
NS mergers at a rate of one per month during its next observing run (Petrov et
al., 2022). However, the coarse localizations expected ($\sim$ 100–1000 deg2)
will continue to be challenging for wide-field optical facilities such as Pan-
STARRS or the Zwicky Transient Facility (ZTF) to follow up, as these searches
and automated filters yield dozens of candidates that require photometric
and/or spectroscopic follow-up. As the interferometers advance in sensitivity,
the rate of events will vastly increase, but many GW localizations will still
have poor precision (Petrov et al., 2022). With the cosmological reach of 3G
interferometers and the corresponding high discovery rate of NS mergers,
follow-up telescopes will struggle to successfully study the population.

Of all the EM counterparts to NS mergers, prompt GRBs are unique in their
propensity for regular, rapid independent localizations of GW-detected events.
They are also the only EM counterparts that have been recovered beyond the
local universe and the only ones that can always be associated to the GW given
the small temporal offset. The GW and GRB localizations can be combined and
distributed to the follow-up community, allowing for earlier and more reliable
recovery of the kilonova and afterglow signatures, as well as precise
localizations enabling host galaxy determination. Additionally, the short time
offset between the GW signal and the GRB can act as signal confirmation and
background rejection, allowing joint searches to be far more sensitive than
those of only one messenger.

Thus, prompt GRB monitors are necessary to provide diagnostic information from
the gamma-ray signatures, to help localize the events for the full community,
and to help identify additional GW events. This work is in addition to the
historic role that GRB monitors played as the sole discovery instrument for 50
years before they were joined by GWs. GRB monitors are absolutely critical to
the study of these events.

The ideal scenario for a GRB monitor is immediate reporting of arcminute-scale
(or better) localizations within 10 seconds, with true, all-sky, sensitive
coverage. With $\sim$0.01 s timing precision, significant sensitivity beyond
existing instruments, long contiguous observing intervals, high temporal
resolution data, broadband coverage ($\sim 10$ keV-$\sim$10 MeV) optimized to
fully characterize short GRBs, and a long mission, all of the above science is
possible. The localization precision could be relaxed if sufficient wide-
field, sensitive follow-up facilities, with massive fields of view and regard,
existed at all other wavelengths. While possible, this would be vastly more
expensive and would necessarily limit fully characterized events far a far
closer horizon, given the trade-off between field of view and sensitivity.

For current or forthcoming facilities, the first major issue for weakened
alert latency and localization precision requirements would be loss of
recovery of X-ray plateau emission. Secondly, if less than one minute alert
latency cannot be attained, this excludes most targeted ground-based radio
searches for delayed pre-merger emission from compact object coalescence. As
rapid localization precision drops, this also prohibits most direct follow-up
from radio facilities. ULTRASAT should allow recovery of UV signatures for
nearby events, but will struggle at greater distances, and cannot provide
localization information on UV-faint events. As the capability to recover each
known or expected signature is lost, so is much of the science described
above.

The development of wide-field infrared, optical, and UV facilities allows for
recovery of follow-up signatures from localizations on the order of $\sim$100
deg2 within a few hours. Note that this introduces an inherent latency from
the need to characterize a potential counterpart as rising or fading. In this
case, advancements will need to be made to understand BNS and NSBH merger sub-
classes, the origin of short GRBs, long GRBs, and GWs, and much of the broader
science mentioned in these sources. It is difficult to project exactly what
science is lost given how much remains unknown about these sources.

We emphasize that having capable GRB monitors, regardless of alert latency, to
help follow-up are still necessary for much of this core science. Sensitive,
all-sky coverage with broadband energy ranges and stable backgrounds allows
identification of extended emission, provides insight into the BNS merger
remnant cases, determines which NSBH mergers are bright or dark, confirms the
progenitors of GRBs and origin of GWs sources, probes the properties of dense
matter, illuminates the physics of ultra-relativistic jets, and provides
breakthroughs in fundamental physics. High-latency localizations enable robust
association of GW and GRBs, and will allow confirmation or rejection of
follow-up counterpart candidates identified before arrival of GRB data. Joint
searches of GW and GRBs, even in high latency, will further broaden this
science.

####  2.2.17 Requested Deliverables from GRB Monitors

Real-time reporting of information from GRB monitors is critical to the
success of TDAMM science, as demonstrated by Swift and Fermi. Swift-BAT
localizations are extremely precise, but BAT will only localize $\sim$1 in 6
joint events. For all other events, automated inclusion of all relevant
localization information would vastly improve the current mode of operation.
This includes generation and distribution of IPN timing annuli as data
arrives, inclusion / exclusion of planetary-occulted regions for detection /
sensitive non-detections of events, and combination of all available
autonomous localizations (i.e. Glowbug and Fermi-GBM).

As one example, INTEGRAL SPI-ACS detects the majority of Fermi-GBM short GRBs,
and its data will soon be available within $\sim$20 s. The Fermi-INTEGRAL
annuli reduces GBM-only localizations by a factor of a few for some short
GRBs. While the latency of data from any individual distant spacecraft of the
IPN is high, when multiple missions are active the latency from one of these
distant spacecraft may be low. In particular, Mars Odyssey gets several
downlinks per day, which enables an annulus with arcminute-level width. These
combined localizations and GRB information should be distributed through a
collated GRB alert stream.

An additional approach is the automatic association and localization
combination of GW and GRB information. The GW-GRB Working Group, comprised of
IGWN, Fermi-GBM, and Swift-BAT, are automating this procedure for GBM and BAT
triggers for the O4 observing run. Combined localizations will be distributed
through the IGWN alert stream in the exact format as the IGWN-only notices.
GBM localizations can reduce the GW localizations by a factor of a few to
several, but these will still generally be large localizations appropriate for
follow-up by wide-field observatories. The creation of a collated GRB alert
stream enables joint GW-GRB localizations with all contributing GRB monitors.

The small time offset between the GW signal and the prompt GRB enables
increased sensitivity in joint searches, which can directly provide
unambiguous proof of additional GW-detected NS mergers. Blind searches are
all-sky, all-time, and have enormous effective trials factors. A localization
and a detection time to inform a search reduces both the spatial and temporal
trials factors. With a fiducial $\sim$25% increase in sensitivity in GW
searches around GRBs (Williamson et al., 2014) we would expect to double the
number of joint detections. This is a major effort within the IGWN, which
produces population-level searches after each observing run (Abbott et al.,
2022). Improved GRB localizations allows for a vast reduction in computation
time for these follow-up searches around GRBs with otherwise poor
localizations as well as improved GW sensitivity on individual bases.

Similarly, the GW network can pass temporal and spatial information to the GRB
monitors. This has been done during O1 and O2 for Fermi-GBM (Hamburg et al.,
2020) and for Fermi-GBM and Swift-BAT during O3 (GW-GRB Working Group, in
prep). This work began due to the expectation that nearby GW-detected mergers
may produce subluminous GRBs (Burns et al., 2016), which was confirmed by GRB
170817A. The O2 version of the GBM search could detect 2.2$\times$ as many
GRBs as the on-board trigger for the purposes of joint detection, while the O3
version could detect 3.7$\times$ as many (Joshua Wood, private communication).
Sustained investment in joint searches directly correlates to an increase in
the number of confirmed multimessenger events. Recent developments include the
construction of true joint searches, combining the data from GWs and GRBs
together directly, without the need for distinct searches of the individual
datasets (Pillas et al., 2023). This should lead to future increases in joint
sensitivity.

Note that a form of these searches are also run in real-time. Joint detections
of multimessenger transients will be reported in low-latency, on the order of
hours, providing the joint localization to the community for follow-up. An
obvious improvement for these searches would be coherent searches of multiple
GRB monitors together, allowing for similar background rejection capabilities
inherent to the ground-based interferometer network.

Additional information could be automatically calculated and provided for
real-time analysis that would help guide follow-up observations. For GW-GRBs,
one can check if the cocoon closure relations Nakar et al. (2012) are met,
which can be used to prioritize X-ray and UV follow-up. The minimum
variability timescale could be calculated on a similar timescale, perhaps
identifying long GRBs from mergers, prioritizing follow-up observations to
characterize these enigmatic events. Continued advancement of distributing
useful information as understanding evolves would greatly benefit the
community, similar to the improvements made to the IGWN alerts in each
observing run.

The community would also benefit from a collated GRB catalog with inclusion of
all localization information. This vastly reduces data gathering efforts of
various collaborations, can improve sensitivity and reduce computational
expense on joint searches, and provides standardized results. A key piece of
information that would be useful is a basic assignment of short or long GRB
probability, or even more fundamental, the probability that a given GRB was
produced by a merger or collapsar.

###  2.3 Collapsars

Supernovae can be powered by either nuclear fusion (thermonuclear explosions
believed to make Type Ia supernovae) or gravitational potential energy
(stellar core-collapse explosions believed to make Type Ib/c and II
supernovae). For CCSN, the energy arises from the collapse of the core of a
massive star down to nuclear densities (and the formation of a proto-NS). GRBs
are also believed to be powered by the release of gravitational energy either
through compact binary mergers or the collapse of massive stars.

The standard engine behind normal supernovae ($\sim 10^{51}\,{\rm erg}$
explosions) is the convection-enhanced, neutrino-driven supernova (Herant et
al., 1994; Fryer & Young, 2007; Takiwaki et al., 2014; Lentz et al., 2015;
Melson et al., 2015; Burrows et al., 2018) that taps the energy in this
collapsed core which escapes as neutrinos. This engine can not produce the
extreme energies and asymmetries in extreme supernovae, a.k.a. hypernovae or
broad-lined (BL) supernovae (Iwamoto et al., 1998; Valenti et al., 2008), or
GRBs. Both hypernovae and long-duration GRBs, although also believed to be
powered by the release of potential energy, require more extreme conditions
than normal supernovae. For these transients, the black hole accretion disk
engine would occur in progenitors that are rotating sufficiently rapidly to
form a disk around the collapsed core (Woosley, 1993). Magnetic fields
generated in this disk drive a powerful jet that produce both the asymmetries
and higher total explosion energy.

One of the leading progenitors behind this black hole accretion disk scenario
for long-duration GRBs is the collapsar model (Woosley, 1993) that invokes the
collapse of a rapidly spinning massive star to a black hole. A number of
scenarios have been suggested to spin up massive stars (Fryer et al., 1999).
In addition, mechanisms have been suggested that merge compact objects with
massive stars to produce the same conditions as a collapsar, e.g. the helium-
merger model (Fryer & Woosley, 1998).

The jet disrupts the star, producing a supernova/hypernova associated with the
long GRB (Sobacchi et al., 2017; Barnes et al., 2018). Radio observations of
the Ic-BL SN 1998bw (Galama et al., 1998), which revealed a relativistic
outflow Kulkarni et al. (1998), built support for this collapsar or collapsar-
like progenitor. Historical observations of SNe Ic-BL do not always reveal
direct evidence of a GRB, but may still show implicit evidence of a GRB
central engine (see Berger et al., 2002; Soderberg et al., 2010; Pignata et
al., 2011; Milisavljevic et al., 2015; Corsi et al., 2022), suggesting a jet
may form but fail to reach ultrarelativistic velocities. Despite its success,
a number of unsolved gaps in our understanding of the collapsar model for long
GRBs persist. We will discuss these unsolved problems in the following
subsections.

While short GRBs are a focus of the field given the advent of GW
multimessenger astronomy, long GRBs have long been the preferred class of GRBs
to study as they are far more energetic, are detected more frequently, and
generally have higher fidelity data. Study of these events is key to
understanding engine-driven supernovae and the collimated, ultrarelativistic
jets that power GRBs.

####  2.3.1 Collapsar and Collapsar-Like Progenitors

The requirements to make a black hole accretion disk engine work in the case
of collapsars or other massive star progenitors are three-fold:

  * •

Efficient conversion process: The jet energy is correlated with the disk
energy, arguing both for high accretion rates and BH, not NS, central objects
(Popham et al. 1999; although see Metzger et al. 2011).

  * •

High angular momentum: There must be enough angular momentum to form an
accretion disk, which argues for a black hole engine, instead of a NS, because
achieving the high angular momenta is easier with black hole progenitors
(Woosley, 1993)

  * •

Baryon-free jets: The high Lorentz factors in the jets of long GRBs require
baryon-free jets; therefore, the engine must be able to clean out the jet
region. To do this, it is assumed that the engine itself lasts longer than the
timescale for the jet to propagate through the star and the jet itself will
first create this baryon free region. This limits the stellar size to a
compact star (MacFadyen & Woosley, 1999).

These restrictions led to a number of predictions: the angular momentum
requirements should only occur in a small subset of collapsing stars which
explains the low rate, long GRBs from these rare and massive progenitors will
form primarily in star-formation regions, and the stellar explosions will
arise from compact He- or carbon/oxygen stars leading to Type Ib/c supernova
associations. Furthermore, a supernova from this system will be produced as
pressure waves from the jet unbind the star, producing very peculiar BL
emission in the supernova spectrum.

With the identification of SN 1998bw following GRB 980425 (Galama et al.,
1998) as well as subsequent detections of unusual supernova in GRB error boxes
(Cano et al., 2017), the collapsar progenitor became the standard model for
long GRBs. The basic model explains the rate of long GRBs, their distribution
within their host galaxies (Bloom et al., 2002), and the fact that the