progenitor must be compact arguing for a progenitor stripped of its hydrogen
envelope (MacFadyen & Woosley, 1999). But it does not explain why the GRB-
associated supernovae appear to be stripped of their helium envelopes as well
(GRB-associated supernovae are Ic, not Ib supernovae). Progenitor models also
struggle to produce the needed high angular momenta (Woosley & Bloom, 2006;
Fryer et al., 2007). For many of the progenitor scenarios, the model also
predicts a large number of Type I and Type II BL supernovae with baryon-loaded
jets that do not produce prompt GRB emission, which is in tension with the
rates of optically-identified GRB afterglow (Ho et al., 2022). Of related
interest is whether the progenitor systems of cosmological collapsars is the
same as those of ultra-long GRBs and low-luminosity GRBs, as discussed below.

There is a sub-class of CCSN that are relativistic, identified by radio
emission from a faster moving ejecta powered by a central engine (Soderberg et
al., 2010), but the inferred rate appears to be on par with the GRB rate
rather than the large number predicted by these scenarios (Corsi et al.,
2022). Also, while the exact requirements (e.g., the baryon-loading fraction)
to produce a successful GRB jet are still not well understood, we now know GRB
980425 is part of a class of low-luminosity (or sub-energetic) GRBs (Bromberg
et al., 2011). In fact, many GRBs with associated supernovae have low
luminosities (Cano et al., 2017), as both can only be seen to limited
cosmological scales. Some of these low-luminosity GRBs are thought to be
emitted by relativistic shock breakout (Campana et al., 2006; Waxman et al.,
2007; Nakar et al., 2012) while others require a different, non-thermal
emission mechanism, likely the typical internal dissipation mechanism of GRBs
(Chand et al., 2020). To make things more complex, for normal CCSN,
relativistic CCSN, low-luminosity GRBs, and fully successful collapsars, the
ejecta velocity and kinetic energies of the slow moving ejecta component are
comparable, while the same parameters for the faster moving ejecta span a
continuum over orders of magnitude (Margutti et al., 2014).

One of the biggest unsolved mysteries of collapsar or collapsar-like GRBs is
understanding the nature of the progenitor. Given the low rate of GRBs and BL
supernovae, it is clear that jet-driven supernovae are rare; therefore, the
process to produce them is uncommon. Spin-down of giant stars could explain
why no Type II supernovae are known to have relativistic ejecta (see Smith et
al. 2012 for an argued jet-driven Type II). But to explain why He-stars also
do not produce long GRBs is more difficult. It has been argued that a large
fraction of jets are “choked” inside the star (or “failed”; Bromberg et al.
2012; Lazzati et al. 2012), which could be because they become so baryon-
contaminated that the shock is not relativistic; however, there is no decisive
observational evidence that any given supernova truly harbors a choked jet.

While the discovery of collapsars and low-luminosity GRBs is the domain of GRB
monitors, optical facilities are key to identifying supernovae of interest.
These include searching for BL Type Ib and Ic supernovae. While this began
nearly 20 years ago (Berger et al., 2003), modern optical facilities and the
modern TDAMM ecosystem for follow-up to characterize the fast ejecta have
vastly increased identification rates of these events (e.g. Corsi et al.,
2017; Ho et al., 2020a; Corsi et al., 2022).

To add another creature to this mix, the discovery of AT2018cow (Prentice et
al., 2018) and similar events (Coppejans et al., 2020; Ho et al., 2020b;
Perley et al., 2021; Yao et al., 2022) led to the recognition of “luminous
fast blue optical transients” (LFBOTs; Margutti et al. 2019; Metzger 2022). It
is generally accepted that these events involve an active compact object or
central engine (Ho et al., 2019; Margutti et al., 2019), with mildly
relativistic speeds in some cases (Coppejans et al., 2020; Ho et al., 2020b).
These LFBOTs have been argued to involve shocked jets and may also be key
neutrino and VHE sources (Fang et al., 2019; Guarini et al., 2022). However,
no prompt emission has been detected.

The nature of LFBOTs is uncertain. They may arise from massive stars, e.g.,
failed supernovae (Perley et al., 2019; Margutti et al., 2019) or stellar-mass
tidal disruption events (TDEs) (Kremer et al., 2021; Metzger, 2022).
Alternatively, they may represent intermediate mass black hole TDEs (Kuin et
al., 2019; Perley et al., 2019). In order to understand the origin of LFBOTs,
continuous and sufficiently sensitive coverage is necessary to recover their
shock breakout or low-luminosity GRB emission or to fully rule them out.

Probing the evident continuum from normal CCSN through relativistic CCSN, low-
luminosity GRBs, and ultrarelativistic collapsars must be a key goal in TDAMM
this decade given the broad range of facilities coming online to identify and
characterize these events. Understanding whether and where X-ray flashes,
LFBOTs, and other exotic supernovae fit into this picture will increase our
understanding of both supernovae and GRBs. The exact questions to be answered
are vague due to the lack of knowledge of these sources. Mapping out the
relation between supernovae and GRBs explores one of only three supernova
explosion mechanisms known. Understanding why the lower energy form of engine-
driven supernova appear similar to the traditional CCSN will undoubtedly drive
advances in modeling of these events. Observation and characterization of
shock breakout will provide the most direct diagnostic and understanding of
the structure of massive stars near the end of their life. Understanding when
the escaping jets transition from only shock breakout to the traditional GRB
emission gives a handle on the necessary conditions for jet formation,
propagation, and energy dissipation.

The high-energy monitors are to be responsible for detection and
characterization of the initial prompt signatures from these events. For
ultrarelativistic collapsars, the signature is that of a traditional long GRB.
The signature for low-luminosity GRBs are generally softer, longer, and less
luminous. For relativistic supernova the shock breakout will have peak
energies around a few keV and durations of hundreds or thousands of seconds.
For normal CCSNe, the shock breakout can peak as low as $\sim$0.1 keV.
Identifying a population of these events is critical to meeting the Decadal
recommended TDAMM science. The information carried in these signals is
impossible to recover through other means, i.e. observations of the shock
breakout in UV (as it cools below the initial $\sim$X-ray temperature) will
probe the circumburst material surrounding the star, but only the X-ray
signature can probe the stellar structure itself. We do not understand massive
stars at the end of their lives, and this is the only known method to directly
study them.

Successful collapsars are served with a range of a few tens of keV to a few
MeV. Low-luminosity GRBs often have peak energies between 10 and 100 keV.
X-ray flashes, likely to be part of the continuum towards less successful
jets, have been observed between $\sim$ 1 – 10 keV. This is the range
appropriate to study shock breakout of relativistic supernova. Observing
normal supernovae push the requirement down to 0.1 keV. Ideally these events
are reported fast enough to recover the UV signatures of the shock-cooling as
well as the potential VHE signatures (discussed later), requiring
localizations with tens of deg2. For characterization of more distant events,
this precision requirement is more stringent.

Sensitivity requirements are generally similar to that of studying short GRBs.
Existing sensitivity is sufficient, but deeper sensitivity, similar to BATSE
from 30 years ago, would provide far greater detection rates and deeper
characterization of detected events. The study of long GRBs requires greater
background stability on the order of their duration, with a median value of 30
s, but a tail extending to several hundred and sometimes thousands of seconds.
This timescale is well-matched to that of the durations of the initial shock
breakout emission of supernova.

As mapping out the GRB to supernova continuum is a key goal, the GRB monitors
must be designed as partners to the UV, optical, and infrared time-domain
surveys. While fortunate events discovered by these facilities may have rise
times constrained to tens of minutes, a more typical value is on the order of
a day, and a less optimistic scenario is an uncertainty of a few days. This
drives the need for all-sky coverage (or at least coverage of the full night
sky), sensitivity, and long contiguous viewing windows. The sensitivity must
be sufficient to exclude traditional long GRB emission of these events, which
is ideally an order of magnitude more sensitive than Konus-Wind, the current
most sensitive all-sky instrument. The contiguous viewing windows should be
longer than a week, as quantified in Section 3.3. This additional dimension to
the observing fraction requirement is specific to longer-duration discovery
windows.

####  2.3.2 Dirty Fireballs and Optically-identified Transients

Transients resembling the optical afterglows from collapsar GRBs are now
routinely detected via their afterglow signatures independently of prompt GRB
emission (Cenko et al., 2015; Stalder et al., 2017; Bhalerao et al., 2017; Ho
et al., 2020; Andreoni et al., 2021; Ho et al., 2022). Ho et al. (2022) show
that the majority of optically-identified GRBs either have associated prompt
GRB emission or the GRB upper limits are insufficient to exclude a prompt
signature. This implies that, if dirty fireballs have a similar energy per
solid angle as clean fireballs, the rate of dirty fireballs is within the rate
of GRBs. However, it is possible that less energetic jets may be more likely
to become baryon loaded, and that it is more difficult for a mass loaded jet
to escape the progenitor star. Therefore, absence of evidence is not evidence
of absence: dirty fireballs may exist, but with less luminous optical
afterglows to clean fireballs. Ongoing optical searches, and particularly
searches for soft X-ray transients (expected from a dirty fireball), will
resolve this question.

Matching the discussion for general engine-driven supernova, the start times
of these transients are sometimes as fast as tens of minutes but more often
are on the timescale of a day, given the nightly observing cadence of many
facilities. Confirming an associated prompt GRB or providing constraining
upper limits requires contiguous viewing intervals over the full sky covering
the entire start time uncertainty. Determining if there is an associated GRB
requires checking the data of several facilities with varying amounts of
coverage, livetime, and sensitivity. Comparison of optically-identified with
gamma-ray-identified collapsar jets is informative on whether they arise from
the same underlying population.

####  2.3.3 Origin of Short GRBs

As noted, recently there have been convincing cases of long GRBs that arose
from a merger origin. Conversely, there are short GRBs that arise from
collapsars, one of which is GRB 200826A (Ahumada et al., 2021). The burst was
discovered in the prompt phase by Fermi-GBM and in the afterglow phase by ZTF.
Despite the $\sim$1 s duration, the expectation was a merger origin, but
follow-up identified an excess inconsistent with a kilonova origin but
consistent with a supernova.

The prompt duration is generally shorter than is expected to be possible from
collapsars. One explanation is the jet was only partially successful at
escaping. It is also feasible that the inferred duration is incorrect due to
the tip-of-the-iceberg effect. This drives a need for vastly improved
sensitivity over Fermi-GBM. While these events cannot be particularly common,
otherwise Swift would have identified more, it is unclear how to prioritize
follow-up of these events. However, the focus on characterizing short GRBs
will be beneficial in finding more of these events, which provide insight into
what makes a successful collapsar jet.

####  2.3.4 Ultra-long GRBs

On the other end of the collapsar prompt GRB duration distribution, ultra-long
GRBs have particularly long durations (Levan et al., 2013). The exact
delineation from normal long GRBs is not agreed upon, but is on the order of
1,000-3,600 s. The most extreme event surpasses 10,000 s in duration as seen
by Konus-Wind (Golenetskii et al., 2011), with evidence from follow-up
observations for a prompt duration of up to 25,000 s (i.e., 7 hours; Gendre et
al., 2013). It is generally believed that normal long GRBs arise from compact
Wolf-Rayet progenitors, which should not be able to power the longest duration
bursts. It is an open question if ultra-long GRBs arise from different stellar
progenitors, including some cases where no supernova is identified to deep
limits. Intriguingly, there also appear to be different types of ultra-long
GRBs, e.g. a precursor pulse followed by quiescence before the main emission
(e.g. GRBs 160625B, 221009A) and others that follow a single pulse with an
exponential decay, which may be indicative of external shock from jets that
did not have internal dissipation (see discussion in the appendix of Kann et
al., 2018).

Resolving these questions are key to understanding collapsars. That is, the
mechanism that powers their supernova is all the same, but identifying
differences in the prompt emission, their shock breakout, or supernova signals
will lead to understanding of all the types of progenitor stars capable of
ending their lives this way and which stars are incapable of doing so. For
example, why has a Type Ib supernova never been found following a long GRB? It
is perhaps related to the size of the star, which may suggest these events
appear as LFBOTs or other engine-driven supernova. This is another step to
understanding the exotic zoo of relativistic transients.

These events are identified either in cases where they remain bright enough to
be recovered by Swift-BAT over multiple orbits (Lien et al., 2016) or through
the unbiased coverage by Konus-WIND (Svinkin et al., in prep). Having two
large GRB monitor with background stability over several hours (requiring a
non-LEO orbit) would allow for discovery and confirmation of additional GRBs.
This would determine whether an ultra-long sample fully distinct or the
extreme of the general long GRB population based on prompt properties (i.e.
via duration).

An additional capability that would be a large advancement in the study of
these sources would be rapid classification of a given burst as belonging to
this class. In a few cases, Swift-BAT has re-triggered on the same burst over
multiple orbits, but a dedicated search identifies additional events (Lien et
al., 2016). Konus is capable of identifying these events, but the high
downlink latency generally prevents follow-up at sufficiently early times.

####  2.3.5 High Redshift GRBs

High-redshift GRBs are one of the most sought after events as they provide a
key cosmological probe of the high-redshift universe (Lamb & Reichart, 2000).
They have been detected out to a redshift of $\sim$9 (Salvaterra 2015; Figure
6), encompassing the era of reionization and beyond. In principle, they can be
recovered deeper into the universe than other objects, e.g., the BOAT could be
seen with existing facilities to a redshift of $\sim$15-20 (Frederiks et al.,
2023).

Figure 6: The observed redshift distribution of long GRBs (from De Cia, 2011,
updated through 2020)

The featureless, power-law behavior of GRB afterglows and their extreme
brightness make high-z GRBs the best probes to understand the epoch of
reionization, which could be mapped with tens of events (Tanvir et al., 2019).
They also uniquely allow follow-up observations to study small, early
galaxies. GRB afterglow measurements also provide a direct measure of the
optical depth of the host galaxy to Lyman continuum radiation and thereby
constrain the fraction of UV emission that escapes to reionize the
intergalactic medium (Kawai et al., 2006).

Spectral observations of the afterglow measure absorption lines from elements
in the host galaxy and the intervening intergalactic medium, probing the
chemical enrichment history of the universe (Hartoog et al., 2015). Follow-up
observations of the host galaxy after the afterglow has faded allows study on
the mass-metallicity relation allowing for tests of the earliest phases of
galaxy formation (Laskar et al., 2011). A sample of these events also allows
inferences on the earliest phases of star formation rate evolution (Robertson
& Ellis, 2011). Lastly, some of these events may arise from the first stars in
the universe.

Several missions have been conceptualized to specifically study and identify
high redshift GRBs. These concepts generally achieve a low-energy threshold of
a few keV with wide-field monitors to handle the effects of extreme
cosmological redshift and design toward extremely sensitive instruments,
necessary to identify events at these distances. High redshift GRBs comprise
perhaps 5-10% of the total GRB population. In order to trigger the large near-
infrared telescopes like JWST and the future extremely large telescopes these
events must be identified rapidly, preferably within 15 minutes. Thus, these
concepts also generally have on-board infrared telescopes to make this
measurement and to recover the $\sim$arcsecond scale localization for spectral
follow-up.

####  2.3.6 Very-High-Energy Emission

The search for Very-High-Energy (VHE) EM emission from GRBs has been a holy
grail in the field for decades. The first potential detection occurred in 2000
with Milagrito (Atkins et al., 2000) but it took nearly 20 years for
unambiguous proof. We separate our discussion into the two main types of VHE
detectors: Imaging Atmospheric Cherenkov Telescopes (IACTs) and water
Cherenkov arrays. We first focus on detections by the IACTs.

IACTs are pointed instruments with fields of view of a few degrees but
excellent sensitivity above tens of GeV. IACT arrays react to GRB detections
by pointing at or tiling the localization regions disseminated by satellites,
resulting in an irreducible delay with respect to the prompt emission. The
first publicly announced VHE GRB was GRB 190114C, discovered by Swift-BAT and
Fermi-GBM, which was detected in the TeV energy range by the MAGIC Telescopes
during the early afterglow (MAGIC Collaboration et al., 2019b). The TeV
emission provided the first potential evidence for inverse Compton radiation —
more specifically, synchrotron self-Compton — which has been long theorized to
be an emission channel during both the prompt and afterglow phases of GRBs.
This synchrotron self-Compton component would provide a measurement of
microphysical parameters of the jet which remain otherwise unknown after
decades of study. Significantly, this GRB has also opened a new channel for
high-redshift GRB identification.

Although GRB 190114C was a particularly bright GRB, the VHE detection of the
early afterglow phase was only possible due to a rapid autonomous IACT
response to the well-constrained localization provided by the _Swift_ -BAT
through GCN. However, late-time observations by IACTs can also be fruitful, as
GRB 190829A (Abdalla et al., 2021) remained detectable in the VHE regime up to
56 hours after the prompt emission, despite being a low-luminosity burst.
Curiously, the VHE emission was more consistent with a synchrotron than a
synchrotron self-Compton origin. If true, this would indicate a strong
incompatibility with the standard one-zone scenario. VHE GRBs therefore offer
a unique opportunity to move beyond the standard simplified scenario and
explore GRB jet properties such as the magnetic field structure, as well as
relativistic shocks in general.

The detection of high-energy emission by the LAT following GRB 211211A, one of
the long GRBs that arises from a compact merger origin, suggests an external
Inverse Compton (EIC) origin (Mei et al., 2022; Zhang et al., 2022). EIC is
where the electrons are from the jet but the seed photons are from another
source, in this case possibly from the kilonova. This detection brings up the
feasibility of recovering EIC from nearby GRBs, e.g. low-luminosity GRBs.

The next-generation IACT array, the Cherenkov Telescope Array (CTA), will be
roughly 10$\times$ as sensitive as the current IACTs and, in sub-hour
timescales, 10,000$\times$ more sensitive in the 75-250 GeV range than Fermi-
LAT (Fioretti et al., 2019). With $\sim$6 VHE detections of GRBs by the
existing IACTs in the past $\sim$5 years and a handful of detections of
$\sim$100 GeV photons by the Fermi-LAT in 15 years of observing, CTA could
detect tens of GRBs per year, so long as it has fast access to sufficiently
accurate localization information. The largest of the telescopes, which have
the lowest energy ranges (25-150 GeV) and are therefore more sensitive to
extragalactic sources (as the highest energy photons are preferentially
absorbed by the Extragalactic Background Light), will be able to repoint to
any position in the sky within 20 s of reception of an
alert111https://www.eoportal.org/other-space-activities/cta. These Large-Sized
Telescopes will have fields of view of 4.3∘ radius, and for poorly localized
sources, CTA can cover a larger region with lower depth by staggering the
pointings of its telescope array, allowing for an adaptive follow-up program.
Thus, CTA can follow-up initial localizations on the order of tens of square
degrees and have robust association of low significance VHE signals through
comparison with higher latency precise localizations of the prompt signal. For
the brightest GRBs, CTA will have the most sensitive view into the GeV energy
band; for the larger number of more moderate GRBs, CTA will provide the
statistics necessary to better characterize the population of VHE GRBs.

The water Cherenkov telescopes are wide-field survey telescopes, and are the
better instrument for VHE observations during the prompt emission. They cover
vast regions of the sky (few sr) at a much shallower depth than the IACTs, but
targeting a higher energy (above a hundred GeV) and with the advantage of
having a much larger duty cycle, as they do not require the dark observing
conditions that IACTs do. The premiere telescopes are the High-Altitude Water
Cherenkov Observatory and the Large High Altitude Air Shower Observatory
(LHAASO), both in the Northern Hemisphere, with a proposed southern
installation called the Southern Wide-field Gamma-ray Observatory. Thus far,
the only detection of a GRB by a water Cherenkov telescope is of GRB 221009A
by LHAASO (LHAASO Collaboration et al., 2023), with confirmed photon energies
of up to 7 TeV and tentatively as high as 18 TeV (Huang et al., 2022). As the
GRB went off in the LHAASO field of view, this also marked the first VHE
detection of a GRB during the prompt emission, though the smooth nature of the
VHE emission was more consistent with being due to forward shock emission.
While this has been the sole detection of a GRB by a water Cherenkov telescope
to date, the enormous number of counts seen by LHAASO suggests other bright
events may be recovered if they are fortuitously observable by these arrays.
If all three water Cherenkov observatories are in operation, then a large
fraction of the sky will be simultaneously observed at any time. These arrays
do not require prompt localizations, only delayed localizations of sufficient
accuracy for association.

However, a VHE detection on its own — whether by IACTs or water Cherenkov
telescopes — is not sufficient to constrain GRB properties such as the
magnetic field structure or the microphysical parameters of the jet. Rather,
this is only achievable when there are longer wavelength observations to
provide the crucial context for the VHE measurements. In particular, sensitive
instruments operating in the UV to GeV range will be absolutely crucial to
characterize the true nature of the transition between the synchrotron and
inverse Compton components. Thus, the prompt localizations need to meet the
needs of telescopes across the EM spectrum.

####  2.3.7 Origin of Neutrinos and Ultra-High Energy Cosmic Rays

There are strong connections of ultra-high energy cosmic rays (UHECRs) with
particle physics and high energy astrophysics. Three messengers, cosmic rays,
gamma-rays, and high-energy neutrinos (HEN), are linked, providing
complementary information about the same underlying physical phenomena in
astrophysical environments. A golden multimessenger era lies ahead of us and
with the next-generation cosmic ray experiments it will be crucial to seek out
neutral ultra-high-energy particles related to transient events in order to
acquire insights into a number of nature’s most energetic processes.

Among the outstanding questions in these interdisciplinary fields is the
origin of UHECRs, of which GRBs are prime suspects. Assuming they produce
UHECRs, then HEN emission is also expected as the UHECRs will collide with
photons (Waxman & Bahcall, 1997). Unlike UHECRs, HEN will travel in a straight
line through magnetic fields, making it possible to pin point their origin. A
HEN signal from a GRB would be considered “smoking gun” evidence for GRBs
being hadronic accelerators.

HEN emission from GRBs has yet to be detected and they have been ruled out as
the main contributor to the all-sky HEN flux (Abbasi et al., 2022). The most
optimistic models for HEN emission have also been ruled out, but more
realistic models are yet to be excluded (Aartsen et al., 2015). Measurements
or limits on the HEN flux of a GRB can be used to constrain the parameter
space of model parameters, such as the baryon loading factor (Murase et al.,
2022). In cases where there is a detection of VHE gamma-rays from a nearby
GRB, a limit or measurement of the HEN flux can set a constraint on how much
of the VHE gamma-rays are from hadronic versus leptonic origins.

The IceCube Neutrino Observatory (Aartsen et al., 2017) is currently the most
sensitive experiment to transient sources of HENs. The most important factors
for improving the sensitivity are increasing the number of GRBs with
localizations near to the angular resolution of IceCube’s through-going tracks
of ${\cal O}(1^{\circ})$. IceCube is undergoing an upgrade and pursuing
support for a second generation instrument. Similarly, in Europe ANTARES is
being upgraded to KM3Net. Baikal-GVD continues improvements in their analysis.
Future treatment of these telescopes as a single effective instrument may
further increase sensitivity. As these non-EM telescopes advance, they require
continued capability in the detection and localization of prompt GRBs.

Related to these works is exploration of whether GRBs can be the origin of
UHECRs even if they do not substantially contribute to the HEN flux. For
example, in the ICMART model of prompt GRB dissipation (Zhang & Yan, 2010) the
internal dissipation radius is substantially larger than in other models,
where the lower density will result in fewer proton interactions, allowing for
contribution to the UHECR flux without producing a luminous HEN signal.
Continued joint observations of the high-energy sky in neutrinos and gamma-
rays is key to understanding the prompt emission mechanism of GRBs, both in
population (Abbasi et al., 2022) and individual studies (Abbasi et al., 2023).

The IceCube observations of GRB 221009A and upper limits from analyses
designed for lower energy studies has also proven informative in new ways
(Abbasi et al., 2023). For example, Murase et al. (2022) show that the non-
detection of GeV neutrinos from IceCube places constraints on the bulk Lorentz
factor of the jet at an earlier phase than has been done before or require an
even more pure (low baryon content) jet than previously shown. As analysis
techniques improve in the existing and forthcoming neutrino telescopes, we
will continue to learn more about GRBs, provided sufficient discovery missions
are still observing.

While the above discussion focused on HENs from fully successful collapsars,
the most promising neutrino source here may be so-called “choked” grbs. These
events would produce significant amounts of neutrinos given the far higher
proton interaction likelihood (Mészáros & Waxman, 2001). Choked GRBs likely
sit in the continuum from normal CCSN to ultrarelativistic collapsars, with
fully choked GRBs appearing as relativistic supernovae and partially choked
GRBs as low-luminosity GRBs. LFBOTs, X-ray flashes, and similar may also be
key neutrino sources. If true, optical identification of these events may
preclude robust association of related neutrinos due to the large temporal
uncertainty on the EM side. The precise timing and spatial information from
detection of shock breakout or GRB emission of a choked GRB will enable
association of even untracked high energy neutrinos. This should be a key goal
of the new multimessenger era.

When speaking strictly about understanding the origin of neutrinos, there is
no strong alert latency driver for high energy EM monitors because the HEN
telescopes are effectively continuously observing the entire sky. It is
sufficient to provide precise localizations, necessary for robust association
of any neutrino signature, in high latency. However, should such a joint
detection occur, it would be one of a few known multimessenger source types
and full temporal and spectral characterization across the full EM spectrum
would be critical. The GRB monitor requirements are thus those described above
for general follow-up of collapsars. Follow-up would be aided by the neutrino
localizations, though tracked IceCube events require tiling from narrow field-
of-view facilities. It is reasonable to assume the first neutrino detections
of GRBs will occur for nearby events, which supports all-sky coverage even
with less sensitivity to GRBs, though low-luminosity events may be of
particular interest in this case.

####  2.3.8 Gravitational Waves

The relationship between collapsars and long GRBs is in a state similar to
that between BNS mergers and short GRBs prior to GRB 170817A. A GW observation
associated with a collapsar, identified by the long GRB, would provide a
watershed of new insights. As the sensitivity of the GW network continues to
improve, such a possibility becomes more realistic. Some specific models for
GW emission, for example, the accretion driven instability model (van Putten
et al., 2014), suggest a long GRB at $\sim 50$ Mpc would already be observable
by the current GW networks (Abbott et al., 2022). Only a small sample of long
GRBs within this distance have been observed to date; however, the rate of
redshift determinations is very low. For example, of the 86 GRBs used in the
analysis for Abbott et al. (2022), none had a measured redshift. Moreover,
gamma-ray brightness is a poor indicator of distance, therefore, the GW
analyses follow up all GRBs, since any one of them may be within the
detectable horizon. While it is expected that detections will not be possible
until the 3G era, observations prior to that epoch will provide some of the
only direct information on what occurs in the interior of collapsars. An
improved sky localization from IPN or other mission could facilitate a follow
up redshift determination to bolster the case for a putative GW detection. The
requirements on high energy monitors are similar to those of multimessenger
searches with high energy neutrinos, as the IGWN also has archival all-sky
observations (when operating). Precise, high-latency alerts are sufficient for
GW-GRB specific science and the GW-detected events will likely be nearby,
favoring all-sky coverage even with lower sensitivity. Similarly,
identification of these events rapidly enough to allow successful follow-up
and detailed characterization is highly beneficial. We additionally note the
effects of failed jet may be detectable in GWs (Gottlieb et al., 2022), but
prompt EM identification likely requires X-ray recovery of the shock breakout.

####  2.3.9 Polarization and the Blandford-Znajek Mechanism

An additional diagnostic in astronomy is polarization, providing insight into
geometry and ordered magnetic fields. Both geometric and intrinsic
polarization may be expected in prompt and afterglow emission. Geometric
polarization will arise when the viewing region crosses structure from the
jet. Intrinsic polarization may arise from the creation of ordered magnetic
fields at the jet launch site by the central engine; these will become
disordered due to turbulence as the jet propagation outwards.

From time-integrated measurements in the prompt phase alone it is not possible
to determine the origin of a significant polarization signal; however, if the
majority of GRB jets have highly ordered magnetic field and their prompt
emission mechanism is synchrotron, it is possible to identify this population
from the rest of GRBs through a population of prompt polarization detections
(Toma et al., 2009). This requires a GRB polarimeter and determination of the
peak energy for a sample of at least tens of GRBs. External determination of
spectra is beneficial for more confident determination of polarization.

To better constrain the prompt emission mechanism and understand jet physics
and evolution, time-resolved polarization is necessary. Historically, theory
has focused on time-integrated measurements as a substantial number of photons
are required for robust spectropolarimetric fits. With the next generation of
polarimeters being built and proposed it may be possible to perform time-
resolved fits for an number of GRBs. Theoretical and simulation work can
explore distinguishable time-resolved spectropolarimetric behaviors, allowing
for advanced understanding on the prompt GRB emission mechanism and on
microphysical jet parameters (e.g. Deng et al., 2016; Gill & Granot, 2021;
Parsotan & Lazzati, 2022).

Very early observations of GRB afterglow may be able to capture intrinsic
polarization and their destruction as the jet propagates, as possibly seen in
GRB 120308A (e.g. Steele et al., 2017). At later times, afterglow polarization
measurements generally capture only geometric polarization, which can be used
to understand jet structure in concert with observations of jet breaks.

Bursts with sensitive polarization observations in both the prompt and
afterglow phase are of particular scientific interest (Negro et al., 2023). If
follow-up observations of the afterglow find low polarization, but the prompt
emission has high polarization, then the prompt phase must arise from a jet
with ordered magnetic fields. As ordered fields will only be destroyed during
propagation they provide insight into the central engine. Jets formed by black
hole and neutrino-antineutrino annihilation central engines (Eichler et al.,
1989; Mészáros & Rees, 1992) will tend to carry most of their energy in the
matter entrained in the jet and will have low prompt polarization. Jets with
ordered magnetic fields arise either from magnetar central engines or black
holes whose energy is extracted to power the jet via the Blandford-Znajek
process (Blandford & Znajek, 1977).

Bursts with magnetar central engines have a maximal energy output set by the
rotation energy imparted on the object. Bursts with total energy, being the
sum of the energy released as light in the prompt phase and supernova (or
kilonova) phases as well as the kinetic energy in the jet and omnidirectional
ejecta, can only be powered by black holes. Thus, an energetic burst with
prompt polarization and no or little afterglow polarization would be direct
proof of the Blandford-Znajek mechanism. While it is known that black holes
are voracious consumers of surrounding material such a result would be proof
that black holes occasionally return significant power to the universe, i.e.
the first proof of a Penrose process (Penrose & Floyd, 1971). This is one of
the few specific questions listed in the Astro2020 Decadal report.

####  2.3.10 Prompt Emission Mechanism

The prior discussions on understanding the origin of the VHE signals, seeking
neutrino counterparts, and applying polarimetry to the study of GRBs in the
prompt and afterglow phase are deeply related to understanding the prompt GRB
emission mechanism. The mechanism, or mechanisms, has escaped confident
determination for decades (see Zhang, 2018, for a general review relevant for
this section). The current state of the field is an on-going debate on whether
a low-energy excess is due to an additional thermal component or an additional
break in a synchrotron spectrum. Working out if the jets are Poynting flux or
matter dominated, how they are launched, whether they have ordered magnetic
fields, and understanding their microphysical parameters are all key
diagnostics to understand the viable prompt GRB emission mechanisms. A sample
of these well-studied events is necessary to understand if more than one
mechanism is viable in distinct GRBs or even if multiple contributions
contribute to the signal seem in individual bursts. The GRB monitors need to
enable rapid, broadband follow-up of the EM spectrum, provide precise
localizations (even in high latency) for the non-EM messengers, and cover a
broad range of the prompt emission, ideally $\sim$1-10,000 keV. Coverage up to
20 GeV would be well-suited to determine the inverse Compton crossover region
in partnership with CTA.

Long GRBs are often easier to use than short GRBs for the study of the prompt
emission mechanism as they are generally brighter, as previously discussed.
Additionally, their durations allow for the possibility to observe the prompt
phase across the EM spectrum (noting it is inherently covered by the neutrino
monitors and IGWN when they are observing). This can occur for any GRB if it
is in the field of view of a given survey instrument, such as the LHAASO
observation of GRB 221009A or the TESS observation of GRB 230307A. However, it
is unlikely that this will occur for multiple survey instruments at the same
time. Swift and Fermi alerts can be received and distributed to the community
on the order of 10–30 s. If observations begin within a minute after trigger,
20% of long GRBs are longer than this. The first success was with a wide-field
monitor observing GRB 990123 (Akerlof et al., 1999), with one case of early
polarimetry observations (Troja et al., 2017), and in total, order tens of
events have been detected with prompt optical observations. Recovery of the
prompt signal from optical (or lower) up to TeV energies would be a stringent
test of any prompt emission model. This requires immediate alerts. Arcminute
localizations are the ideal scenario, and is perhaps the only way to recover
the prompt signature in more than one follow-up facility at a time.

####  2.3.11 Lorentz Invariance Violation

Observations of GRBs provide some of the most sensitive search space for
Lorentz Invariance Violation (LIV), motivated by the goal to test General
Relativity and the search of a quantized field theory of gravity, and is thus
a path towards a grand unified theory (Burns, 2020). Here we use the language
of the Standard Model Extension Framework (Kosteleckỳ & Russell, 2011), which
separates potential LIV into sectors (gravity, neutrino, photon, matter),
dispersive and non-dispersive, birefringent and non-birefringent, and
directional dependent violations.

Perhaps the most well known LIV test using GRBs is using GRB 090510 where the
constraints fall into the dispersive, non-birefringent, photon case (e.g.
Vasileiou et al., 2013). However, the most precise test of LIV with GRBs
arises from the detection of polarization, as birefringent LIV would destroy a
coherent signal with precision scaling with distance (e.g. Stecker, 2011).

In the new age of multimessenger astronomy, these and other EM limits are key
to determination of LIV in the other sectors. For example, the speed of
gravity was determined from the known speed of light, and dispersive LIV tests
in gravity determined from the far more precisely known limits on the photon
sector with GW170817 and GRB 170817A (Abbott et al., 2017). Similar results
have been calculated for SN 1987A (Ellis et al., 2008) and the neutrinos seen
in coincidence with a blazar flare from TXS+0506 (Ellis et al., 2019).

For dispersive tests, coverage of the prompt emission over several orders of
magnitude is critical. Improvements require a range broader than that achieved
for GRB 090510 ($\sim 10$ keV to $\sim$10 GeV) or a shorter timescale ($\sim$1
s), favoring the broadband recovery of prompt emission described in the past
subsections. For more precise birefringent tests sensitive polarimetric
observations is required to recover polarization from events deeper into the
universe.

####  2.3.12 Summary of Capability Requirements

As expected, a great deal of the science that can be achieved through
collapsar detections requires precise localizations. High-latency, precise
localizations allow deep multimessenger searches for neutrinos and GWs, and
can confirm or reject candidate counterparts identified in independent surveys
or from follow-up observations of earlier GRB localizations (e.g. optical,
VHE, or otherwise); if precise enough ($\sim$arcsecond scale), these can
enable late-time follow-up observations to identify host galaxies and thus
redshift determination. A best-case scenario certainly includes rapid
reporting of precise localizations from the high-energy monitor and is
scientifically well-motivated. It allows broadband observations of the prompt
phase to study LIV and the prompt emission mechanism, full characterization of
the spectral energy distribution for VHE detections, observations of the early
polarization decay, measurement of redshift directly from the afterglow, and
recovery of the full range of diagnostics of these events.

The field of view requirement depends on the specific science case. When
paired with other discovery facilities, true all-sky monitoring is preferred,
given the usual limited horizon of other diagnostics (optical, GW, neutrino,
etc). Studies of the high-redshift universe favor depth in a given direction.
These capabilities need not be satisfied with the same instruments.

Discovery of transients in UV, infrared, optical, and radio drives $\gtrsim$1
week contiguous observing intervals. The need to determine if ultra-long GRBs
are part of the long GRB population or are distinct motivates background
stability on the order of at least thousands of seconds, preferably hours. The
timing precision and temporal resolution of collapsar GRBs is generally less
stringent than that of short GRBs or magnetar flares.

Polarization in a GRB monitor is key to understanding the jets themselves and
the prompt GRB emission mechanism. Polarimetry of the afterglow of these
bursts may provide proof of the Blandford-Znajek mechanism in specific cases.
These drive instrument and precise localization requirements.

Fully successful collapsars require the typical historic energy range of GRB
monitors. To entirely map out the continuum through low-luminosity GRBs/X-ray
flashes, shock breakout from relativistic supernovae, and shock breakout from
normal CCSN requires low energy thresholds of approximately 10, 1, and 0.1
keV, respectively. A mission capable of achieving 0.1 keV would be a
revolutionary facility for both GRBs and supernovae.

####  2.3.13 Requested Deliverables from GRB Monitors

Many communities, including IGWN, IceCube, and wide-field survey instruments,
would greatly benefit from collated GRB alert streams and catalogs. This aides
the discovery of multiwavelength and multimessenger events and vastly reduces
the work necessary for multidiagnostic studies. It would also enable real-time
multimessenger searches, e.g., for neutrino-GRB joint detections or comparison
with optically-identified transients. Reduced localization area increases
joint search sensitivity and reduces computational expense. Having the prompt
GRB community deliver this information will result in greater science return.
Providing access to the detections of the full GRB network, or coherent upper
limits for non-detections, will help prioritize follow-up observations of
externally identified relativistic transients.

Similarly, studying data as it arrives with the goal being to determine if a
given burst could be consistent with a shock breakout origin may aid the study
of low-luminosity GRBs, or shock breakouts from supernovae in the case of an
X-ray monitor. The automatic flagging of potential ultra-long GRBs while the
afterglow is still detectable would be a boon in the study of these sources.
Providing information on whether a given event may have broadband spectral
coverage, prompt polarization coverage, or is particularly bright would also
help the follow-up community determine which bursts to target. These are
similar to the findings for merger GRBs; we refer the reader to that section
for additional details.

###  2.4 Miscellany

In addition to the sources where GRB monitors have compiled large samples,
they are also important for rare and unusual events. This is a driver for
sustained coverage of the high-energy sky over the timescales of human
lifetimes, to ensure detection of once-in-a-lifetime events.

####  2.4.1 Jetted Tidal Disruption Events

There have been four relativistic (“jetted”) TDEs discovered to date. Three
were discovered by Swift-BAT, one by the on-board trigger and two in high-
latency analyses by the BAT hard X-ray monitor (Levan, 2015). The fourth was
recovered in optical by ZTF (Andreoni et al., 2022; Pasham et al., 2023). Key
questions to these sources include i) what is the connection between jetted
TDEs and other classes of TDEs found routinely by optical surveys (Van Velzen
et al., 2021; Charalampopoulos et al., 2022; Hammerstein et al., 2022; Yao et
al., 2023)? ii) are optically-identified and high-energy-identified jetted
TDEs distinct or of the same class? iii) how do their jets carry their energy?
Study of these sources allows for observation of a jet where both the
initiation and end of the jet can be observed, similar to GRBs, and provide a
link to active galactic nuclei. Study of the three source classes may allow
for understanding on how relativistic jets are powered, what distinguishes
ultrarelativistic from relativistic, and understanding of the ways these jets
carry their energy.

Discovery of more TDEs by high energy monitors likely requires instruments
beyond the basic scintillator design, which are non-imaging and background
dominated. They can be discovered at a low cadence with coded aperture masks
like Swift-BAT. Lobster-eye telescopes may prove to be more capable but will
generally detect more distant events. However, as optical surveys become ever
more capable it is evident that they will discover additional jetted TDEs.
Determination of a high-energy signature requires either early identification
in optical with rapid follow-up of pointed X-ray telescopes or a highly
sensitive X-ray monitor with at least a $\sim$daily all-sky cadence.

####  2.4.2 Non-Astrophysical Sources

GRB monitors are often used for studies in heliophysics, helping to complete
the multiwavelength picture of solar flares. The discovery of terrestrial
gamma-ray flashes associated with lightning by BATSE has led to
characterization of these sources in X-rays and higher energy gamma-rays by
RHESSI and Fermi-LAT, as well as comparison with lightning detection by
ground-based optical networks. Thus, GRB monitors have a role to play in
multiwavelength studies of solar and Earth science events. As we utilize
instruments on planetary and heliophysics spacecraft, it would appropriate to
ensure the deliverables from the prompt GRB monitors are also designed to
benefit other divisions and cross-divisional studies, e.g. in the study of
solar flares.

####  2.4.3 Unexpected Discoveries

The discovery of GRB 211211A as a long GRB from a merger origin was
unexpected, as was the consistency of the prompt emission arising from a fast-
cooling synchrotron emission (Gompertz et al., 2023). The analysis of GRB
221009A identified a 10 MeV line in the prompt phase of individual, rare
events (Ravasio et al., 2023) which has no previous theoretical expectation.
Even after 10,000 prompt GRB detections, new insights into GRBs are made
though careful study of the prompt phase. As a further example we emphasize
the possibility of GRBs being created by the evaporation of local primordial
black holes (Ukwatta et al., 2016). Such a possibility is best studied with
multiple spacecraft at interplanetary distances. Maintaining various
capabilities of the prompt GRB monitors over decades is key to maximizing
science from rare events, either through the study of their prompt signatures
alone or with multidiagnostic studies common in the current era.

##  3 Capability Requirements

Each sub-topic for the sources discussed in Section 2 discusses the specific
capability requirements necessary to advance scientific understanding on that
topic. Each source also has a Summary of Capability Requirements section
giving the broad needs specific to high energy monitor observations of that
source class, i.e. magnetars in Section 2.1.9, compact mergers in Section
2.2.16, and collapsars in Section 2.3.12. When seeking to understand the set
of requirements necessary for a specific question or source class we refer the
reader to the content of Section 2.

This section takes a more holistic view on the observational capabilities
themselves, commenting on the general importance of various requirement
thresholds. This is useful in considering which technologies to use or
develop. We discuss how easy it is to meet the various requirement levels. We
emphasize all capabilities do not need to be met with each instrument, i.e. a
polarimeter does not also need to provide arcminute-scale localizations alone.

###  3.1 Localization Precision

The science possible with a given localization precision is summarized in
Table 1. For cosmological GRBs, i.e. those from mergers or collapsars, a
position determination to $\sim 1^{\prime\prime}$ accuracy is necessary to
robustly associate the event to a host galaxy and to determine the offset from
the host galaxy. This permits inference of the redshift (though this can be
measured directly from the afterglow in some cases), enabling measurements of
intrinsic energetics and the progenitor environments. This precision is beyond
the capability of wide-field transient monitors and is enabled through
successful recovery of the afterglow in follow-up instruments.

Localization Accuracy | Corresponding Result | Sections
---|---|---
4$\pi$ sr | Detection of gamma-ray transients by all-sky monitors |
| Chance joint detection of transients with other wide-field monitors |
$\sim$1000 deg2 | Follow-up tiling of GRBs by the widest field UV and optical telescopes | 3.1
| Robust association of GWs and GRBs | 2.2
$\sim$100 deg2 | Identification of MGF candidates and potential host galaxy | 5.3.7
| Follow-up tiling of GRBs by wide-field optical telescopes |  2.2.3, 2.3.6
$\sim$30 deg2 | Follow-up tiling of GRBs by wide-field radio, VHE, and IR facilities | 3.1
$\sim$10 deg2 | Associate nearby extragalactic MGFs to ideal host galaxies | 2.1.1
| Robust association of GRBs to neutrinos | 2.3.7
$\sim$1 deg2 | Associate SGR flares to specific magnetars | 2.1.3
| Robust association of UVOIR identified transients to GRB |
$\sim$100 arcminute2 | Extragalactic MGF host galaxy association | 2.1.1
$\sim$30 arcminute2 | Follow-up observations by the majority of telescopes |
$\sim$1 arcminute2 | Follow-up observations by effectively all telescopes |
$\sim 100$ arcsecond2 | Follow-up identification of Galactic magnetars | 2.1.5
$\sim 10$ arcsecond2 | Robust associations of cosmological GRBs to host galaxy and measurement of offset | 2.3.12
Table 1: Brief summary of the follow-up observations and association strengths
for various gamma-ray transient localization precision. Relevant sections
which detail specific rows are provided in the third column.

Thus, achieving precise localizations is tied to a mix of alert latency and
precise localizations. Localizations on the $\sim 10^{\prime}$ scale is
sufficient to be observed with the vast majority of telescopes, though $\sim
1^{\prime}$ scale is necessary for some specialized telescopes like the large
optical and infrared spectrometers. Localizations at this scale are possible
with coded aperture masks (e.g. Barthelmy et al., 2005; Chattopadhyay et al.,
2018). Sub-degree localizations, achievable with Compton telescopes and pair-
conversion telescopes, can be followed-up with a small number of tiled
observations. A common example of this is Swift-XRT and UVOT tilings of Fermi-
LAT detections, which have a typical uncertainty of $\sim 0.5^{\circ}$.

Localizations on larger scales can be followed up with wide-field instruments.
Instruments similar to Fermi-GBM can achieve localizations on the scale of
$\sim 1-50^{\circ}$. As the localization area increases, the number of
facilities capable of observing a significant portion of the localization
decreases rapidly. With the advent of GW astronomy, which can have
localizations of comparable areas, more wide-field optical facilities have
been built (e.g. Groot et al., 2019; Bellm et al., 2018; Steeghs et al., 2022)
that are capable of covering thousands of square degrees per night (e.g.
Coughlin et al., 2019). Similarly, UV coverage by ULTRASAT, VHE coverage by
CTA and the water Cherenkov telescopes, infrared by WINTER, and several wide-
field radio facilities contribute to broad wavelength coverage. While these
facilities exist and unlock new, critical capabilities in the TDAMM ecosystem,
there is a cost to poor high-energy localizations. Successful identification
of the afterglow can take more than a day as the requirement for a fading
transient and observing cadence following Earth’s day/night cycle is a
fundamental limit for ground-based facilities. There is generally a trade-off
between sensitivity and field of view, limiting their depth to the nearer
universe. Especially in the optical, there must be vetting of enormous numbers
of sources.

The independent localizations from joint detections of prompt signals can be
combined to produce an improved localization compared to either independent
localization alone. A key example is the case of joint GW-GRB localizations
where GBM-like localizations will reduce single GW interferometer
localizations by factors of $10-100$x and double interferometer triggers by a
median of $\sim$5x due to the different localization morphologies. Joint
detections are also possible with IceCube, ULTRASAT, TESS (e.g. GRB 230307A),
the water Cherenkov telescopes, etc. Though, again, a precise localization on
the high energy signal is the best-case scenario for the entire TDAMM
community.

The other main role of localization precision is in associating transient
events. For the association of two signals, the significance scales inversely
to the localization area. Localization precision can be relaxed in this role
if the temporal precision is sufficient to associate signals in time. The
temporal offset between the GW signal and GRB is on the order of seconds, so
even large (thousand square degree) localizations are sufficient to achieve
unambiguous association. For optically-identified transients with large
uncertainties on the time of initial explosion, these large localization
regions can be insufficient for associating signals. These often require final
localizations on the order $\sim$1 deg2, with discovery claims requiring
greater precision. The same is true for most expected neutrino signals from
GRBs and CTA follow-up observations.

Figure 7: The 1-sigma width distribution of the third IPN timing annuli,
through 2020. Overlaid is a black vertical line denoting the typical 90%
confidence error for Swift-BAT. The narrowest 90% confidence width in the
distribution is $4^{\prime\prime}$ with a median width of $5^{\prime}$.
Additional larger annuli can be generated for satellite pairs in low Earth
orbit which is not included in the figure.

Timing annuli from the IPN span a wide range of precision, as shown in Figure
7. Typical annuli widths for IPN bursts recovered by distant spacecraft (which
are the bright $\sim$half of GBM bursts) are a few arcminutes wide, though
precision down to $4^{\prime\prime}$ has been achieved. Note that a single
annulus with a few arcminute width can span significant total sky area, and at
least three spacecraft (two annuli) are required to achieve a localization
area comparable to Swift-BAT. IPN localizations have two drawbacks compared to
those possible from coded aperture masks. The reporting latency for distant
spacecraft is most often measured in hours to days, though fortuitous timing
is possible. Second, when the first data from distant spacecraft arrive the
initial localization is very elongated, which is a shape that wide-field
monitors are not generally suited to tiling. Despite this, successful recovery
of interesting events has been demonstrated, most notably with GRB 230307A as
discussed in Section 5.3.2. Initial localizations sufficient for narrow-field
follow-up allowed for recovery of a candidate counterpart, which was confirmed
in part by the higher latency data providing a particularly precise
localization.

For the study of magnetars, the most stringent localization precision
requirement arises for associating extragalactic MGFs to their host galaxies,
where local events require precision of $\sim$10-100 arcminute2 and distant
events require precision approaching a few arcseconds. Association of normal
SGR short bursts to active magnetars requires order $\sim$1 deg localizations.
New magnetars can be found by stacking localizations of several short bursts,
historically achieving accuracy on the order of $100$ arcsecond2 (Hurley et
al., 1999).

###  3.2 Alert Latency

The ideal worldwide system for transient astronomy would be sensitive all-sky
coverage at all wavelengths and messengers. Barring that, the best-case
scenario is immediate reporting of a precise localization from the first
detectable signal(s) of a given source class. For all types of GRBs, SGR/FRB
and magnetar giant flares, and jetted TDEs, this is usually the keV-MeV
signal. In the pursuit of multiple diagnostics from the same event, observing
the immediate aftermath is absolutely critical to fully understand these
sources and unlock multimessenger science. The definition of precise
localization in this subsection refers to $<10^{\prime}$, sufficient for
follow-up with a single pointing in most facilities. Achieving this accuracy
can be done directly from some high energy monitors, or in concert with high
energy monitors and follow-up tiling, as described above. Various telescopes
across the EM spectrum can observe with a wider field of view; see Section 2
for specific details. The results possible with a given timescale are
summarized in Table 2.

Alert Latency | Corresponding Result | Sections
---|---|---
$>$10,000,000 s | Origin of short GRBs, MGF candidate identification | 2.1.1
| Origin and physical mechanisms of FRBs | 2.1.4
| MGF QPOs and NS equation of state | 2.1.2
| Sources of GWs |  2.1.3 2.3.8
| Discovery of new magnetars | 2.1.5
| Magnetar formation channels, properties, and burst physics |  2.1.6 2.1.7 2.1.8
| Determination of SGRB progenitor fractions | 2.2.6
| GRB classification of GW sources | 2.2.8
| Speed of gravity measures | 2.2.15
| Determination of GRB counterpart to orphan afterglow, dirty fireballs |  2.3.1 2.3.2
| Origin of neutrinos, ultra-high energy cosmic rays | 2.3.7
1,000,000 s | Guide fast radio burst searches of active Galactic magnetars | 2.1.4
| Capture rise of supernova | 2.3
100,000 s | Follow-up classification of long GRBs from mergers |  2.2.7 2.2.9
| Latest reliable recovery of afterglow, potential redshift determination, cosmology |  2.2.10 2.2.11 2.2.13
| Guide follow-up of externally-identified transients based on prompt GRB signal |  2.3.1 2.3.2
| Capture rise of red kilonova | 2.2.4
10,000 | Key diagnostic information on relativistic jets | 2.2.14
| X-ray recovery of plateau emission in afterglow | 2.2.1
| Tests of gravity parity violation | 2.2.15
| Follow-up observations for VHE emission | 2.3.6
| Capture rise of blue kilonova | 2.2.3
1000 s | X-ray observations of fading tail after Galactic MGFs | 2.1
| Discrimination of origin of early UV emission in mergers | 2.2.3
| Observation of prompt phase of ultra-long GRBs | 2.3.4
| Blandford-Znajek test via afterglow polarization observations | 2.3.9
100 s | Multiwavelength characterization of BNS merger classes and associated science | 2.2.4
| Critical early observations of EM-bright NSBH mergers | 2.2.5
| Prioritized follow-up based on GW merger classification | 2.2.8
| X-ray observations of fading tail after extragalactic MGFs | 2.1
| X-ray recovery of merger GRB extended emission | 2.2.1
| Full tests of dense matter, origin of heavy elements | 2.2.12
10 s | Recovery of higher radio frequency (low dispersion measure) precursors | 2.2.2
Table 2: Summary of the science possible with various alert latency
thresholds. Implicit in these timescales are the time to recover the precise
position for narrow-field telescopes to use, which may add delays for poorly
localized prompt GRB detections. Complete understanding of these transients
requires reporting on the timescale of 10-100 s, though some key science can
be done with high latency data access.

The fastest realistic latency is 10 s, commonly achieved by Swift-BAT. This
would allow for the recovery of effectively all signals discussed in Section
2, given that capable follow-up telescopes are ready. After $\sim$100 s the
radio precursor signature in NS mergers may be lost, as are observations of
the early plateau decays (though some plateaus are recoverable for $10^{5}$
s). Also after 100 s the chances to observe prompt emission at other
wavelengths begins to rapidly decrease out to $\sim$10,000 s. After 1000 s the
chance to recover fading, pulsating X-ray tails from extragalactic MGFs with
current facilities becomes extremely unlikely. On a similar timescale, the
prospects of discriminating some of the potential sources of the early UV
emission in NS mergers begin to fade.

A latency of 10,000 s results in the loss of the brightest portion of
afterglow, making recovery of VHE emission and early polarimetric observations
of the afterglow less likely to succeed. Observations at this time are still
likely recover follow-up signals, capture the rise of redder kilonova
emission, and to observe the pre-break timescales of the jet. After 100,000 s
(i.e. about a day), each of these three are less likely but still possible,
but beyond 1,000,000 s these successful observations become quite unlikely.
High latency reporting is still valuable for the purposes of confirming or
rejecting candidate counterparts found in other wavelengths and messengers.

Reporting latency is generally less critical for the monitoring of Galactic
magnetars. It is necessary to inform the radio community when specific
magnetars are active, but high-latency matching of SGR and FRB flares is
sufficient, provided other FRB counterparts are not identified. The exception
is the detection of magnetar giant flares, as noted, with possible recovery of
the fading tail in X-rays if observations begin within minutes of an
extragalactic event and within somewhat longer timescales for Galactic events.

###  3.3 Field of View, Livetime, and Contiguous Viewing Intervals

The study of nearby events presents critical diagnostics to provide leaps in
the understanding of a given source class. The detection of MeV neutrinos and
nuclear gamma-rays from SN 1987A set our current understanding of the
convective engine paradigm in typical CCSN explosions. The detection of SN
1998bw following GRB 980425 proved the theoretically predicted collapsar model
for GRBs and supernova explosions. The detection of the off-axis GRB 170817A
with GW170817 and AT2017gfo led to breakthroughs in several fields of physics
and still sets our standard for the study of NS mergers in astrophysics. The
association of an SGR 1935+2154 X-ray short burst to bright $\sim$ms radio
flashes proved that at least some cosmological FRBs originate from magnetars.
The limits from the IceCube non-detection of GRB 221009A are more informative
on prompt GRB models than the stacking of several thousand bursts. This is an
effect of a multi-diagnostic Malmquist bias, where the nearest maximum
detection distance determines the rate of events that can have complete
observations. Whenever possible, a key component of the transient and
multimessenger ecosystem should approach complete and uniform coverage of the
sky. This is approximately met by IceCube and should be met by the IGWN
network beginning in $\sim$2027.

Sources | Field of View | Livetime | Contiguous Interval | Sections
---|---|---|---|---
Prompt GRBs |  $\sim$100% |  $\sim$100% |  |  2.1 2.2 2.3
GWs, FRBs, High-energy neutrinos |  $\sim$100% |  $\sim$100% |  |  2.1.3 2.2.8 2.3.8 2.1.4 2.3.7
MeV Neutrinos, Shock Breakout |  $\sim$100% |  $\sim$100% |  |  2.3 2.3.1
Relativistic supernovae |  $\sim$80% (Night Sky) |  $\sim$100% |  $\sim$1 week | 2.3.1
Orphan Afterglows, LFBOTs |  $\sim$80% (Night Sky) |  $\sim$100% |  $\sim$1 week | 2.3.2
Unknown Unknowns | 100% |  $\sim$100% |  $\sim$1 week | 2.4
Table 3: The Field of View, Livetime, and Contiguous observing intervals
necessary for various sources of interest for GRB monitors.

The use of high-energy monitors in multi-diagnostic astronomy are necessary
for the vast majority of future astrophysical advances, and all-sky field of
view is often more important than depth for a given effective area of
background-dominated instruments, as summarized in Table 3. This is because
nearly all other diagnostics have a more limited detection horizon. NS mergers
can only be recovered by current GW interferometers to a few hundred
megaparsecs, to an even shorter distance by current UVOIR surveys, and to a
smaller volume when observing afterglows than when observing the prompt phase.
This effect for GW-GRB joint detections was shown directly in the NASA GW-EM
Task Report prior to the Astro2020 Decadal (Racusin et al., 2019). For
collapsars, low-luminosity GRBs are detectable to only a tiny fraction of the
observable volume of typical long GRBs. VHE observations are limited in
distance by extinction due to pair production with the extragalactic
background light. The first transient neutrino sources will almost certainly
be (cosmologically) local. In these cases, all-sky sensitivity for the gamma-
ray transients is favored over depth. Exceptions include any hope of recovery
of SGR X-ray flares around extragalactic FRBs (perhaps possible with pointed
X-ray telescopes) and high-redshift GRBs.

Historically, discovery of relativistic transients was the domain of high-
energy monitors. As the survey capabilities of other wavelengths and
messengers advanced, they have begun to play a leading role in the discovery
of these events. Because the prompt gamma-ray signatures are reasonable
contemporaneous or precede other signals, archival coverage of the full event
is necessary to make strong statements in the case of sensitive non-detection
and to maximize the chance of signal recovery.

For short-duration, well-localized transients, the chance that a GRB monitor
will have coverage is determined by its instantaneous field of view multiplied
by its livetime. An example of this is observation of a Swift-BAT detected GRB
by GBM. For spatially extended localizations, this fraction decreases further
when the field of view is under 100%. For example, the long arcs of two-
interferometer IGWN localizations generally precludes total coverage by
individual low Earth orbit satellites, driving the need for sensitive all-sky
instantaneous field of view. This, and livetime, is driven by the need to
provide complete coverage for neutrino events, FRBs, external prompt GRB
detections, and other short-duration transients.

For externally identified events with imprecise start times, this fraction
decreases even further when the livetime is under 100%. The lack of
sufficiently long contiguous observing intervals prevents direct statements on
the gamma-ray emission from individual relativistic transients identified by
optical surveys that have start time uncertainties on the order of $\sim$1
day. This may prevent significant statements on particularly rare events.
Additionally, it limits the population comparison of a high-energy-identified
and optically-identified transients of the same source class (i.e. collapsar
jets, relativistic supernova). With $N$-day fixed contiguous viewing intervals
and 1 day as the representative start time uncertainty, the effective livetime
scales as $(N-1)/(N+1)$, i.e. a 3-day window has full coverage of only
$\sim$50% of events and a 7-day window of $\sim$75%. This an additional
dimension to the observing fraction requirement, specific to longer-duration
transients. Thus, the development of ground-based wide-field survey telescopes
drives the need for long contiguous viewing intervals in GRB monitors.

Similar needs will be needed as the capability to discover transients occurs
in new wavelengths and messengers. As one example we point to the millimeter
and sub-millimeter wavelengths where orphan afterglows can be found (Whitehorn
et al., 2016). The discovery space here is emphasized by the Decadal
recommendation for time-domain capability in CMB-S4.

True 100% all-sky coverage and livetime is ideal to have in prompt high-
energy monitors. This need is generally met by IPN, albeit to a shallower
sensitivity than some of the more sensitive low Earth orbit satellites. The
best individual instrument, in this regard, is Konus-Wind, which has all-sky
coverage, near 100% livetime, and is more sensitive than the more distant IPN
spacecraft. Achieving these properties is effectively impossible to meet with
satellites in low Earth orbit, unless there is a fleet. For a mission
dedicated to detecting transients that have complete multiwavelength
observations, it could be designed to focus on the $\sim$80% of the sky
visible at night on Earth, as most ground-based facilities cannot observe near
the Sun.

There is one particularly notable exception to the need for a massive field of
view, which is the search for high redshift GRBs. Identifying a sample of
these events requires great sensitivity to recover them as far back in cosmic
time as they occur. This sensitivity is likely more easily achieved with a
narrower field of view.

###  3.4 Background Stability

A related requirement is background stability, referring to the range of time
over which background is stable. Stability over $\sim$100 s is key to
identification of extended emission following short GRBs, key to much of the
science possible with NS mergers. This requirement can be met with inertially
pointed instruments in low Earth orbit as demonstrated by the greater recovery
of extended emission in BAT and BATSE and lower recovery rate in GBM. To
provide a definitive answer on the overlap of traditional and ultra-long GRBs,
and to prompt identify ultra-long GRBs while their emission is still
observable, background stability for $\gtrsim$6 hours is necessary. While
Swift-BAT has identified some ultra-long GRBs over multiple orbits, the
orbital timescale imprints an effective cut in the timescales. For the study
of ultra-long GRBs having multiple instruments with sufficient sensitivity and
background stability is key for confirming and localizing these bursts.

Background Stability Interval | Source Type | Sections
---|---|---
10 s | SGR flares and FRB counterparts, short GRBs |  2.1, 2.2
100 s | Short GRBs with extended emission | 2.2.1
1,000 s | Long GRBs | 2.3
10,000 s | Ultra-long GRBs | 2.3.4
100,000 s | TDEs, longest GRBs |  2.4, 2.3.4
Table 4: Brief summary of the background stability necessary for various
source classes.

###  3.5 Timing Capabilities

Absolute timing refers to the preciseness of the data compared to a given time
standard, generally UTC, with accuracy limited by the capability of the
onboard clock. For scientific purposes, absolute timing of $\sim$1 ms is
sufficient for nearly all of the discussed outcomes, with the comparison of
the emission times of SGR X-ray flares and FRBs imposing the most stringent
case requirement. This capability is lost at an absolute timing precision of
$\sim$10 ms. A precision of $\sim$100 ms is an important limitation on
measuring the speed of gravity and LIV. Achieving $\sim$1 ms timing precision
for low Earth orbit satellites is easy, given the capability to update onboard
clocks with pings from the Global Positioning System (GPS). It is more
difficult for satellites beyond the reach of GPS, but the adoption of atomic
clocks and/or tiny X-ray telescopes for pulsar timing and deep space
navigation (Ray et al., 2014; Winternitz et al., 2016; Ray et al., 2017)
should prove revolutionary in this regard. Additional approaches to more
precise absolute timing would also prove beneficial for these studies. The IPN
would 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.

Relative Timing | Corresponding Result | Sections
---|---|---
10 $\mu$s | Measure shortest duration pulses observed in SGR flares | 2.1.8
100 $\mu$s | Measure shortest duration pulses observed in cosmological GRBs |
| Search for QPOs in the range of interest for neutron stars | 2.1.8
Absolute Timing | Corresponding Result | Sections
100 ms | Subdominant limitation to most speed of gravity measures | 2.2.15
1 ms | Enable minimal (statistical) timing annuli widths |
| Comparison of FRB and SGR arrival time, physical mechanisms | 2.1.4
Table 5: Brief summary of the relative timing and absolute timing
requirements, with relevant sections.

Relative timing precision sets how accurately the offset between specific
events can be measured. There are two main types of temporal data in high
energy monitors, time-tagged event (TTE) and binned data. TTE data is an array
of a time and energy channel for each event registered, and the timing
precision is limited by the instrument and electronics. Binned data are time-
series spectral histograms, with predefined temporal resolution, which is
nearly always larger than the native timing precision. Most onboard clocks
undergo a slow drift away from a reference time, meaning relative timing is
often far more precise than absolute timing.

An effective temporal resolution of 10 $\mu$s is sufficient for any high-
energy monitor. The shortest pulses ever observed have durations of 70 $\mu$s
and 100 $\mu$s (Bhat et al., 1992; Roberts et al., 2021), which may be lost
for instruments with only 100 $\mu$s timing. This relative timing precision
allows for QPO searches up to $\sim$10 kHz, beyond the range of most NS
oscillation eigenmodes. These searches will miss much of the QPO frequency
range of interest for these modes at 10 ms relative timing precision. At 10 ms
the complete searches for SGR flares will be negatively affected. At 100 ms
they will be severely limited, and there will be significant losses in the
detection of cosmological short GRBs.

A key example of the importance of temporal resolution is the sub-threshold
searches for SGR and short GRBs in Fermi-GBM and Swift-BAT data. With the
planned continuous binned data resolutions of these instruments, searches for
untriggered SGR flares would be impossible, and these provide a more complete
picture on the waiting time and energetics distributions of these events. Many
short GRBs would also be lost, preventing associations between GRBs and GW
signals. The downlink of continuous TTE data for Fermi-GBM and the saving of
Swift-BAT TTE data around externally identified times of interest by GUANO
(Tohuvavohu et al., 2020) has allowed substantially more sensitive searches
for, and better localizations of, these types of events. This method of
requested TTE is an option that should be explored in all future GRB monitors
as the flexibility and resolution of TTE data is a critical asset for TDAMM
studies.

Timing capabilities are intimately tied to the operation of the IPN. The
instrument with the lowest absolute timing resolution and relative temporal
resolution will dominate the systematic uncertainty. Extragalactic MGFs can,
in principle, be timed to sub-millisecond accuracy, setting a floor on this
requirement. A level of $\sim$1-10 ms is likely sufficient for the majority of
bright GRBs. There are two hurdles implicit to IPN operation in this regard.
The first is the relatively inaccurate absolute timing precision of distant
spacecraft. In fact, using the position of known GRBs and inverting the IPN
calculation is used to map the timing accuracy of planetary spacecraft.
Second, the limited bandwidth through the Deep Space Network (DSN), and
similar communication solutions, places a floor on the temporal resolution of
the binned data, and has precluded unbinned data. These can be ameliorated in
future instruments, as discussed in Section 4.

TTE data has a critical use in the IPN. Timing annuli are generally derived
through a cross-correlation function comparison of two datasets. The ability
to analyze data are arbitrarily small timescales ensures minimal timing annuli
widths. For example the Konus data has a 2 ms binning, but the time offset
between GBM and Konus for GRB 200415A is 1.3 ms. There should always be LEO
satellites with TTE data. TTE could be recovered from distant spacecraft with
advanced communications solutions, or through limited TTE around intervals of
interested identified by an on-board trigger.

$\sim$1 ms timing precision is necessary for joint SGR-FRB studies and allows
for more accurate triangulation of SGR X-ray flares. This drives both absolute
timing precision as well as minimum effective temporal resolution.

###  3.6 Energy Range

Standard GRBs typically have the highest signal-to-noise ratio over the
$\sim$50-300 keV energy range. Broadening this energy range to lower values
brings detection of additional relativistic transient classes. Broadening the
range in either direction brings greater characterization of detected
transients. These are summarized in Table 6.

Energy Range | Corresponding Result | Sections
---|---|---
50–300 keV | Detection and localization |
High Energy Threshold |  |
1 MeV | Constraint on peak energy, total energetics for long GRBs | 2.3.10
10 MeV | Constraint on peak energy, total energetics for short GRBs and MGFs | 2.3.10
| Evidence for additional spectral components in prompt GRB emission | 2.3.10
1 GeV | New understanding of additional spectral prompt components | 2.3.10
Low Energy Threshold |  |
10 keV | Increase in SGR flare detections, FRB counterparts |
| Identification of short GRBs with extended emission | 2.2.1
| Detection of low-luminosity GRBs | 2.3.1
| Identification of multiple prompt GRB spectral components | 2.3.10
1 keV | Insight on prompt GRB emission mechanism | 2.3.10
| Detection of X-ray plateaus | 2.2.1
| Detection Shock breakout of relativistic supernovae | 2.3.1
| Detection of X-ray flashes | 2.3.1
0.1 keV | Detection of shock breakout of most supernovae, full mapping of relativistic transients | 2.3.1
Table 6: Corresponding results possible with specific low and high energy
instrument thresholds. Each column implicitly assumes sufficient sensitivity
for a given topic.

Observing at higher energies improves the spectral characterization.
Understanding the spectrum and energetics is vastly improved when spectral
curvature can be measured. With a high-energy threshold of 1 MeV curvature
will be measured for most collapsar GRBs. At 10 MeV it will be measured for
the majority of all GRBs and MGFs, although the curvature has been measured
above 10 MeV in some rare cases. Extending the energy range higher is also
important to seek potential additional spectral components, in particular an
extra power-law component that is likely indicative of afterglow radiation.
Sensitivity up to 20 GeV would match the expected low-energy threshold of CTA
and thus provide complete spectral coverage to the VHE regime. This complete
spectral coverage would have the ability to determine the spectral turnover
from base synchrotron to inverse Compton humps.

Decreasing the low-energy threshold is also crucial for characterization of
GRBs. Sensitivity to $\sim$10 keV allows recovery of the observed low-energy
excess in some bright GRB spectra, the origin of which is still unknown. It
also enables recovery of extended emission and mapping of the low-energy
quasi-thermal tails seen after GRBs 170817A and 150101B (Goldstein et al.,
2017; Burns et al., 2018). Extension down to 0.1–1.0 keV would enable routine
measurement of three separate regimes in the prompt synchrotron
interpretation, increase the recovery rate and insight on extended emission,
and increase the recovery rate of the cooling of thermal tails and shock
breakout emission.

Additionally, the low-energy threshold increases the capability of the
instrument to recover additional transient classes. If the threshold is at 50
keV, the majority of SGR short bursts are missed, while many are recovered
with a threshold of 10 keV. Given the peak flux-temperature connection for
SGRs, pushing to even lower values has limited return. A 10 keV threshold
allows recovery of many low-luminosity GRBs, with near-complete sampling if a
1 keV threshold can be achieved. This threshold would also allow recovery of
relativistic supernova and X-ray flashes, likely helping to understand the
exotic zoo of relativistic transients. Pushing down to $\sim 0.1$ keV would
also capture shock breakout from normal CCSNe, providing direct insight into
most types of massive stars at their end, fulfilling an additional key science
goal of the Astro2020 Decadal. These capabilities would also unlock
identification of X-ray bright tidal disruption events and undoubtedly uncover
new transient classes.

Fermi GBM and LAT combine to provide unparalleled broadband observations of
GRBs. The GBM low-energy threshold of 8 keV has allowed for the identification
of structure in the low energy end of GRB spectra ($\sim$10s of keV) which may
be evidence for an additional thermal component an an extra break in the
broader curvature. The high energy response from Fermi shows an expected
turnover below the LAT energy range in several GRBs. Together they have shown
evidence for the onset of a power-law component during the prompt phase,
likely attributable to the onset of external shock. Pushing to even lower
energies and covering the gap of $\sim$10–50 MeV promise ever further advances
in understanding the prompt GRB components.

###  3.7 Sensitivity

The sensitivity of high energy monitors drives both the number of events they
will observe and the statistics available for detections of a given
brightness. Over typical GRB energy ranges, the order of magnitude sensitivity
of Fermi-GBM and Swift-BAT triggers is $\sim 1\times 10^{-7}$ erg s-1 cm-2.
BATSE triggers and Swift-BAT subthreshold triggers reach to a few$\times
10^{-8}$ erg s-1 cm-2. An approximate sensitivity for the distant instruments
of the IPN (i.e. beyond Konus) is $\sim 1\times 10^{-6}$ erg s-1 cm-2.

Bright bursts are easily recoverable by the IPN instruments, but the IPN
sensitivity strongly limits scientific return. The non-detection of GRB
170817A by any spacecraft more distant than INTEGRAL demonstrates that GW-GRB
joint detections require deeper sensitivities. The current all-sky coverage of
optically-identified afterglows can exclude most of the viable parameter space
for prompt GRB counterparts, but it is not sufficient to fully exclude such a
signal, preventing direct inferences on the potential existence of dirty
fireballs.

The deeper sensitivities of GBM and BAT enable far higher detection rates and
recovery of events far deeper into the universe. They also allow recovery of
far greater numbers of magnetar bursts, though this is also enabled by high
temporal resolution data. Neither instrument, even with deep subthreshold
searches, is capable of recovering GRB 170817A beyond 100 Mpc, while IGWN can
already recover BNS mergers out to $\sim$300 Mpc, a $\sim$30$\times$ larger
volume. The deeper BATSE sensitivity resulted in a measurement of the turnover
in the cumulative fluence distribution of long GRBs, proving the cosmological
origin of GRBs. For temporal and spectral studies, the BATSE data is often far
superior than modern instruments (though it has a narrower energy range than
GBM). Achieving $\sim 1\times 10^{-8}$ erg s-1 cm-2 with modern flight
software detection algorithms and TTE data would enable breakthroughs.

Increases in sensitivity well beyond BAT and GBM are key to success to
observing high redshift objects as well enhanced studies of magnetars in the
local universe. However, we emphasize that the characteristic sensitivity
values quoted here do not apply to X-ray transients, which generally have
lower absolute values as they are integrated over a narrower energy range and
we are quoting energy (not photon) fluxes.

###  3.8 Maximum Photon Rate

The most stringent requirement on a maximal photon rate comes from a potential
energetic Galactic MGF which, in the most extreme scenario, may exceed
$1\times 10^{8}$ photons s-1 cm-2, although the SGR 1806 event had a peak flux
of 1.5$\times 10^{7}$ photons s-1 cm-2 (Frederiks et al., 2007a) and the other
two nearby MGFs were more than an order of magnitude less luminous. The
highest rates of extragalactic MGFs, normal Galactic SGR short bursts, and the
BOAT would be perfectly measured if an instrument could handle $1\times
10^{4}$ photons s-1 cm-2; roughly one GRB per year exceeds $\sim 1\times
10^{3}$ photons s-1 cm-2.

The maximum photon rate a given detector can handle is determined by the
instrument size and various aspects of its readout implementation. For
example, it is far easier to saturate BATSE than a smallsat with much smaller
detectors. It is easier to saturate slower scintillators than fast
scintillators. The electronics readout speed is also important. When the rate
limit is exceeded there are various instrumental effects that can occur
including data losses, data gaps, and spectral distortion. These can sometimes
be modeled and corrected, but this process is often imperfect and model
dependent. Achieving full, unaffected data of a Galactic giant flare likely
requires a tiny dedicated detector built for this purpose. For all other
purposes, large instruments can likely make clean observations, but there is a
cost to accommodating sufficiently high photon rates; however, exceptionally
luminous and energetic events often provide greater physical insights,
strongly motivating this additional cost.

###  3.9 Spectral Resolution

High-energy transients usually have smooth continuum spectra owing to their
origin from incoherent radiative processes in relativistic plasmas. Instrument
energy resolution is typically 10%–50% over $\sim$50–300 keV, which has proven
sufficient for nearly all purposes. For future study of physically motivated
models or possible lines in prompt GRB emission (Ravasio et al., 2023) higher-
energy resolution may be beneficial. This will be informed by the forthcoming
COSI mission. We add that this requires spectral readout into a sufficient
number of energy channels in order to maximize sensitivity to transients and
enable spectral analysis. If a high-energy monitor low-energy threshold were
extended down to $\sim$1 keV, then spectral resolution capable of resolving
lines, e.g. iron lines, would be scientifically well motivated.

High spectral resolution benefits constraints on fundamental physics. For
instance, constraining populations of low-mass black holes for dark matter by
lensing of fast transients (such as MGFs) potentially requires higher spectral
resolution $\Delta E/E\ll 10\%$ (Katz et al., 2018).

###  3.10 Polarization

The study of polarization in prompt GRBs is still relatively new. With
properly calibrated instruments, scientific advancements can be made with a
minimum detectable polarization down to 10% for bright bursts. Recent works
have shown that tests of specific prompt GRB models are best done with time-
resolved spectropolarimetric analysis. This requires either studying only the
very brightest bursts or vastly increased sensitivity to polarization than
prior instruments, e.g. POLAR. Precise requirements will be more easily
defined after continued advancements in theory and simulation and with respect
to the specific specialized instruments designed for this purpose.

Magnetar bursts at all energies (but particularly above 50 keV) are expected
to be highly polarized. Models of such bursts are at an early stage but
forthcoming.

##  4 Actionable Items for Missions and Instruments

Earlier sections detailed the scientific potential achievable through high-
energy monitor observations of transient events, as well as the requirements
needed to reach this potential. Several actions could be taken by NASA to
significantly augment this multi-diagnostic, interdisciplinary science. Some
specific actions are summarized below.

We emphasize that this report has implicitly assumed that space missions will
be capable of downlinking data and uplinking new commands with the typical
latencies currently in use today. The TDAMM Communications SAG will
investigate the space-based communications needs for broader science and
instruments than are considered here. We comment directly on how alterations
in the use of DSN may improve IPN operations below.

###  4.1 Active Missions

The Astrophysics Advisory Committee (APAC) recently recommended NASA’s
Astrophysics Division (APD) to perform a reanalysis of their current portfolio
to determine how to maximize TDAMM capabilities. Here we discuss some specific
actionable items where NASA APD could enhance return of its own assets as well
as assets in other Divisions, similar to its funding a TDAMM enhancement of
the Near-Earth Object Surveyor mission. The key enhancements are all
effectively different ways to get more data accessed as fast as possible. We
note that Swift and Fermi have historic and planned improvements, per the last
round of Senior Review, to continue enhancing the TDAMM science return of
these workhorses. For example, the plan to lower the on-board triggering
threshold of GBM for short GRBs would allow for great scientific returns in
partnership with the IGWN.

Perhaps the most immediate example is the over-guide request from Swift for
additional ground-station passes in the last round of Senior Review. This
would reduce the latency of full data downlink from BAT (both standard and
GUANO data), XRT, and UVOT. Identifying counterparts of GWs from any of these
instruments $\sim$hours earlier would contribute to understanding of specific
NS mergers, and allow the location information to be passed to telescopes
worldwide that are ready to characterize these events. The lower latency of
data would be generally useful in the study of all Swift time-domain sources.
For example, analysis of the Swift UV grism observations of the recent
supernova in M101 (the closest in a decade) were key to the decision to
repoint Swift for additional observations and as guidance for other UV and
X-ray telescopes, but the existing system resulted in delays. The cost is
minimal compared to the scientific gain. The choice of Senior Review to avoid
decisions on TDAMM-related over-guides and lack of strategic TDAMM planning
since then has overlooked this obvious improvement. Yet, O4 has already begun.
This is perhaps the greatest immediate priority action NASA could take to
benefit TDAMM.

Additional ground-station (or other downlink) passes could benefit additional
NASA missions with respect to gamma-ray transients. This requires discussion
with the relevant instrument teams to ensure it is possible and beneficial
within the existing mission architecture. Fermi is a specific case where
additional downlink passes would accelerate spectral and temporal analysis of
on-board triggers and sub-threshold searches for additional events rapidly
enough to guide follow-up observations of its core transient classes.

One of the most important improvements would be additional downlinks for the
distant spacecraft of the IPN. Today the IPN includes 11 gamma-ray detectors,
of which four are beyond low Earth orbit. Additional types of detectors can be
used for particularly bright events (on the order of one per year). The most
distant gamma-ray detector alternates between the Mercury Gamma-ray and
Neutron Spectrometer (MGNS) on-board BepiColombo Mercury Planet Orbiter, and
the High Energy Neutron Detector (HEND) on-board Mars Odyssey. The recent
switch of the MGNS data on BepiColombo, en route to Mercury, from 250 ms to 50
ms has enabled current IPN localizations to rival Swift-BAT positions. HEND
on-board Mars Odyssey has 250 ms resolution. Konus on-board the US Wind
satellite has 2.944 s background resolution and 2 ms resolution for on-board
triggers. Additional downlinks would support more rapid IPN localization in
the multimessenger era and could possibly support higher temporal resolution.
However, we emphasize that any change is not trivial and must be done in
concert with interest from the instrument and mission teams. Potential issues
include aging flight recorders, the necessity to maneuver the spacecraft for
downlinks, and potential onboard processor or storage limitation.

Beyond decreasing the latency for data to arrive on the ground, improvements
can be made to data access. For Fermi-GBM, distribution of HEALPix maps that
account for systematics of the ground localization would be beneficial for
observations beginning under 15 minutes, as would removal of Earth occulted
positions from localization maps. Serializing the roboBA localization into the
alert itself rather than waiting for the delay added by HEASARC would allow
more rapid use of GBM data. Further, distribution of retraction Notices due to
the astrophysical nature or misclassification of events would be beneficial to
guide follow-up efforts. Some of these enhancements can be done outside of the
instrument team pipelines and could be handled by the IPN (see Section 5), as
discussed in Section 5.

Support for improving the robustness of existing IPN data pipelines and
enhancing the information shared is key to automating the process and reducing
ground-based contributions to alert latencies. Support for creating instrument
responses for the purposes of GRB studies is necessary for inclusion in any
multi-mission analysis.

###  4.2 Forthcoming Missions

There are numerous forthcoming missions of relevance. NASA APD has selected
COSI as an upcoming Small Explorer mission, StarBurst as an upcoming Pioneer,
and is funding additional technology demonstration missions through APRA.
Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a
French-Chinese mission designed with a similar ideology as Swift. China is
launching a series of GECAM satellites. Several gamma-ray spectrometers on
planetary and heliophysics spacecraft are planned to be built and launched by
American and Russian scientists over the next decade.

StarBurst will have several times the effective area of Fermi-GBM, launching
as a ride share. StarBurst is using silicon photomultipliers, which will limit
the overall instrument lifetime at high inclination orbits due to increased
radiation exposure. COSI will have a dedicated mission launch and requires a
$<2^{\circ}$ inclination orbit. Pairing StarBurst to the COSI launch would
significantly enhance the scientific return of StarBurst by increasing
observing livetime due to fewer passages through high rate particle background
regions. It would also increase the operating lifetime of StarBurst by
reducing the damage of the silicon photomultipliers. Additionally, increasing
the number of ground contacts and bandwidth would enable the downlink of all
StarBurst TTE data, instead of TTE limited to on-board triggers.

COSI is slated to launch in 2027, well-timed with the planned beginning of the
IGWN O5 observing run. The onboard COSI trigger for GRBs will be initiated by
count rate increases in the anticoincidence shield, which will trigger prompt
downlink of a limited number of Compton events allowing for a $\sim$deg scale
localization within an hour of the trigger. Enhancing the amount of data that
can be rapidly downlinked by COSI would improve that initial localization.
Enabling the downlink of the full dataset would result in final localizations
at a much higher cadence, enhancing the transient science return of COSI. We
also support the downlink of all possible data from the COSI shields and
single-site events in the main instrument, regardless of likely event class or
the likelihood of being recovered as Compton events. COSI will contribute to
the IPN with events detected by its shields and will help characterize the IPN
systematics through facilitating the recovery of arcsecond localizations.
There may also be possible TDAMM enhancements for the COSI student-led BTO
instrument which will broaden the energy coverage and can also contribute to
IPN.

The Johns Hopkins Applied Physics Laboratory is building the Psyche Gamma-Ray
and Neutron Spectrometer (GRNS), the Mars-moon Exploration with GAmma rays and
NEutrons (MEGANE) instrument for the JAXA Martian Moons eXploration (MMX)
mission, and the Dragonfly Gamma-ray and Neutron Spectrometer (DraGNS). All
three spectrometers use germanium which must be cooled. DraGNS will utilize
the temperature of the Titan atmosphere for passive cooling, preventing use
for the IPN.

The Psyche GRNS will contribute to the IPN for $\sim$6 years while MEGANE on
MMX should contribute for $\sim$2 years. These instrument and mission
operations have been adapted for use in the IPN. Each has an onboard trigger
that will achieve 50 ms resolution for a fixed time period. At Psyche’s
greatest distance of 3 AU this allows a statistical timing annulus width of
$\sim 10^{\prime\prime}$. Additional TDAMM support from NASA could ensure the
data is accessed as quickly, easily, and reliably as possible, and to support
the generation of GRB response matrices.

The Ioffe Institute in Russia is launching gamma-ray spectrometers on three
future spacecraft (Ulanov et al., 2019). Konus-UF will consist of two detector
units on the World Space Observatory Ultraviolet, providing all-sky coverage
and possibly TTE data from a geosynchronous orbit. If so, it would be the
first TTE data from beyond low Earth orbit. The twin InterhelioProbe
(Kuznetsov et al., 2016) spacecraft will each contain one detector unit and
will be among the few spacecraft to significantly diverge from the ecliptic,
which will be valuable for IPN localizations. US contributions to these
missions may bolster the IPN and meet the Decadal suggestion of supporting a
TDAMM program through international partnership.

As this report is being delivered, the review of the current Mission of
Opportunity call is underway. Both missions, LEAP and MoonBEAM, are dedicated
GRB monitors, with different focus areas. We support consideration of
additional TDAMM enhancements should one or both instruments be selected.

###  4.3 The Decadal-Recommended High Energy Monitor

The top priority sustaining activity recommended by the Astro2020 Decadal
Pathways to Discovery in Astronomy and Astrophysics for the 2020s (National
Academies of Sciences, Engineering, and Medicine, 2021) was: “NASA should
establish a time-domain program to realize and sustain the necessary suite of
space-based EM capabilities required to study transient and time-variable
phenomena, and to follow-up multi-messenger events. This program should
support the targeted development and launch of competed Explorer-scale or
somewhat larger missions and missions of opportunity.” While much of TDAMM can
be done from the ground, the wavelengths only observable from space are
critical for the TDAMM ecosystem, as noted by the Decadal: “The most important
of these are wide-field gamma-ray and X-ray monitoring, and rapid and flexible
imaging and spectroscopic follow-up in the X-ray, ultraviolet (UV), and far-
infrared (far-IR).” We also highlight the following quote to emphasize the
introduction of the concept of a workhorse mission to NASA APD: “NASA’s
workhorse hard X-ray and gamma ray transient facilities (Swift and Fermi,
respectively) are aging and their longevity is uncertain. Higher sensitivity
all-sky monitoring of the high-energy sky […] is a critical part of our vision
for the next decade in transient and multi-messenger astronomy.” Other NASA
science divisions have sustaining capabilities that are ensured across
generations of missions; for example, total and complete coverage of the solar
flares from the Sun. The message is clear: NASA astrophysics should ensure
continuous coverage of the sky with wide-field high-energy monitors.
Furthermore, the total NASA TDAMM program was recommended a budget of
$500M-$800M for use over the coming decade.

Relativistic transients are a major component of time-domain and
multimessenger astrophysics, containing the multi-diagnostic events of
supernovae, collapsars, NS mergers, TDEs, FRBs, magnetars, and some signals
whose origin is sill unknown. The science and corresponding requirements
detailed in this document provide motivation and an outline for a strategic
high-energy monitor. In the context of a field where every other wavelength is
facing a major upgrade in TDAMM capabilities (e.g., the Decadal recommended
next-generation Very Large Array, the Square Kilometer Array, CHORD, Roman,
the Vera Rubin Observatory, ULTRASAT, Einstein Probe, and the Cherenkov
Telescope Array) including simultaneous upgrades of ground-based GW
interferometers and HEN telescopes, a TDAMM-focused high-energy monitor is the
missing critical link in the full ecosystem. It is important to bolstering the
return of NASA’s investment in other missions and infrastructure.

NASA’s current largest investment in a high-energy monitor is COSI. COSI will
achieve $\sim$degree scale localizations of GRBs that will be reported within
1 hour, meeting some goals outlined in Section 3 and the temporal and
polarization requirements listed below. COSI’s prime design driver; however,
is nuclear astrophysics, and although it will revolutionize this field, COSI
will not replace Fermi or Swift for TDAMM studies. As highlighted in the
Decadal, another future high-energy monitor is SVOM, a French-Chinese mission
being built with similar instrument designs and science goals as Swift, but
with worse sensitivity and localization. SVOM is set to launch in 2024 and
will provide localizations at $\sim 10^{\prime}$ precision for 50-60 GRBs per
year. Over the nominal 3 year mission and potential 2 year extension, SVOM
will produce a sample of well-studied events, but its capabilities will not
match that of Swift or Fermi. Additionally, as noted in Astro2020, the US has
no significant involvement.

A number of nations have built, or are in the process of building, low-cost
scintillator-based instruments to search for GRBs from GW sources. A veritable
fleet will be operating soon. The largest of these are Glowbug and StarBurst,
funded by NASA as a technology demonstration and Pioneer, respectively. These
missions will both be more sensitive than Fermi-GBM, a key piece towards
improving the number of GW-GRB detections, but have narrower energy ranges,
face the same limitations of operating in LEO, and it is impossible to
substantially lengthen mission lifetimes, which are $\sim$1 year each, given
the instrument design. Additionally, they are designed for one area of
transient gamma-ray science. They will both provide new insight into the
universe in partnership with GW detectors, but they are not Fermi or Swift
replacements, which is not surprising given the orders of magnitude lower
total cost.

Based on the requirements outlined herein, and the currently planned future of
this field, there are some obvious missing existing capabilities. A high-
energy monitor covering the majority of the sky and capable of $\sim$arcminute
scale localizations, provided within $\sim$10 s, from an instrument that
continuously observes at a sufficient sensitivity would fill the missing gap
in the transient ecosystem. The operational mode of the IPN precludes this
possibility, given light travel time from distant spacecraft. Such a dedicated
mission is within the capability of existing technology. If the instrument
could achieve 10’ scale localizations, a low-energy threshold down to $\sim$1
keV and transient spectral sensitivity to several MeV, while maintaining
background stability, it would additional bring the recovery of the initial
shock breakout (necessary to map the properties of the progenitor star) and
high-cadence monitoring of the X-ray sky (also highlighted in the Decadal).
This seems feasible with developing technologies (e.g. Chattopadhyay et al.,
2018) and achievable as the TDAMM mission at the scale recommended by the
Decadal. Such a mission would prove a critical partner to all other transient
facilities including nearly the entire astrophysics fleet of NASA (and the
rest of humanity) as well as the low and high energy neutrino facilities, low
and high frequency GW interferometers, and the vast numbers of ground-based
telescopes used in follow-up observations.

Such a mission, through collaboration with worldwide partners, would unlock
new knowledge on fundamental physics including gravity parity violation, dark
matter, antimatter, the speed of gravity, and Lorentz Invariance violation.
Some of these tests using astrophysics are orders of magnitude more sensitive
than any other proposed method. It would provide the most precise measures on
the behavior of dense matter by determining the maximum mass of NSs, the
lifetime of meta-stable NSs, the radius of NSs, insight into the proto-NSs
formed during core-collapse, and the compactness of NSs. The maximum mass of a
NS is an asymptotic value, providing a uniquely constraining test of dense
matter, and could be measured to 1% precision (Margalit & Metzger, 2019),
which is a factor of several better than non-asymptotic tests expected through
currently active or selected missions. This mission would probe the origin of
GRBs, FRBs, GWs, astrophysical neutrinos, low-luminosity GRBs, ultra-long
GRBs, unusual supernovae and LFBOTs, every other relativistic transient class
discovered with forthcoming surveys, and rare and unusual individual
transients. It will provide insights into how jets form, whether black holes
return power to the universe, insights into the physics of magnetars, and the
transition regime between dense and condensed matter. It will support multiple
diagnostics for identifying different source classes, standardizing NS mergers
for precision cosmology throughout the universe, support proper inference on
the origin of the elements, standardize CCSN to understand how they explode,
and resolve the neutrino mass hierarchy. The all-sky field of view is critical
to identify all nearby transients where the diagnostics with the most limited
detection distances can still be recovered, a capability that has never
existed with prompt and precise localizations. Implementing the requirements
will provide significant advances in astrophysics, cosmology, gravity, and
fundamental physics, and enable interdisciplinary work in nuclear, particle,
plasma, and atomic physics.

The level of support for space-based TDAMM missions recommended by the
Astro2020 Decadal is absolutely necessary to unlock this magnificent breadth
of science. While no mature mission concept is capable of these requirements,
informed estimation accounting for the cost of fiducial detectors to achieve
the requisite sensitivity and including the requirement for achieving at least
a high Earth orbit suggest it may not fit within the Small Explorer mission
budget cap. The Decadal also highlights the need for “…rapid and flexible
imaging and spectroscopic follow-up in the X-ray, ultraviolet (UV), and far-
infrared (far-IR)”, which are wavelengths only accessible by instruments in
space. Key to the modern TDAMM system is the decoupling of discovery monitors
from the follow-up telescopes onto separate spacecraft. This design principle
implies the necessity of dissemination of high-energy alerts to the full
community, which can route to low-latency commanding of the follow-up
telescopes, as shown by Swift. This allows the high-energy monitors to
maintain background stability and vastly reduces the moment of inertia of the
narrow-field telescopes. Together the cost is likely to fall into the range of
the $500-800M$ recommended in Astro2020.

If the above described monitor is sufficiently sensitive, it will also support
high-redshift GRB science; however, this may not be technically feasible, and
a dedicated $\sim$1 sr field of view instrument may be required. It is
critical for this instrument to overlap with JWST, allowing for follow-up
characterization of the low-mass galaxy hosts in the early universe. In the GW
3G era, it will be critical to recover merger GRBs deep into the universe to
build the precision Hubble diagram out to a redshift of a few. Thus, such a
mission should be designed to launch before the end of JWST and to overlap
with the future GW interferometers. The characterization of explosive
transients by the previously described near-universe monitor will prove
important to science of this far-universe mission.

###  4.4 The Future of the InterPlanetary Network

The IPN has utilized more than 50 instruments, and outlived 31 of them,
providing true all-sky coverage for nearly 50 years. It is the existing
example of a workhorse network in NASA Astrophysics. Regardless of dedicated
GRB missions, the use of all-sky monitoring with relatively precise
localizations will always be needed.

Note that precise localizations of GRBs cannot be routinely accomplished by a
network of detectors entirely contained to low Earth orbit. A given annulus is
described as (RA, Dec, radius, width). The radius is the angle $\theta$,
defined relative to the vector joining the two spacecraft, calculated as
$\cos(\theta)=c\delta T/D$ with $c$ the speed of light, $\delta T$ and $D$ the
arrival delay time and distance between the two spacecraft, respectively. The
width is calculated as $c\sigma(\delta T)/D\sin(\theta)$ where $\sigma(\delta
T)$ is the uncertainty on the delay time (Hurley et al., 1999). The
interplanetary baselines are of order $10^{8}$ km. LEO provides baselines
$\lesssim 10^{4}$ km. While the temporal resolution of distant IPN spacecraft
can limit timing annuli accuracy, GRB lightcurves are not delta functions and
have a floor on the arrival time uncertainty determination.

A representative 10 ms arrival time uncertainty in LEO gives a best-case
1$\sigma$ annulus width of $\sim$10 deg. Even for particularly bright bursts
where 1 ms timing is possible, this is still $\sim$1 deg annulus width.
Improved methodologies cannot avoid this fundamental fact. Additionally, small
detectors will also generally result in poorer timing precision given the
smaller effective area. Careful simulations of a network show that they will
achieve degree-scale localizations similar to those provided by Fermi-GBM
alone (e.g. Thomas et al., 2023; Hurley, 2020). Hurley (2020) also raises the
issue that increasing the size of the network will decrease the average
distance between viewing detectors, thereby limiting the spatial precision
gain possible.

What is required from LEO satellites is a deep sensitivity and TTE data, which
is a critical complement to the more distant instruments in the IPN. The more
distant instruments will launch predominantly on planetary and heliophysics
spacecraft, and perhaps on astrophysics satellites that are more often
venturing to the Sun-Earth L2. Often these instruments are enhanced beyond
their primary purpose for use in the IPN: planetary missions use gamma-ray
spectrometers to map elements on the surface of other bodies in the solar
system and heliophysics missions can use them to study the X-ray and gamma-ray
emission in solar flares. Enhancements included increasing the temporal
resolution of all data and adding onboard GRB triggers (with a pre-trigger
buffer) for higher temporal resolution during specific times. Additional
enhancements that require very little modification to the existing systems
could include prioritization of the GRB data to be downlinked first during
scheduled communication passes and contribution of additional DSN time to
allow for greater temporal resolution.

Swift has demonstrated the recovery of TTE data around externally identified
times of interest, e.g. GW triggers. An addition to missions that host
contributing instruments to IPN would be large onboard storage, which would
allow days worth of TTE data to be saved onboard and requested during specific
times of interest.

A more disruptive enhancement may be scheduling DSN downlinks with greater
frequency. Given the large distances, only directional antennae are viable,
requiring reorientation for communication during coast or acceleration times.
This can result in downlink delays of $\sim$1 week, which are substantial
limitations. If DSN time can be scheduled and mission operations can
accommodate this change, this would be a vast improvement in IPN operations.

There are a few potential game changers identified in Origins, Worlds, and
Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 for
planetary science that would also enable capability leaps of the IPN. The
first is the planned Deep Space Optical Communications experiment on Psyche,
seeking the first demonstration of optical communications beyond the Moon.
Success would mean potential bandwidth increases by 10-100 fold above the
current radio solutions. This would greatly enhance the temporal resolution of
distant IPN instruments and may additionally support more frequent contacts.
The other development is the potential success of Starship and similar mega-
rockets. These heavy launchers would alleviate the greatest limitation of
launching distant spacecraft: mass, potentially allowing the launch of
multiple planetary spacecraft with one rocket. This would allow for larger
gamma-ray spectrometers for the study of element distributions on planetary
bodies. Further, it may allow the return of dedicated GRB monitors on distant
spacecraft which was routine in the field for decades but has not occurred
with U.S. led missions since the launch of Ulysses more than 30 years ago.
Russia plans to contribute dedicated GRB monitors to three spacecraft, an area
where the US could explore contribution of downlinks in support of the IPN and
the Astro2020 Decadal suggestion of partnering on foreign missions for TDAMM.

The last major change is in the reduction of systematic uncertainty that has
previously limited IPN precision. One advancement is the demonstration of
precise atomic clocks in space and, in particular, the plan to launch one on
the forthcoming VERITAS probe to Venus in 2028. Historically, the onboard
clocks of distant IPN spacecraft have been verified or their drift quantified
using the GRB instruments (e.g. Hurley & Sommer, 1994). The precise absolute
calibration is critical for IPN operation and key to knowledge of the true
spacecraft position. Inclusion of atomic clocks can solve this problem.
Alternatively, both timing and position could be enhanced with pulsar timing
positioning systems (e.g. Ray et al., 2014; Winternitz et al., 2016; Ray et
al., 2017). As discussed, the suggested clocks program in Thriving in Space:
Ensuring the Future of Biological and Physical Sciences Research: A Decadal
Survey for 2023-2032 would vastly exceed the precision needed for the IPN. The
other improvement since Ulysses is the external verification of true source
positions, which supports the verification of IPN precision or modeling of an
irreducible systematic uncertainty. The 90% annulus width of
$4^{\prime\prime}$ was derived from the collaboration between the GRB monitor
on Ulysses, a distant heliophysics satellite, and BATSE, but this was limited
by temporal resolution of these instruments and the systematics of the IPN at
the time.

If optical communications, megarockets, and atomic clocks become pillars of
future distant spacecraft, then the future of IPN could be scientifically
fruitful, perhaps achieving arcsecond scale localizations. There are $\sim$20
distant spacecraft launched by NASA or partner space agencies. If even half of
future distant spacecraft had dedicated GRB monitors, roughly $\sim$10
instruments would be available. Even if each has an average downlink latency
of days, then a few will downlink data each day. This could support precise
localizations with latency sufficient for successful follow-up of most
sources. The addition of a GRB monitor to any of the future Venus probes would
be beneficial.

At a few AU, e.g. the distance achieved by Ulysses and expected for Psyche,
TTE data would enable statistical localizations on the order of 0.1′′ for
particularly bright and variable bursts. 1′′ precision is possible with
$\sim$10 ms temporal resolution. If the atomic clocks and systematics are
carefully modeled, the resulting localizations are sufficiently precise that
host galaxy association and redshift determination could be made without the
need for successful follow-up recovery of the afterglow; only optical spectra
follow-up would be desirable. This could produce a complete mapping of
intrinsic energetics distributions and source evolution and complete the
Hubble diagram alongside 3G GW interferometers.

This range of enhancements, from non-disruptive to the intentional
construction of a dedicated IPN, requires inter-division discussions at NASA.
Elevation of IPN to a strategic initiative could be a necessity, as discussed
in the next section. This could prioritize taking advantage of new
capabilities and options, such as a dedicated monitor onboard Gateway enabling
TTE data from $\sim$1 lightsecond away.

As an additional example, Origins, Worlds, and Life: A Decadal Strategy for
Planetary Science and Astrobiology 2023-2032 recommend the Uranus Orbiter and
Probe mission as the highest priority among new flagship missions. If $\sim$2%
of the total mission mass could be allocated to a dedicated GRB monitor and
NASA APD funds the instrument, an on-board trigger, and the technology for
precise timing, then sub-arcsecond timing annuli would be available to the IPN
for nearly two decades ($\sim$13-15 year cruise, $\sim$4.5 year prime
mission). This would be a foundation for a new era of the IPN. With an
appropriate paired instrument (preferably outside of the ecliptic), host
galaxies could be found without requiring an afterglow detection phase. Other
opportunities exist including the Venus probes. Additionally opportunities may
arise pending the outcome of the Heliophysics Decadal.

##  5 Actionable Items for the Gamma-ray Transient Network

The needs of the various communities interested in studying magnetars, compact
mergers, and collapsars also lead to a number of requested products from the
ideal gamma-ray transient network. We find the best path to creating these
materials to be enhancing the operations of the IPN.

###  5.1 Support for Community-led Strategic Initiatives

Of critical importance to NASA’s implementation of the TDAMM aspects of the
Decadal is long-term, sustained funding in support of community-led strategic
initiatives, software development, and formalized renewal of mission-oriented
projects that are too small for a Senior Review. There is no general software
development call, preventing NASA from tapping expertise in our broad
community. These issues have precluded enhancement of the IPN due to limited,
sporadic, and fractured support.

Kevin Hurley’s operation of the IPN included half of the real-time
localization and reporting, archiving of the GRB and SGR flare catalogs,
maintenance of the entire system, and providing requested products to various
institutions (e.g. catalogs to LIGO). In his later years, this was supported
through Fermi and Swift GI proposals, proposed annually, providing a total
support on the order of $100 k per year. This was both insufficient to support
modern development of an improved system and the year-to-year funding nature
required time spent on proposals that otherwise could have been used to
deliver an obvious community project. Prior roles as participating scientists
in planetary missions allowed fostered communication and support from the
relevant instrument and mission teams, but the proposed work must be directly
for the individual funding missions, resulting in bifurcated community
products (e.g. Hurley et al., 2005, 2010, 2011a, 2011b, 2017).

Making the IPN, and the products described above, a community-led strategic-
initiative supported by NASA with review on a reasonable cadence would enable
far greater science with existing and forthcoming missions for a fraction of
the cost of a new mission. Such a decision would also allow the IPN to act in
an appropriate manner related to its importance in the field. For example,
allowing the IPN to request NASA-led press directly.

The advancement of astronomy has necessitated ever greater group sizes. In
many cases, the lone astronomer has been replaced by groups that have
sometimes evolved into larger collaborations, and a few of those have
coalesced into consortia. This change has been particularly prevalent in the
area of TDAMM, requiring broad expertise to capture a full story of specific
events. Perhaps the most visible is the formalization of IGWN from the LIGO
Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration. This
integration has greatly benefited these groups and the astronomical community
as a whole. Similarly, the creation of the SNEWS from the global set of MeV
neutrino telescopes will be critical to discover and characterize the next
Galactic supernova. There are also initial discussions on potential
integration of the HEN telescopes.

The gamma-ray transient detectors have not integrated to the same degree. The
IPN and the GW-GRB Working Group are the two relevant multinational consortia.
The IPN has operated as a university-led project within the US for decades.
This setup has allowed for continued operation through the evolution of
restrictions related to geopolitics, use of private data that is unlikely to
be accessible outside of the IPN, and is a key example of community-led
mission-oriented work. The GW-GRB Working Group is now more than a decade old
and has been co-led by various individuals at universities, non-profits, and
labs.

The integration of these groups is on-going. While leadership could rotate to
individuals at NASA institutions, it is preferable to maintain the broad
community-led nature of these consortia. There is a sufficient support
mechanism to facilitate growth of these projects to meet the needs of the
broader TDAMM community outlined in this report. NASA has supported key
aspects related to these goals through the Internal Scientist Funding Model,
but this method is not accessible for university-led projects. Given that the
deliverables described herein would result in maximal TDAMM science return via
gamma-ray scientists making otherwise publishable results immediately
available to the community, long-term sustained support would act as a proper
reward system for these scientists. Otherwise the individual benefit is
maximized by restricting public release to ensure authorship on
multiwavelength papers.

The purpose of this report is to gather the needs of the wider TDAMM
community, and to ensure those needs are met. The specific products to
maximize science with these instruments is listed in the next subsection. NASA
has facilitated the technical capability for many of these projects through
various grants and internal funding; however, to fully implement the community
requests, support for a greater integration of the gamma-ray transient
community is necessary. The specific implementation likely requires working
through the existing consortia, which enable the sharing of otherwise private
data.

###  5.2 Products

The analysis of data from multiple missions is generally limited to joint
spectral analysis or localization via the IPN. The products listed here would
greatly benefit the community, as previously noted in the Multimessenger
Astronomy Science Analysis Group (Brandt et al., 2020).

####  5.2.1 Event-based Real-time Alert Stream

The community has developed signal-based reporting to facilitate coordinated
observations and analysis of specific events. This is most evident in the
fixed naming conventions of supernovae, GRBs, FRBs, GWs, etc, which are
generally distinct from the internal identifiers of each observing instrument.
For example, GRB 170817A is the event-based GRB name for the GBM trigger
bn170817529. IGWN has created a single alert stream for GW alerts, allowing
the community to receive the best available real-time information, especially
spatial information, in a fixed format. SNEWS has similarly created this for
temporal and spatial information of MeV detections of neutrinos.

No such stream exists for GRBs. This would be one important benefit the high-
energy monitors could deliver to the community: providing a single access
point to real-time GRB information to the full follow-up community. This would
unify the schema of the data presentation, allowing a single script to be
written to listen in to this alert stream rather than requiring work for each
individual existing stream. The greatest benefit would be improved spatial
constraints to aide follow-up efforts. We emphasize this stream would support
equitable access to data in the field. Currently large institutions have an
advantage in developing frameworks to handle the vast heterogeneous alert
streams. A single access point allows institutions with less technical
capability to access the full information from a field with a single script.
The Unified Schema from GCN will be a great equalizer in this regard by
facilitating ease of access to information from all contributing TDAMM fields.

As currently implemented, the community is losing key early information in
rare and interesting events. This is perhaps best exemplified by GRB 230307A
(Section 5.3.2). It was iterative localized by IPN as data became available,
but limitations introduced by processing the real-time data and the manual
steps required resulted in delaying the alerts. Despite this, the localization
was tiled with both Swift, a space-base facility, and ULTRACAM, a large (3.5