###### Abstract

Since their first discovery in the late 1960s, Gamma-ray bursts have attracted
an exponentially growing interest from the international community due to
their central role in the most highly debated open questions of the modern
research of astronomy, astrophysics, cosmology, and fundamental physics. These
range from the intimate nuclear composition of high density material within
the core of ultra-dense neuron stars, to stellar evolution via the collapse of
massive stars, the production and propagation of gravitational waves, as well
as the exploration of the early Universe by unveiling first stars and galaxies
(assessing also their evolution and cosmic re-ionization). GRBs have
stimulated in the past $\sim$50 years the development of cutting-edge
technological instruments for observations of high energy celestial sources
from space, leading to the launch and successful operations of many different
scientific missions (several of them still in data taking mode nowadays). In
this review, we provide a brief description of the GRB-dedicated missions from
space being designed and developed for the future. The list of these projects,
not meant to be exhaustive, shall serve as a reference to interested readers
to understand what is likely to come next to lead the further development of
GRB research and associated phenomenology.

###### keywords:

$\gamma$-ray astrophysics; Gamma-ray bursts; X-ray polarimetry; X-ray surveys;
X-ray instrumentation

1 1 0 https://doi.org/ Future Perspectives for Gamma-ray Burst Detection from
Space The future of GRBs from space Enrico Bozzo1,†*, Lorenzo Amati2, Wayne
Baumgartner3, Tzu-Ching Chang4, Bertrand Cordier5, Nicolas De Angelis6,7,
Akihiro Doi8, Marco Feroci7, Cynthia Froning9, Jessica Gaskin3, Adam
Goldstein10, Diego Götz5, Jon E. Grove11, Sylvain Guiriec12, Margarita
Hernanz13,14, C. Michelle Hui3, Peter Jenke15, Daniel Kocevski3, Merlin Kole6,
Chryssa Kouveliotou12, Thomas Maccarone16, Mark L. McConnell17,18, Hideo
Matsuhara8, Paul O’Brien19, Nicolas Produit1, Paul S. Ray11, Peter Roming9,
Andrea Santangelo20, Michael Seiffert4, Hui Sun21, Alexander van der Horst12,
Peter Veres15, Jianyan Wei21, Nicholas White12, Colleen Wilson-Hodge3, Daisuke
Yonetoku22,8, Weimin Yuan21,23, Shuang-Nan Zhang24,25. Most authors provided
inputs on behalf of a larger collaboration. Correspondence:
<EMAIL_ADDRESS>
###### Contents

  1. 1 Introduction
  2. 2 Einstein Probe
    1. 2.1 Mission Overview
    2. 2.2 Instrument Design
    3. 2.3 Expected Performance
  3. 3 eXTP
    1. 3.1 Mission Overview
    2. 3.2 Instrument Design
    3. 3.3 Expected Performance
  4. 4 Gamow
    1. 4.1 Mission Overview
    2. 4.2 Instrument Design
    3. 4.3 Expected Performance
  5. 5 HiZ-GUNDAM
    1. 5.1 Mission Overview
    2. 5.2 Instrument Design
    3. 5.3 Expected Performance
  6. 6 LEAP
    1. 6.1 Mission Overview
    2. 6.2 Detector Design
    3. 6.3 Expected Performance
  7. 7 MoonBEAM
    1. 7.1 Mission Overview
    2. 7.2 Detector Design
    3. 7.3 Expected Performance
  8. 8 POLAR-2
    1. 8.1 Mission Overview
    2. 8.2 Detector Design
    3. 8.3 Expected Performance
  9. 9 StarBurst
    1. 9.1 Mission Overview
    2. 9.2 Instrument Design
    3. 9.3 Expected Performance
  10. 10 STROBE-X
    1. 10.1 Mission Overview
    2. 10.2 Instrument Design
    3. 10.3 Expected Performance
  11. 11 SVOM
    1. 11.1 Mission Overview
    2. 11.2 Instrument Design
    3. 11.3 Expected Performance
  12. 12 THESEUS
    1. 12.1 Mission Overview
    2. 12.2 Instruments Design
    3. 12.3 Expected Performance
  13. 13 Conclusions

##  1 Introduction

About 57 years have already gone by since the first discovery of a Gamma-ray
burst (GRB) in 1967. This occurred genuinely by chance, while the _Vela_
military USA satellites were looking for evidence of nuclear bomb testing in
space following the 1963 partial test-ban treaty (which prohibited testing in
the atmosphere, outer space and underwater). The existence of GRBs was later
revealed to the scientific community, stimulating a fast growing excitement
for this fortunate discovery (Klebesadel et al., 1973). A number of
experiments were quickly developed and launched into space, mainly by the
Soviet Union and the United States, to boost the detection of similar events
and understand their true nature. The inevitable requirement of a space-based
instrumentation to catch the unpredictable short and bright flash of high
energy radiation from these events, largely contributed to boost the
international race in the development of cutting-edge technological detectors
with exponentially increasing sensitivity, as well as the provision of more
and more advanced platforms to ensure their most proficient operations (see,
e.g., Vigliano and Longo, 2024, for recent hystorical reviews on GRBs).

A major leap forward in the GRB research was first provided by the Konus-WIND
satellite (Aptekar et al., 1995; Tsvetkova et al., 2021) and the BATSE
experiment on-board CGRO (Harmon et al., 2002; Band et al., 1993), which
allowed the determination of the dichotomy into short and long GRBs,
established the isotropic distribution of the events in the sky and led to the
first characterization of their non-thermal emission (see, e.g., Kaneko et
al., 2006, and references therein). Subsequently, the Italian-Dutch Beppo-SAX
satellite, launched in 1996, unveiled the cosmological nature of these events
by identifying for the first time the GRB X-ray afterglows and providing
arcsec-accurate localizations of their host galaxies (enabling also multi-
wavelength observations with ground-based observatories; Frontera et al.,
2009). This was possible thanks to an on-board powerful suite of instruments
covering a large energy-band (2–700 keV) and providing simultaneously a high
sensitivity to faint sources via dedicated pointed observations and monitoring
of a large fraction of the sky at once via large field-of-view (FoV)
instruments (achieving up to 20$\times$20 deg with full imaging and good
localization capabilities of typically a few arcminutes; Boella, G. et al.,
1997).

The size of the GRB community and its efforts to probe and understand the
physics of these uniquely powerful cosmological explosions have been growing
ever since. GRBs are also widely recognized to have a key role in many of the
most debated aspects of research in Astronomy and Astrophysics. These range
from problems of fundamental physics, such as the equation of state of supra-
density matter, to stellar evolution, via the collapse of massive stars and
the production of both kilonovae and supernovae, to jet formation and
dissipation, to the generation and propagation of gravitational waves, up to
the quest for key Cosmological parameters (see, e.g. Demianski et al., 2017;
Abdalla et al., 2022, and references therein). In the most recent years, GRBs
have also gained renewed interest as key sources in the field of multi-
messenger Astrophysics due to their association with Kilonovae and
Gravitational Wave (GW) sources (see the well known cases of GRB 170817A, GRB
211211A and GRB 230307A; e.g., Abbott et al., 2017; Troja et al., 2022;
Dichiara et al., 2023, and references therein).

Following the decommissioning of _Beppo-SAX_ in 2002, a number of successful
missions have provided dramatic advancements in the fields related to GRBs.
Among these, AGILE (Tavani et al., 2009), Astro-SAT (Singh et al., 2014),
Fermi (Atwood et al., 2009), INTEGRAL (Winkler, 1994), and the Neil Gehrels
Swift Observatory (Gehrels et al., 2004), are still operational and work in
synergy also with ground-based multi-wavelength observatories to boost the
detection and characterization of bright impulsive cosmological events (beside
transients of different kinds). While an appraisal of the most relevant
achievements of these missions can be found in recent reviews, the aim of the
current paper is to provide interested readers an overview of the world-wide
efforts from different teams to develop the GRB instruments of the future.
These are expected to lead the scientific community toward the next leaps
forward in the understanding of GRBs and associated phenomena. Different
missions are presented in alphabetical order in the following sections. The
list is not meant to be exhaustive and, given the heavily dynamic and fast-
evolving nature of GRB research, it is likely (and desirable) that additional
relevant missions set to provide important contributions to GRB research are
being developed by other teams.

##  2 Einstein Probe

###  2.1 Mission Overview

The Einstein Probe111https://ep.bao.ac.cn (EP) is a mission designed to
monitor the sky in the soft X-ray band. It will perform systematic surveys and
characterization of high-energy transients and monitoring of variable objects
at unprecedented sensitivity and monitoring cadences. It has a large
instantaneous FoV (3600 sq. deg.) that is achieved by using established
technology of novel lobster-eye micro-pore optics (MPOs). EP also carries a
conventional X-ray focusing telescope with a larger effective area to perform
follow-up observations and precise localization of newly discovered
transients. Public transient alerts will be issued rapidly to trigger multi-
wavelength follow-up observations from the world-wide community.

The primary science objectives of EP mission are: (1) Discover and
characterize cosmic X-ray transients, particularly faint, distant and rare
X-ray transients, in large numbers. (2) Discover and characterize X-ray
outbursts from otherwise normally dormant black holes. (3) Search for X-ray
sources associated with gravitational-wave events and precisely locate them.
EP is an international mission led by the Chinese Academy of Sciences (CAS) in
collaboration with the European Space Agency (ESA), the Max Planck Institute
for extraterrestrial Physics (MPE) in Germany and the Centre National d’Etudes
Spatiales (CNES) in France. The mission was successfully launched on 2024
January 9 with a nominal lifetime of 3 years (5 years as a goal).

Figure 1: Preliminary configuration of the EP payloads alignment, with twelve
WXT modules surrounding two FXT telescopes.

###  2.2 Instrument Design

EP carries a Wide-field X-ray Telescope (WXT) with a large instantaneous FoV,
which adopts a novel lobster-eye MPO technology. Complementary to this wide-
field instrument is a Follow-up X-ray Telescope (FXT) with a large effective
area and a narrow FoV. Figure 1 shows the configuration of the EP payload.

  * •

Wide-field X-ray Telescope (WXT): There are 12 almost identical WXT modules,
each with a FoV of $\sim$ 300 sq. deg. Each WXT module includes a lobster eye
MPO mirror assembly with a focal length of 375 mm, a focal plane detector
array and an electronics unit. The mirror assembly of each module comprises 36
MPO optics mounted on a supporting alloy frame. An MPO optic is made of a thin
plate with millions of square micro-pores perpendicular to the surface,
slumped into a spherical shape. Incoming X-rays at a grazing-incidence angle
are reflected off the walls of the pores and are brought onto a focal sphere
with a radius of half the curvature of the optic. It produces true imaging
with a characteristic cruciform point-spread function. The detector array of
each module is composed of four black-illuminated CMOS sensors, each of 6 cm
by 6 cm in size and 4 k by 4 k in pixels. An aluminium layer of 200 nm
thickness is coated on the surface to block incident optical and UV light. The
nominal detection bandpass of WXT is 0.5–4 keV.

  * •

Follow-up X-ray Telescope (FXT): FXT is a set of two telescopes of the
Wolter-I optics, which have almost the same design and are co-aligned. The
design of the mirror assembly is similar to that of the eROSITA telescopes,
which has a focal length of 1.6 m. The focal plane detectors are built from
pn-CCDs, and a set of thin and thick filters of aluminium layers are mounted
on a filter wheel.

Figure 2: Representative grasp (effective area times FoV) of WXT as a function
of photon energy (black). As a comparison, the grasp parameters of several
X-ray focusing telescopes are overplotted. Adapted from Figure 10 in Yuan et
al. (2022).

###  2.3 Expected Performance

The angular resolution of WXT is about 5 arcmin (FWHM) for the central focal
spot and the effective area in the range of 2–3 $\rm cm^{2}$. The grasp
parameter of WXT is shown in Figure 2. In a 1000 s exposure, a sensitivity of
approximately (2–3)$\times 10^{-11}$ $\rm ergs\,s^{-1}\,cm^{-2}$ in the 0.5–4
keV band can be achieved at the 5-$\sigma$ level. Such sensitivity and spatial
resolution improve by one order of magnitude or more upon the previous and
current wide-field X-ray monitors.

Operating in the 0.3–10 keV energy range, FXT has a narrow field of view (60
arcmin in diameter) and an effective area is about 300 cm2 at 1 keV (for one
unit). The spatial resolution (PSF) is about 23 arcsec in half-power diameter
(HPD), which gives a source localization precision of 5–15 arcsec (90 % c.l.)
depending on the source intensity. The FXT is responsible for the quick
follow-up observations (within 5 minutes) of the triggered sources from WXT,
and will also observe other interested targets as target of opportunity (ToO)
observations.

Once a transient is detected with WXT, the spacecraft will slew to point FXT
to the target for quick follow-up observations. Meanwhile the alert
information of the transient will be down-linked quickly to the ground segment
and made public to trigger follow-up observations. Quick command up-link for
time-critical ToO observations is also possible.

With the unprecedented capability of WXT and FXT, EP is expected to
characterize the cosmic high-energy transients over wide time-scales and at
high cadences, revealing new insights into a diverse set of systems including
dormant black holes, neutron stars, supernova shock breakouts, active galactic
nuclei, X-ray binaries, gamma-ray bursts, stellar coronal activity, and
electromagnetic-wave sources and gravitational-wave events. In particular, EP
will provide valuable data for the prompt emission of GRBs in the soft X-ray
band, which has not been common in previous GRB detections. Meanwhile, EP will
also monitor the variability of several types of X-ray sources in large
samples all over the sky.

##  3 eXTP

###  3.1 Mission Overview

The enhanced X-ray Timing and Polarimetry
mission222https://www.isdc.unige.ch/extp/ (eXTP) is designed to study mainly
the state of matter under extreme conditions of density, gravity, and magnetic
field Santangelo et al. (2023).The core science objectives of the mission are
focused on the determination of the equation of state of matter at supra-
nuclear density and the study of the dynamics of accretion/ejection flows
under the influence of strong gravitational fields. Thanks to an extensive
suite of innovative instruments, eXTP is designed also to be a general
observatory for astrophysics, providing broad capabilities in the X-ray domain
(0.5–50 keV) to conduct timing, spectral, and polarimetric observations of a
wide variety of Galactic and extra-Galactic sources. Although the mission is
not focusing on GRB science, the availability of a large FoV and sensitive
instrument, the WFM (see below), makes eXTP an important contributor to the
possible detection and study of these events in the future.

eXTP is planned to be the next flagship-class mission led by China and it is
currently in an advanced design phase, where virtually all payload elements
have reached a relatively mature technology and could be ready for
implementation (see Figure 3). The mission is being studied by a large
collaboration including a wide scientific community in China and many European
member states. At present, a launch of opportunity has still to be identified,
and programmatics are being cleared in order to possibly bring eXTP into space
in 2028-2029.

Figure 3: An artist impression of the eXTP satellite. In the image we show the
13 nested shells optics of the SFA (9 telescopes) and PFA (4 telescopes), as
well as the 6 cameras with coded masks composing the WFM. The LAD is placed on
deployable panels equipped with solar shades. There are a total of 40 modules
composing the LAD and each module hosts 16 SDDs.

###  3.2 Instrument Design

The eXTP scientific payload consists of four main instruments: the
spectroscopic focusing array (SFA), the large area detector (LAD), the
polarimetry focusing array (PFA), and the wide field monitor (WFM). The SFA
comprises 9 identical Wolter-I grazing- incidence X-ray telescopes, and is
mainly used for spectral and timing observations of celestial sources in the
energy range 0.5–10 keV. In its current design, the SFA total effective area
is of 7400 cm2 (at 2 keV) and the instrument FoV is circular with a diameter
of 12 arcmin. The instrument achieves an energy resolution of 180 eV at 6 keV
and a timing resolution of 10 $\mu$s in the whole operational energy band. The
LAD is designed to perform photon-by-photon observations of X-ray sources in
the 2–30 keV energy range. The LAD is exploiting the technology of innovative
large-area Silicon Drift Detectors (SDDs; Rachevski et al., 2014) to provide
an unprecedented large effective area, reaching 3.4 m2 at 8 keV. The
instrument is not designed for imaging purposes, and the FoV is limited to 1
deg using micro-channel collimator plates (Mineo et al., 2014) to
simultaneously reduce source confusion and background. The LAD achieves an
energy resolution of 260 eV at 6 keV and a timing resolution of 10 $\mu$s in
the whole operating energy range. A similar instrument is also planned on-
board the STROBE-X mission (see Sect. 10). The WFM exploits the same SDD
technology coupled with coded masks to provide a coverage of about 5.5 sr of
the sky at any time in the energy range 2–50 keV. The instrument is capable of
localizing X-ray sources within an accuracy of $<$1 arcmin and perform timing
(spectral) investigations on these objects with a time (energy) resolution of
10 $\mu$s ($<$300 eV at 6 keV). The WFM is equipped with an automatic alert
system to promptly broadcast to the ground (within few tens of seconds at the
most) the onset time and position of bright impulsive events detected on-
board, including GRBs. A similar instrument is also planned on-board the
STROBE-X mission (see Sect. 10). The PFA provides polarimetric capabilities in
the energy range 2–8 keV, exploiting X-ray optics optimized for polarimetric
observations and a similar detector technology compared to that currently
flying on the IXPE mission (the so-called gas pixel detectors, GPDs,
detectors; see, e.g., Soffitta et al., 2023, and references therein).

Figure 4: The large FoV of the eXTP/WFM (red line) overlapped on a background
map of the high energy sky that has been provided as a courtesy from the MAXI
team (T. Mihara, RIKEN, JAXA). The red line corresponds to the total FoV
covered by the WFM in the configuration used for eXTP as illustrated by in’t
Zand et al. (2019).

###  3.3 Expected Performance

Given the capabilities of the eXTP/WFM (see Figure 4), a detection rate of
about 100 GRB/year has been estimated. The WFM will be able to measure with
good accuracy the spectral shape of the detected events and follow the hard to
soft evolution of the prompt emission in the 2–50 keV band, where different
GRB models are known to make different predictions (Zhang and Mészáros, 2002;
Ghirlanda et al., 2007). This could provide information on the composition and
magnetization of the emitting plasma, the geometry of the emission, and the
structure of the jet and surrounding material. The combination of sensitivity
and soft energy coverage of the WFM provides the opportunity to detect a few
high redshift (z$>6$) GRBs per year and many of the so-far elusive absorption
features in tens of medium bright GRBs, probing (among others) the
surroundings of GRB progenitors (see, e.g., Amati et al., 2000; Gehrels et
al., 2009). The WFM is also expected to detect each year up to 40 X-ray
flashes (see, e.g., Pélangeon et al., 2008).

##  4 Gamow

###  4.1 Mission Overview

The Gamow Explorer mission (White et al., 2021) is optimized to: 1) probe the
high redshift universe (z$>$6) when the first stars were born, galaxies formed
and Hydrogen was re-ionized; and 2) enable multi-messenger astrophysics (MMA)
by rapidly identifying electro-magnetic (EM) counterparts to gravitational
wave (GW) events. GRBs have been detected out to z$\sim$9 (see, e.g.,
Salvaterra, 2015; Fryer et al., 2022, and references therein) and their
afterglows are bright beacons lasting a few days that can be used to observe
the spectral fingerprints of the host galaxy and intergalactic medium. Gamow
is designed to detect and rapidly identify high-redshift events. Rapid follow-
up spectroscopy with the James Webb Space Telescope (JWST) and $>$6 m ground-
based telescopes provide NIR R$\sim 2500$ spectra to determine the IGM neutral
fraction versus redshift using the damping wing of the Lyman-$\alpha$
absorption line (see Figure 5 from Lidz et al., 2021).

Figure 5: Recent measurements of the neutral fraction $<$XHI$>$ versus
redshift, z, by various techniques, including GRBs (green; see Ota et al.,
2017). The forecast Gamow results are shown as red (Lidz et al., 2021). The
Fisher matrix forecast assumes 26 GRBs at z$>$5\. We assume follow-up spectra
with a signal-to-noise ratio (SNR) of 20 at the continuum per R=3000
resolution element and assume accurate host redshift determinations via metal
absorption lines. The results illustrate that Gamow can map the re-ionization
history in detail from z$\sim$6–14. Predicted re-ionization curves (blue and
dashed red lines) illustrate the degree of variance between theoretical models
compared to the Fisher forecasting.

GRB afterglows are particularly advantageous for this measurement versus,
e.g., using QSOs. GRBs have a featureless power-law spectrum, ideal for
fitting the Ly-$\alpha$ absorption profile, are hosted by low mass galaxies
directly tracing the typical ionization state of the IGM, and can be seen out
to redshifts of 20, whereas the abundance of QSOs drops steeply with redshift.
The same spectra will be used to determine metallicities from absorption lines
to trace the early chemical enrichment and measure the escape fraction of
ionizing photons that escape the galaxies to ionize atoms in the IGM. The co-
moving GRB rate generally follows the star-formation rate (SFR). At higher
redshifts, the GRB rate exceeds the SFR derived by other means. The greatly
improved measurements of high-redshift GRB rate by Gamow, will provide crucial
information to probe potential changes of the initial mass function of massive
stars and star-formation rate at high redshift, and constrain GRB progenitors
and their properties (e.g., luminosity distributions, progenitor binary
fractions, etc.) in the high redshift Universe (Fryer et al., 2022).

###  4.2 Instrument Design

The Lobster Eye X-ray Telescope (LEXT) with a wide FoV of $\sim$1350 sq deg
detects GRBs and locates them with arc-minute precision. The LEXT utilizes an
array of slumped micro pore optics (MPOs) with 40 $\mu$m square pores and a
focal length of 30 cm (Feldman et al., 2021). The focal plane uses heritage
MIT-Lincoln Labs CCDs. The required performance is to detect at least 20 z$>$6
long GRBs over a 2.5 year prime mission. A rapidly slewing spacecraft
autonomously points the Photo-z Infra-Red Telescope (PIRT) within 100 s to
identify high redshift (z$>$6) GRBs. The photo-z technique takes advantage of
the Hydrogen Lyman-$\sigma$ absorption which creates a sharp blue-ward drop
out. A 30 cm aluminum RC telescope passively cooled to 200 K feeds a dichroic
prism beam splitter to place five images onto a single H2RG detector covering
the 0.6 to 2.5 $\mu$m band (Seiffert et al., 2021). The FoV is 10 arcmin
square. This design provides the required 15 $\mu$Jy (21 mag AB) 5-$\sigma$
sensitivity in a single 500 s exposure (see also Fausey et al., 2023). The
high redshift science objectives require rapid follow-up observations to
provide NIR medium resolution spectroscopy (R$\sim$3000) to measure the
profile of the Ly-$\sigma$ absorption line and metal absorption lines from the
host galaxy. To obtain sufficient signal to noise in a reasonable observing
time requires 6 m class or greater telescopes. Ground-based telescopes begin
observations within an hour and JWST and the ELT within 2–3 days (Kann et al.
2024, to be submitted). An L2 orbit provides $>$95 % observing efficiency with
pointing optimized for these follow-up observatories. A low bandwidth
continuous S-band connection provides real time alerts to the ground.

Figure 6: The ability of Gamow LEXT to detect the afterglows of on-axis BNS
and NSBH mergers. X-ray luminosity (left axis) and X-ray flux at 600 Mpc
(right axis) for Swift short GRBs at known redshift. The black line is the
LEXT sensitivity starting at 0 s (base-line). In practice, it will frequently
be earlier than this (short-slew), and sometimes before the BNS merger (pre-
alert; figure from White et al. (2021) and see Chan et al. (2018); Nitz and
Canton (2021); Banerjee et al. (2023)).

###  4.3 Expected Performance

The Gamow capabilities are also optimized for the identification of EM
counterparts to binary neutron star (BNS) and neutron star black hole (NSBH)
mergers detected by the A+ generation of GW detectors. Within $<$200 s of a GW
alert the real time low bandwidth link uploads commands to re-point the LEXT
at the GW uncertainty region so as to detect and locate to 3 arcmin precision
the accompanying short GRB afterglow. Figure 2 from White et al. (2021) shows
measurement predictions are based on extrapolation of Swift short GRB
afterglow detections to the LEXT pass-band and 600 Mpc source distances (the
horizon of the LVK A+ GW detectors). Approximately 80 % of Swift short GRBs
have X-ray afterglow detections. These demonstrate that they will be
detectable by Gamow at BNS-appropriate distances. If a transient X-ray source
is detected, an autonomous PIRT pointing will refine the position to 1 arcsec
precision and use simultaneous five-band photometry to follow the early phase
of the merger. The X-ray and Optical-NIR multi-band and multi-wavelength
capabilities combined with an agile spacecraft will enable a broad swath of
time domain astronomy science, very similar to Swift (Gehrels and Cannizzo,
2015). While waiting for GRBs and GW events we envision Gamow will undertake a
community driven comprehensive Time Domain Astronomy program. The Gamow
Explorer was proposed to the 2021 NASA MIDEX call. While it was not
successful, there are considerations to re-propose to future NASA
opportunities.

Figure 7: a) A schematic view of the HiZ-GUNDAM WFXM (top; copyright MEISEI
ELECTRIC Co., LTD.). b) Schematic views of the HiZ-GUNDAM NIRT (middle and
bottom).

##  5 HiZ-GUNDAM

###  5.1 Mission Overview

The High-z gamma-ray bursts for unraveling the dark ages mission, HiZ-
GUNDAM333https://www.isas.jaxa.jp/en/missions/spacecraft/future/hiz-
gundam.html, is a future satellite mission, a competitive medium-class mission
in ISAS/JAXA, designed to advance time-domain astronomy through the
observation of high-energy transient phenomena Yonetoku et al. (2020a, b). Two
scientific goals are defined: (1) the exploration of the early universe with
high-redshift gamma-ray bursts and (2) contribution to multi-messenger
astronomy. These scientific objectives require observational capabilities to
detect high-energy transients and to carry out automatic/rapid follow-ups in
the near-infrared band. This Section provides the description of the current
specifications and designs of the satellite and mission payloads, but it is
important to note that they are subject to change in subsequent studies and
developments.

###  5.2 Instrument Design

HiZ-GUNDAM has two types of mission payloads: the wide-field X-ray monitor
(WFXM) and the near-infrared telescope (NIRT). These payloads are optimized
and minimized to achieve the scientific goals Li et al. (2020); Ogino et al.
(2020); Sawano et al. (2020); Goto et al. (2022); Tsumura et al. (2020). We
show a schematic view in Figure 7, and basic specification of WFXM and NIRT in
Table 1 and 2, respectively.

The wide-field X-ray monitors (WFXM) consist of Lobster Eye optics arrays and
focal imaging sensors (pnCCD). They are designed to detect high-energy
transients within a wide field of view of $>0.5~{}\mathrm{steradian}$ in the
0.5–4 keV energy range. The current design of a single module of WFXM includes
a lobster-eye optic array of $3\times 3$ with a 300 mm focal length and a
pnCCD with a pixel size of approximately 100 $\mu$m on a $55\times 55$ mm2
format at the focus. Multiple modules are installed aboard the spacecraft to
monitor the wide FoV.

The near-infrared telescope (NIRT) has an aperture size of 30 cm in diameter
and simultaneously observes at five wavelength bands between 0.5–2.5 $\mu$m
using a dichroic mirror and Kösters. The NIR telescope is cooled to $<200$ K
by radiative cooling to maintain the best sensitivity up to the 2.5 $\mu$m
band. The mirrors and structure of NIRT are primarily made of aluminum alloy
to ensure imaging performance remains consistent even if the temperature of
the telescope changes in orbit, i.e., an athermal configuration.

Table 1: HiZ-GUNDAM Wide Field X-ray Monitor. Item | Specification
---|---
Optics | Lobster Eye
Focal Length | 300 mm
Focal Detector | pnCCD
Size of Focal Detector |  $55\times 55$ mm2
Energy Band | 0.5–4.0 keV
Field of View |  $>0.5$ str (in total)
Sensitivity (100 s) |  $\sim 10^{-10}$ erg cm-2 s-1
Localization Accuracy |  $\sim 3$ arcmin
Time Resolution |  $\sim 0.1$ s
Table 2: HiZ-GUNDAM Near Infrared Telescope. Item | Specification
---|---
Telescope Type | Offset/Athermal
Aperture Size | 30 cm
Telescope Temp. |  $<200$ K
Focal Detector | HyViSi (Optical)
| HgCdTe (NIR)
Field of View |  $15\times 15$ arcmin2
Band | Sensitivity (2 min$\times$5)
0.5–0.9 $\mu$ m | 21.4 mag(AB)
0.9–1.3 $\mu$ m | 21.3 mag(AB)
1.3–1.7 $\mu$ m | 21.4 mag(AB)
1.7–2.1 $\mu$ m | 20.8 mag(AB)
2.1–2.5 $\mu$ m | 20.7 mag(AB)

###  5.3 Expected Performance

In Figure 8, we show schematic views of HiZ-GUNDAM satellite. The size of
satellite bus is about 1 m3 except for the solar paddles and mission payloads.
For orbital placement, we have chosen a sun-synchronous polar orbit along the
twilight line. This choice is based on the favorable thermal conditions for
the NIRT, which is cooled down to $<200$ K. However, X-ray observation may
face disturbances from both South Atlantic Anomaly and aurora belt at high
latitude polar region. The nominal operation of HiZ-GUNDAM follows the
sequence outlined below:

  1. 1.

Set the satellite attitude to a solar separation angle of 120 degrees and a
forward movement of 50 degrees.

  2. 2.

Maintain the inertial pointing direction for approximately 560 s, during which
HiZ-GUNDAM monitors X-ray transients.

  3. 3.

After the monitoring time, the satellite slews to the same attitude
configuration as described in (1), but with a different pointing direction to
prevent thermal radiation from the Earth exposing the NIRT.

This nominal sequence is repeated, and the observation of eight different
fields of view are performed in every orbit. The satellite continuously
monitors the X-ray sky, except during the maneuver. This optimized sequence
facilitates follow-up observations with the NIR telescope. HiZ-GUNDAM can
execute this operation for 97 % of the GRBs discovered by itself, ensuring
more than 10 minutes of follow-up time in each case.

Figure 8: Schematic view of the HiZ-GUNDAM satellite.

##  6 LEAP

Figure 9: The LEAP payload includes seven independent polarimeter modules.
When mounted on the outside of the ISS, the array of modules points toward the
local zenith and scans the sky for GRBs.

###  6.1 Mission Overview

The LargE Area burst Polarimeter444https://sti.usra.edu/in-the-news/leap-
mission-study/ (LEAP) is a mission that was proposed in 2021 to NASA’s
Astrophysics Mission of Opportunity Program. The LEAP instrument is a wide FoV
Compton polarimeter that measures GRB polarization over the energy range from
50 keV to 1 MeV and performs GRB spectroscopy from 20 keV to 5 MeV. If
approved (a final decision on the selection is expected during the first
quarter of 2024), it will be deployed as an external payload on the
International Space Station (ISS) in 2027 for a three year mission McConnell
et al. (2021); Oñate-Melecio et al. (2021). The LEAP science investigation is
based on the ability to distinguish between three classes of GRB models Toma
et al. (2009). The baseline science investigation requires the observation of
65 GRBs with a minimum detectable polarization (MDP) of 30 % or better.
Evidence of polarized $\gamma$-ray emission in GRBs ($>$ 100 keV) has been
accumulated in recent years, but the limited sensitivity of these measurements
does not yet yield a clear picture of the underlying physics McConnell (2017);
Tatischeff et al. (2019); Chattopadhyay (2021). A sensitive and systematic
study of GRB polarization, such as the one that will be provided by LEAP, is
needed to remedy this situation.

###  6.2 Detector Design

The LEAP payload (Figure 9) consists of an array of seven independent
polarimeter modules, each with a 12$\times$12 array of optically isolated
high-Z and low-Z scintillation detectors read out by individual PMTs. Each
polarimeter module includes 84 plastic scintillator elements, 58 CsI(Tl)
scintillator elements, and two 60Co calibration sources. Each calibration
source consists of a small plastic scintillator doped with 60Co, which permits
electronic tagging of each decay via the coincident $\beta$ particle. As a
Compton polarimeter, scatter events recorded by the scintillator array within
each module are used to measure polarization. Within each module, the
arrangement of plastic and CsI(Tl) scintillation detectors is designed to
optimize the polarization response. The dominant type of scatter event is one
involving only two detector elements, in which incident photons scatter from a
low-Z plastic detector element into a high-Z CsI(Tl) element. The distribution
of azimuthal scatter angles for these events provides a polarization
signature. The total effective area for polarimetry is $\sim 1000$ cm2 at
energies above 100 keV. Since the total energy deposit is a sum of the energy
deposits in all triggered elements, spectroscopic information is provided by
all types of events of any multiplicity (both singles event and scatter
events). To characterize the GRB parameters, spectroscopic measurements
(20-5000 keV) are obtained using all event types (both multiple and singles
events), with a total effective area that reaches $>3000$ cm2 between 50 and
500 keV.

To accurately reconstruct the source spectrum and polarization, LEAP self-
sufficiently determines the source direction using singles events from all 420
CsI(Tl) elements, whose relative response provides the source localization.
Localization errors of $<5^{\circ}$ (1$\sigma$) are obtained at a rate of
about 40 per year. This is sufficiently precise to enable rapid followup by
many ground-based instruments using the rapid burst response messages that
will be generated and distributed (in real time) by LEAP.

###  6.3 Expected Performance

As a wide FoV instrument, LEAP maintains some level of polarization
sensitivity out to at least 75∘ off-axis, providing an effective FoV of
$\sim$1.5$\pi$ sr. Minimal obstructions from ISS within the FOV maximize sky
exposure and minimize photon absorption and scattering effects. However,
scattered flux from ISS structures (such as solar panels) must always be
considered in the analysis of both the spectrum and the polarization. Response
simulations spanning a range in energy, spectral shape, and incidence
direction have been used in estimates of instrument performance. Figure 10
shows, for a three year mission, the number of expected GRBs as a function of
MDP. These estimates show that LEAP will attain its requirement of 65 GRBs
with $<30$ % MDP, with significant margin.

Figure 10: The number of GRBs measured with a polarization degree $>30\%$
versus the number of GRBs with an MDP $<30\%$ provides a convenient way to
distinguish between different model classes. For the case shown here, 65 GRB
measurements are required to distinguish the three model classes. LEAP will be
able to measure a total of 135 GRBs with MDP $<30\%$ (and 23 GRBs with MDP
$<10\%$) during its baseline 32 months of science operations. Figure 11: Six
scintillating detectors positioned for an instantaneous all-sky field of view,
no slewing required. Coupled with a cislunar orbit, MoonBEAM provides an
unprecedented all-sky sensitivity that cannot be achieved in Low Earth Orbit.
Figure 12: MoonBEAM’s 1-second limiting flux sensitivity across the entire sky
in any instant, providing the continuous sensitivity needed to study
astrophysical jet formation and emissions.

##  7 MoonBEAM

###  7.1 Mission Overview

Moon Burst Energetics All-sky Monitor (MoonBEAM) is a proposed gamma-ray
mission to observe the entire sky instantaneously for relativistic
astrophysical explosions from a cislunar orbit. It is designed to explore the
behavior of matter and energy under extreme conditions by observing the prompt
emission from GRBs, identifying the conditions capable of launching transient
relativistic jets and the origins of high-energy radiation from the
relativistic outflows. MoonBEAM provides the essential continuous all-sky
gamma-ray observations for time-domain and multi-messenger astrophysics by
reporting on any prompt emission of a GRB and by providing the critical first
alerts to the community for contemporaneous and follow-up observations.

###  7.2 Detector Design

MoonBEAM achieves instantaneous, all-sky coverage by positioning six gamma-ray
detector assemblies at the corners of the spacecraft to minimize blockage (see
Figure 11), and by deploying the observatory in an Earth-Moon Lagrange Point 3
cislunar orbit instead of Low Earth Orbit (LEO) to reduce particle background
from radiation belts, atmospheric interactions, and planetary occultation from
30 % to $<<1$ % of the sky.

Each detector assembly consists of a NaI(Tl)/CsI(Na) phoswich scintillator
coupled to flat panel photomultiplier tubes. The phoswich design allows for
both localization improvement and increased effective area for spectroscopy.
It is sensitive to 10–5000 keV photons, with an energy resolution better than
12 % at 662 keV.

###  7.3 Expected Performance

MoonBEAM is expected to detect more than 1000 GRBs over 30 months of
operation, with an intrinsic localization capability similar to that of the
Fermi Gamma-ray Burst Monitor. In the absence of a detection, MoonBEAM
provides unprecedented sensitive gamma-ray upper limits for any externally
detected transients such as mergers seen in gravitational waves and supernovae
detected in optical wavelengths, estimated to be in the order of thousands
over the operation time of MoonBEAM. Figure 12 shows the 1-second limiting
flux sensitivity of MoonBEAM across the entire sky.

The wide-field, sensitive, and continuous gamma-ray coverage is necessary to
advance our current understanding of astrophysical jet formation, structure,
and evolution. MoonBEAM achieves sensitivity improvement over current missions
in LEO because of the combined advantage of its cislunar orbit and instrument
design. It will join the Interplanetary Gamma-Ray Burst Timing Network Hurley
et al. (2013) as one of the few missions outside of LEO with gamma-ray
sensitivity and the only one outside of LEO with the capability of on-board
transient localization.

##  8 POLAR-2

###  8.1 Mission Overview

Figure 13: An exploded view of one of the 100 POLAR-2 detector modules which
make up the instrument. The detector measures the polarization of the incoming
photons when these undergo Compton scattering in the segmented scintillator
array. The scintillators are read out by a temperature-controlled Silicon
photomultipliers (SiPM) array connected to its own front-end electronics.
Taken with permission from De Angelis (2023).

POLAR-2555https://www.unige.ch/dpnc/polar-2 is a dedicated GRB polarimeter
foreseen to be launched in 2027 towards the China Space Station (CSS). The
detector is being developed by an international collaboration consisting of
teams from Switzerland, Poland, Germany, and China. The project is currently
ready for the production of the flight model after a prototype was
successfully tested for physics performance and space qualification in 2023.
POLAR-2 is a successor of the POLAR mission which detected 55 GRBs between
October 2016 and April 2017 and performed polarization measurements in the
energy range of 50–500 keV of 14 of these Kole et al. (2020) as well as of the
Crab pulsar Li (2022).

Although the measurement results of POLAR are the most constraining GRB
polarization measurements to date, they are only able to constrain the
polarization degree to be below $\approx 40$ %. Therefore the results remain
consistent with the majority of the existing theoretical predictions Gill et
al. (2021). This, along with the hint of an evolution of the polarization with
time observed in 2 of the GRBs Kole et al. (2020), indicates the need for a
significantly more sensitive detector. For this purpose the POLAR-2 detector
was initiated in 2017, followed by an approval for launch towards the CSS,
through a United Nations Office for Outer Space Affairs call in 2019.

###  8.2 Detector Design

Compared to its predecessor, the POLAR-2 detector will be a factor 4 larger in
size, thereby employing a total of 6400 plastic scintillator bars. These
scintillators are read out in groups of 64, using segmented SiPM arrays
connected to their own front-end electronics as indicated in Figure 13. As the
GRB photons enter the detector they can undergo Compton scattering in the
detector array, followed by photo-absorption in a second scintillator. Their
azimuthal scattering angle can be constrained using the relative position of
the two scintillators in which the photon interacted. As the photons will
scatter preferentially perpendicular to their initial polarization, this
measurement allows to determine the polarization of the incoming photon flux.

While the increase in detector area provides an increase in effective area of
a factor 4 compared to POLAR, further improvements to its design allow for
additional improvements in its sensitivity. Particularly the switch from PMTs
as used in POLAR, to SiPMs results in a significant increase in sensitivity at
lower energies.

Figure 14: The MDP (99 %) for 1 s GRBs as a function of GRB fluence for the
POLAR and POLAR-2 instruments. In addition, the sensitivity of GAP, the first
dedicated gamma-ray polarimeter, is indicated. For reference, the fluence of
the very weak GRB 170817A is indicated with a dotted line.

###  8.3 Expected Performance

Thanks to all the design improvements POLAR-2 is approximately an order of
magnitude more sensitive compared to POLAR as indicated in Figure 14. As a
result POLAR-2 will be able to perform constraining polarization measurements
for GRBs with fluences as low as $10^{-6}$ erg cm-2, while measurements able
to constrain the polarization degree below 10% will be possible for about
10–15 GRBs per year.

The POLAR-2 detector will also contribute to transient alerts thanks to its
large effective area which exceeds $2000\,\mathrm{cm^{2}}$. This large
effective area, combined with continuous observations of half the sky and
almost continuous communication to ground will allow POLAR-2 to send alerts
within one minute from the onset of about 1 GRB every 2 days. As the
instrument furthermore has access to a GPU onboard of the CSS, studies are
currently ongoing on how to optimize GRB spectral and location information
within such alerts Kole et al. (2023).

##  9 StarBurst

###  9.1 Mission Overview

The StarBurst Multimessenger Pioneer is a highly sensitive and wide field
gamma-ray monitor designed to detect the prompt emission of gamma-ray bursts.
StarBurst is designed as a SmallSat to be deployed to LEO as a secondary
payload using the Evolved Expendable Launch Vehicle Secondary Payload Adapter
(ESPA) Grande interface for a nominal 1-year mission starting in 2027 to
coincide with LIGO’s scheduled 5th observing run. StarBurst will utilize
NASA’s Tracking and Data Relay
Satellites666https://www.nasa.gov/mission/tracking-and-data-relay-satellites/
(TDRS) system to report possible electromagnetic counterparts to gravitational
wave mergers with low-latency via the GCN system. StarBurst is among the first
of NASA’s new Pioneer class missions, intended to do compelling astrophysics
science at a lower cost than missions in the Explorers Program. The Pioneers
program provides opportunities for early-to-mid-career researchers to propose
innovative experiments and lead space or suborbital science investigations for
the first time.

Figure 15: a) The StarBurst instrument (left). b) The integrated StarBurst
observatory shown mounted to an ESPA-Grande ring (right).

###  9.2 Instrument Design

StarBurst relies heavily on the heritage of the GBM, Glowbug, and BurstCube
instruments, consisting of an array of 12 NaI(TI) scintillator detectors that
utilize new, low mass and low voltage SiPMs to cover an energy range from 30
keV to 2 MeV. The StarBurst detectors are arranged to form a half cube, with
two detectors mounted side-by-side to form four sides of the cube and four
detectors comprising the top, as shown in Figure 15. This configuration
maximizes the available active surface area of the instrument given the volume
constraints imposed on ESPA-Grande Rideshare Payloads and provides coverage of
the entire unocculted sky. Each of the individual StarBurst detectors consists
of a 24 cm $\times$ 24 cm $\times$ 1.6 cm NaI(TI) scintillator read out by an
array of SiPMs and enclosed in a housing with a thin (1-mm) aluminum window
transparent down to 30 keV and a beryllium-copper back shield to provide
strong attenuation below $\sim 200$ keV. The scintillation light is read out
by a 2 $\times$ 38 linear array of 6 mm $\times$ 6 mm J-Series SiPMs from ON
Semiconductor (formerly SensL) optically coupled through an elastomeric
silicone optical pad to a single edge of the NaI(Tl) crystal. Edge readout of
the NaI(TI) exploits the planar geometry of the crystal to pipe scintillation
light to the SiPM array. Each of the detectors has a dedicated bias voltage
supply and Multi-Channel Analyzer (MCA) for independent operation. The MCA is
a Commercial-off-the-Shelf (COTS) Bridgeport Instruments slimMorpho with an
onboard 20 MHz oscillator and flash ADC.

###  9.3 Expected Performance

The scientific performance of the StarBurst instrument was assessed through
Monte Carlo simulations using Geometry and Tracking 4 (GEANT4, Agostinelli et
al. (2003)). The peak StarBurst effective area between 50 and 300 keV,
averaged over azimuth, is roughly 3000 cm2, or roughly 500 $\%$ that of GBM
(Figure 16. This provides an estimated detection efficiency of $>90\,\%$ for a
64-ms peak flux (photons cm-2 s-1) of 0.9 for SGRBs ($<2$ s) and a 1024-ms
peak flux of 0.25 for long GRBs ($>2$ s). Because StarBurst will have a
similar field of view and duty cycle as GBM, it is estimated that StarBurst
will detect 158 SGRBs per year, compared to the 40 and 8.6 SGRBs detected by
GBM and Swift, respectively. StarBurst is estimated to achieve a localization
uncertainty within 8∘ (1$\sigma$) for an SGRB with a 1 photon cm-2 s-1 64-ms
peak flux. For SGRBs bursts with a 64-ms peak flux comparable to GRB 170817,
or about 3 photons cm-2 s-1, StarBurst is expected to achieve a localization
uncertainty of $<3^{\circ}$ (1 $\sigma$). Employing the technique developed in
Howell et al. (2019), along with the StarBurst detection efficiency and the
projected A+ (LHVKI) BNS detection efficiency provides an estimate joint GW-
SGRB detection rate of roughly 7 joint GW-SGRBs detections per year.

Figure 16: a) The StarBurst effective area averaged over azimuth, as a
function of angle from the instrument boresight (left). a) The StarBurst
effective area as a function of energy (right).

##  10 STROBE-X

###  10.1 Mission Overview

The Spectroscopic Time-Resolving Observatory for Broadband Energy
X-rays777https://strobe-x.org/ (STROBE-X) is a mission proposed to the NASA
call for an X-ray probe-class ($1B PI-managed cost cap) mission responding to
the recommendations of the 2020 Astrophysics Decadal Survey (Astro2020). It
has a broad range of science goals with a focus on time-domain and transient
events in the era of multi-wavelength and multi-messenger astronomy. It
combines huge collecting area, high throughput on bright sources, broad energy
coverage, and excellent spectral and temporal resolution in a single facility.
With its wide field of view, agile spacecraft, and low-latency communications,
it would be a critical component of the NASA Time-Domain and Multi-Messenger
(TDAMM) program.

###  10.2 Instrument Design

Figure 17: Rendering of the STROBE-X spacecraft, showing the 3 science
instruments. LEMA and HEMA are co-aligned narrow FoV instruments, while WFM
has an instantaneous FoV of 1/3 of the sky.

STROBE-X carries three instruments. The Low-Energy Modular Array (LEMA) covers
the soft or low-energy band (0.2–12 keV) with an array of lightweight optics
(3 m focal length) that concentrate incident photons onto small solid-state
detectors with CCD-level (85–175 eV) energy resolution, 100 ns time
resolution, and low background rates. This technology has been fully developed
for NICER and will be scaled up to take advantage of the longer focal length
of LEMA, which provides a factor of 8.5 improvement in effective area over
NICER with over 1.6 m2. The High-Energy Modular Array (HEMA) covers the harder
or higher-energy band (2–30 keV or beyond), with modules of Si drift detectors
and micropore collimators originally developed for the European LOFT and eXTP
mission concepts. HEMA provides a factor of 5.5 improvement in effective area
(3.4 m2) and $\sim 3$ in spectral resolution (200–300 eV) over RXTE/PCA. The
Wide-Field Monitor (WFM) comprises a set of coded-aperture cameras operating
in the 2–50 keV band that have a combined instantaneous field of view of 1/3
of the sky, with arcmin localization capability. It will act as a trigger for
pointed observations of X-ray transients and will also provide high duty-
cycle, high time-resolution, and high spectral-resolution monitoring of the
dynamic X-ray sky. WFM will have 15 times the sensitivity of the RXTE All-Sky
Monitor, enabling multi-wavelength and multi-messenger investigations with a
large instantaneous field of view, down to a new, order-of-magnitude lower
flux regime. Onboard processing will detect bursts in real time and provide
notifications to the ground as well as triggering autonomous slews to get the
pointed instrument on source in a few minutes.

###  10.3 Expected Performance

Figure 18: The expected cumulative onboard detection rate of canonical gamma-
ray bursts (GRBs) and X-ray flashes (XRFs) by the STROBE-X/WFM as estimated by
folding observed GRB and XRF spectra through the WFM responses and accounting
for the effective field-of-view of the WFM. It will detect $\sim 100$ long
duration GRBs, $\sim 7$ short duration GRBs, and $\sim 12$ XRFs onboard per
year. The onboard detection rate of long GRBs exceeds that of the Swift/BAT,
while the short GRB detection rate is comparable. A unique capability is the
downlink of event data to the ground for the WFM, enabling sub-threshold
searches to double the number of short GRB detections (purple shading). The
detection rate of XRFs exceeds that of previous instruments and is a
particular science focus for the WFM. The blue shading shows the region of
signal-to-noise where high-fidelity spectroscopy can be performed in the
prompt X-ray for these sources.

STROBE-X will make major advances in several areas of GRB and multimessenger
astronomy. The WFM will detect and localize $>5$ short GRBs per year and
distribute the position accurate to 2 arcmin, brightness, and timing to the
ground in $<5$ minutes. In some cases it will measure the redshift directly
from the X-ray data (from the location of absorption edges in their spectra).
STROBE-X will study the plateau emission of both short and long GRBs and
provide unique diagnostics of whether the emission comes from millisecond
magnetars or structured jets, revealing the nature of the central engines.

STROBE-X will also be triggered by ground-based GW detections and get the
pointed instruments on source within $<11.5$ minutes, given access to early
afterglows with the tremendous collecting area and CCD-quality spectral
resolution of LEMA.

The large grasp and softer response than many GRB missions will give STROBE-X
access to unusual GRB phenomena that have not been well studied so far,
including X-ray flashes (XRFs) and ultra-long GRBs (ULGRBs). Detecting and
localizing $>10$ XRFs per year will increase the samples of these sources and
break the degeneracies in the models for the source mechanisms.

##  11 SVOM

###  11.1 Mission Overview

The Space based Variable astronomical Object Monitor (SVOM) mission is
dedicated to gamma-ray burst studies and in general to Time Domain
Astrophysics, including multi-messenger Science. It is optimized to detect and
follow-up all type of GRBs, but particularly tailored for high-redshift GRBs
thanks to its low energy triggering threshold around 4 keV. One of the main
goals of SVOM is to produce a complete catalogue of GRBs, including their
redshift measurements. For this purpose SVOM will follow an anti-solar
pointing strategy, and will avoid the galactic plane, in order to facilitate
the observations from ground based observatories and robotic telescopes. That
is why SVOM is composed by a space segment, as well as a few ground based
dedicated follow-up facilities. SVOM alerts will be promptly transmitted to
the ground through a dedicated network of VHF antennas and through the Chinese
Beidou inter-satellite communications link. The goal is that ground observers
receive SVOM alerts less than 30 s after the GRB is detected on board. The
SVOM satellite will be launched from China around mid 2024 with a LM 2-C
rocket and injected in a low Earth orbit (h$\sim$600 km) with an inclination
of about 30∘. It will carry four co-aligned instruments. Two instruments
(ECLAIRs and GRM) are sensitive in the hard-X/soft gamma-ray energy range and
have wide FoV, in order to monitor vast regions of the sky and detect gamma-
ray transients. Two narrow FoV instruments (MXT and VT) will be used to
follow-up and characterize the afterglow emission. The SVOM integrated payload
is shown in Figure 19.

Figure 19: The fully integrated SVOM payload.

###  11.2 Instrument Design

ECLAIRs is a coded-mask telescope, composed of a 54$\times$54 cm2 pseudo-
random coded mask made of a Ti-Ta-Ti sandwich (10/0.6/10 mm) placed 45.8 cm
above a pixellated detection plane made of 80$\times$80 CdTe crystals
(4$\times$4$\times$1 mm3). Its FoV is about 2 sr (89${}^{\circ}\times$89∘)
wide. ECLAIRs is sensitive in the 4 keV–150 keV energy range, and it comprises
an on-board software to detect and localize (to better than 13 arcmin) in
near-real-time the GRBs that appear in its FoV. Once a new transient is
detected, ECLAIRs issues an alert and requests the platform to slew so that
the error box can be observed by the narrow-field instruments.

ECLAIRs is complemented by the Gamma-Ray Monitor (GRM), a set of three 1.5 cm
thick NaI scintillators of 16 cm in diameter, each one offset by 120∘ w.r.t.
each other and with a combined FoV of $\sim$ 2.6 sr. The GRM has poor
localization capabilities, but it extends the SVOM spectral range up to about
5 MeV, and increases the probability of simultaneous detection of short GRBs
and GW alerts.

The Microchannel X-ray Telescope is a light ($<$ 42 kg) and compact (focal
length $\sim$ 1.15 m) focusing X-ray telescope; its sensitivity below 1 mCrab
makes it the ideal instrument to detect, identify and localize down to the
arcmin level X-ray afterglows of the SVOM GRBs. Its optical design is based on
a “Lobster-Eye” grazing incidence X-ray optics, inspired by the vision of some
crustacean decapods. It is composed of 25 square MPO plates of 40 mm each
arranged in a 5$\times$5 configuration. Although Lobster-Eye optics have been
originally developed for large FoV telescopes (several tens of square
degrees), the MXT optical design is optimized for a (relatively) small FoV of
58$\times$58 arcmin2. The “Lobser-Eye” technique results in a peculiar point
spread function (PSF), made by a central peak and two cross arms. The MXT
optics is couples with a focal plane based on a pnCCD sensor, cooled at -65∘,
sensitive in the 0.2–10 keV energy band. MXT will localized GRB afterglows to
better than the arc minute for the majority of them.

The Visible Telescope (VT) is a Ritchey-Chretien telescope with a 40 cm
diameter primary mirror. Its field of view is 26$\times$26 arcmin2 wide,
adapted to cover the ECLAIRs error box in most of the cases. It has two
channels, a blue one (400–650 nm) and a red one (650–1000 nm), and a
sensitivity limit of $M_{V}=22.5$ in 300 s, allowing the detection $\sim$ 80 %
of the ECLAIRs GRBs. The main characteristics of the SVOM space segment are
summarized in Table 3.

Table 3: Summary of the characteristics of the SVOM space instruments. | ECLAIRs | GRM | MXT | VT
---|---|---|---|---
Energy/Wavelength | 4–150 keV | 15–5000 keV | 0.1–10 keV | 650–1000 nm
Field of View | 2 sr | 2.6 sr (combined) | 58′$\times$58′ | 26′$\times$ 26′
Localization accuracy |  $<$ 12′ |  $<$20∘ |  $<$2′ |  $<$1′′
Expected GRBs year-1 | 60 | 90 | 50 | 40

The SVOM mission is also provided with a number of dedicated telescopes on
ground. In particular here we mention:

  * •

the Ground-Based Wide Angle Cameras (GWACs), a set of 36 optical cameras with
a combined FOV of 5400 deg2, located in Ali (China), whose goal is to catch
the prompt optical emission for the ECLAIRs GRBs;

  * •

the Chinese Ground Follow-up Telescope (C-GFT), a robotic 1-m class telescope,
with a 21$\times$21 arcmin2 FOV, located in Xinglog (China) and sensitive in
the 400-950 nm wavelength range;

  * •

the French Ground Follow-up Telescope (F-GFT, Colibri), a robotic 1-m class
telescope, with a 26$\times$26 arc min2 FOV, located in San Pedro Martir
(Mexico), and with multi-band photometry capabilities over the 400-1700 nm
wavelength range.

Other robotic telescopes will be part of the SVOM follow-up system, but they
will not be fully dedicated to SVOM.

###  11.3 Expected Performance

The understanding of the GRB physics requires observations in the largest
spectral domain and in the largest temporal interval, from the possible
precursor up to the transition between the prompt and afterglow emissions.
Simultaneous observation of the prompt GRB event in the gamma-ray, X-ray and
visible bands, combined with narrow field observations of the afterglow in
X-rays, visible and near infrared immediately after the beginning of the event
will enable a better understanding of mechanisms at work in such events. New
measurements (gravitational wave, gamma-ray polarization, neutrinos) may also
become possible in the future, and their impact on the physical understanding
of the GRB phenomenon will be maximized if these measurements are made for
bursts whose “standard” properties, including the distance, are well measured.
The SVOM mission is well adapted to these objectives. Compared to previous
missions, it offers simultaneously (i) the capacity to trigger on all types of
GRBs (especially on X-ray rich, and ultra-long ones); (ii) an excellent
efficiency of the follow-up and the redshift measurement; (iii) a good
spectral coverage of the prompt emission by ECLAIRs+GRM allowing a detailed
modeling. For a significant fraction of SVOM GRBs, GWAC will provide in
addition a measurement or an upper limit on the prompt optical emission; (iv)
a good temporal and spectral coverage of the prompt and afterglow emission
thanks to MXT, VT, and the GFTs.

Concerning Multi-Messenger Astrophysics, SVOM with its ground and space
instruments will offer a large and complementary follow-up capability through
ToOs. GWAC with its 5000 sq. deg. coverage can start the observation since the
alert reception. The GFTs with their small FoV will confirm GWAC candidates
and will be able to do follow-up for well localized events. To activate the
satellite instruments, we will rely on a specific ToO program to send the
observation program using S-band stations. This program guarantees less than
12 hours between the alert and the start of space observations (less can be
expected for most cases) and can be activated around 20 times per year. From
space, MXT and its 1 sq. deg. FoV will have the possibility to cover larger
sky portion using a specific tiling procedure.

Figure 20: Examples of THESEUS capabilities for multi-messenger and time-
domain astrophysics.

##  12 THESEUS

###  12.1 Mission Overview

The Transient High Energy Sky and Early Universe
Surveyor888https://www.isdc.unige.ch/theseus/ (THESEUS) mission concept aims
to fully exploit GRBs for investigating the early Universe, around the epoch
of re-ionization, and substantially advancing Multi-Messenger Astrophysics
(see Figure 20). THESEUS is planned to also simultaneously increase the
discovery space of high energy transient phenomena and allowing tests of
fundamental physics. The core science goals of THESEUS are summarized as
follows:

  * •

Investigating the first billion years of the Universe through high-redshift
GRBs, thus shedding light on main open issues in modern cosmology, like: (i)
the population of primordial low mass and luminosity galaxies; (ii) the
drivers and evolution of cosmic re-ionization; (iii) the star formation rate
(SFR) and metallicity evolution up to the ”cosmic dawn” and across Pop-III
stars.

  * •

Providing a substantial advancement of multi-messenger and time-domain
astrophysics by enabling the identification, accurate localisation and study
of: (i) electromagnetic counterparts to sources of gravitational waves and
neutrinos, which will be routinely detected in the mid-30’s by the second and
third generation Gravitational Wave (GW) interferometers and future neutrino
detectors; (ii) all kinds of GRBs and most classes of other X/gamma-ray
transient sources.

The achievement of these scientific objectives will be possible by a mission
concept including: (a) a set of innovative wide-field monitors with
unprecedented combination of broad energy range, sensitivity, FOV and
localization accuracy; (b) an on-board autonomous fast follow-up in
optical/NIR, arcsec location and redshift measurement of detected
GRB/transients.

THESEUS has been selected twice by the European Space Agency (ESA) for a phase
A study (in 2018 and 2022), aiming at demonstrating its technological and
programmatic feasibility within the boundaries of a medium-size mission.
THESEUS is currently undergoing its second phase A study, with a further
selection step expected in 2026 and eventually a launch planned in 2037.
Nominal scientific operations are being planned for four years but the lack of
on-board consumable makes it feasible for the mission to extend operations in
space well beyond the nominal lifetime.

Figure 21: Left: Distribution of long GRBs with redshift determination in the
peak isotropic luminosity versus redshift plane now (yellow points and hatched
histogram) and after the nominal operation life of THESEUS (purple points and
full histogram). Right: A different version of the left side figure where the
GRBs discovered by THESEUS and those detected up to 2020 are displayed in a
cone representing the Cosmic evolution.

###  12.2 Instruments Design

Three instruments are planned on-board THESEUS. The Soft X-ray Imager (SXI)
uses lobster-eye wide-field ($\sim$0.5 sr) focusing optics to increase the
mission’s sensitivity to fast transients in the 0.3–5 keV energy band. The use
of such optics provides uniform sensitivity across a very large field of view
while maintaining arcminute localisation accuracy. The X and Gamma-ray Imaging
Spectrometer (XGIS) is a GRB and transients monitor providing an unprecedented
combination of exceptionally wide energy band (2 keV–10 MeV), imaging
capabilities and location accuracy. The latter achieves $<15$ arcmin up to 150
keV over a FoV of 2$\pi$. The instrument is also characterized by an energy
resolution of few hundreds eV at energies $<30$ keV and a time resolution of
few $\mu$s over the whole energy band. Finally, the Infra-Red Telescope (IRT)
is mainly conceived to detect, identify and measure the redshift of GRB
afterglows detected by the SXI and the XGIS, especially those at high
redshifts ($z>6$). The IRT is a 70 cm Korsch telescope, optimized for an off-
axis line of sight (LoS) of 0.884 deg. The optical design will implement two
separated FoV, one for photometry with a minimal size of 15$\times$15 arcmin
(potentially extendable to 17$\times$20 arcmin), and one for spectroscopy of
2$\times$2 arcmin. On the photometric field of view the IRT will be able to
acquire images using five different filters (I, Z, Y, J and H) and on the
spectroscopic field of view will provide moderate resolution (R$\sim$400)
slit-less spectroscopy in the 0.8–1.6 $\mu$m range.

###  12.3 Expected Performance

THESEUS has two main core science objectives: the characterization of the
physics of the high redshift universe and the electromagnetic characterization
of gravitational wave transients (above all, the neutron star binary mergers).
Long and short GRBs will be exploited to this scope. The first objective is
instrumental to the understanding of the emergence of the first structures in
the Universe, including the mass function of the high redshift galaxies, and
the metal enrichment in the early stages of the Universe (at the epoch of the
reionization). The second objective has transformational potential, building
upon the first ever gravitational wave multimessenger detection thus far, i.e.
GW170817. THESEUS observations and discoveries in this field are expected to
unveil the nature of ultra-dense matter, pin down the physics of relativistic
jets, and help us understanding the nucleosynthesis of heavy elements (in
turns probing fundamental aspects of general relativity and cosmology). We
show in Figure 21 the total number of GRBs expected to be detected during the
nominal lifetime of the mission (corresponding to 3.45 yrs of scientific
operations) compared to the total number of GRBs discovered from 2005 up to
2020. The transformational capabilities of THESEUS in these respects can well
be appreciated from this figure.

##  13 Conclusions

In this paper we provided an overview of a number of missions, either close to
beginning scientific operations or being planned for the coming/far future,
that are expected to dramatically widen our capability of detecting and
characterizing bright impulsive transient events of Astrophysical interest, as
(but not limited to) GRBs. The list of missions is not meant to be exhaustive,
especially in view of the recent fast-growing interest of the international
community in the fields of time-domain and multi-messenger Astrophysics which
is driving many parallel developments in several countries around the globe.
The missions presented here have been or are being conceived based on largely
overlapping science objectives which have, as main celestial targets,
transient and fast variable sources (down to time scales as short as fractions
of a second). Catching and studying these objects typically requires the
simultaneous availability of large FoV X-/$\gamma$-ray instruments efficiently
(i.e. with high duty cycles) monitoring the high energy sky and either
narrower field instruments providing higher sensitivity measurements in
complementary energy domains (from space as well as from the ground) or
advanced detector technologies paving the way to poorly explored regions of
the relevant parameter space (e.g., high energy polarimetry, simultaneous
energy-time resolved spectroscopy, etc.). Given the largely different
programmatic states of all summarized missions and the dynamic evolution of
the design of at least those missions which are still in the early design and
programmatic stage, providing an exhaustive comparison of their performance
capabilities in different scientific fields is hardly achievable with
sufficient confidence and thus beyond the scope of the present paper.

At the time of writing, EP is the first of the described mission to come on-
line, as the launch was successfully executed on 2024 January 9th and
information on the first results are expected to be publicly available soon.
SVOM is planned to be the second in line, with a launch planned for June 2024.
As described in the previous sections, both missions are likely to boost the
number of GRBs, as well as other high energy transients, to be discovered,
characterized, and possibly followed-up in different energy domains during the
next (at least) $\sim$5 years. The lifetime of both EP and SVOM will
significantly overlap with the scientific runs of the current generation of GW
detectors, as LIGO and VIRGO, possibly providing the discovery of
electromagnetic counterparts of merging binary systems. A similar conclusion
applies to the case of StarBurst, which launch is currently planned in 2027
and the 1-year expected duration of science operations shall match the so-
called LIGO’s scheduled fifth observing run. In 2027, also the installation of
POLAR-2 onto the CSS is being planned and this should bring to the community a
much deeper insight onto the polarization properties of GRBs, extending the
outcomes of previous investigations in this domain from its predecessor,
POLAR. Less certain is at present the path to the beginning of operations for
a few more missions presented, eXTP, Gamow, HiZ-GUNDAM, LEAP, and MoonBeam.
The advanced design state of all these missions could possibly bring them to
space in between late 2020s to early 2030s, making them largely complementary
and uniquely valuable for discoveries in the field of time-domain and multi-
messenger Astrophysics to EP, SVOM, and StarBurst. STROBE-X and THESEUS are
being planned for scientific operations not earlier than mid- to late-2030s.
STROBE-X is competing with several other candidate missions in both the X- and
IR domain within the NASA 2023 Probe mission
call999https://explorers.larc.nasa.gov/2023APPROBE/., while THESEUS is
competing against two further candidate missions for a launch opportunity in
$\sim$2037 within the context of the ESA seventh call for medium-sized
mission101010https://www.esa.int/Science_Exploration/Space_Science/Final_three_for_ESA_s_next_medium_science_mission..
Although the possible launch dates of STROBE-X and THESEUS are projected
relatively far in the future, their timeline could interestingly match the
planned observational runs from the 3rd generation of GW detectors, such as
the Einstein Telescope (ET; see, e.g., Punturo et al., 2010; Maggiore et al.,
2020, and references therein) and Cosmic Explorer (CE; see, e.g., Reitze et
al., 2019, and references therein). The enhanced sensitivity of these
instruments would greatly increase the number of possibly detected
electromagnetic counterparts of GW sources, reaching up to a few tens per year
as reported, e.g., in the case of THESEUS (Stratta et al., 2022).

E. Bozzo coordinated the overall work; W. Yuan and H. Sun have provided
information on Einstein Probe; E. Bozzo, S.N. Zhang, M. Feroci, M. Hernanz,
and A. Santangelo have provided information on eXTP; N. White, S. Guiriec
T.-C. Chang, M. Seiffert, W. Baumgartner, C. Kouveliotou, and A. van der Horst
have provided information on Gamow; D. Yonetoku, A. Doi, H. Matsuhara have
provided information on HiZ-GUNDAM; M.L. McConnell, J. Gaskin, C. Wilson-Hodge
and P. Veres have provided information on LEAP; C. Michelle Hui, C. Wilson-
Hodge, A. Goldstein and P. Jenke have provided information on MoonBEAM; M.
Kole, N. Produit and N. De Angelis have provided information on POLAR-2; D.
Kocevski and Jon E. Grove have provided information on Starburst; P. Roming,
C. Froning, P.S. Ray and T. Maccarone have provided information on STROBE-X;
D. Götz, B. Cordier, and J. Wei have provided information on SVOM; E. Bozzo,
L. Amati, D. Götz, P. O’Brien and A. Santangelo have provided information on
THESEUS.

This research received no external funding.

This paper contains information about different missions either being planned
for the future, or starting nominal operations soon. Further details on all
missions considered can be found on the corresponding websites (if applicable)
or on complementary material in the literature.

###### Acknowledgements.

We thank all referees for their constructive comments and suggestions. LA
acknowledges support from the Italian Ministry of University and Research
through grant PRIN MIUR 2020 - 2020KB33TP METE and INAF grant programme 2022.
The authors declare no conflict of interest. References

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