EP250610a
GCN Circular 40703
Subject
EP250610a: Upper limit from Fermi-GBM Observations
Date
2025-06-12T14:32:39Z (4 months ago)
From
mariaedvige.ravasio@ru.nl
Via
Web form
M. E. Ravasio (Radboud Univ.), E. Burns (LSU), and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:
Fermi-GBM had full spatial coverage of the transient EP250610a detected by EP-WXT (Zhang et al., GCN 40660, Lian et al. GCN 40669). There was no Fermi-GBM onboard trigger around the EP starting time T0=2025-06-10T04:23:24 UTC.
The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [T0-50;T0+500] s, seeking signals between 64 ms and 32.768 s in duration. No signal consistent with the EP transient, both temporally and spatially, is identified, as confirmed also by visual inspection of the data.
Assuming a “soft” spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7), and a duration of 8.192 s, we derive a flux upper limit of 2.8e-08 erg/cm2/s in the energy band 10-1000 keV.
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 40696
Subject
EP250610a: continuous follow-up observations with EP-FXT
Date
2025-06-12T08:52:14Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Q. Cheng (NAO, CAS), Y. J. Zhang (THU) , T. Y. Lian, C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
Following the detection of the fast X-ray transient EP250610a (Zhang et al., GCN 40660, Lian et al. GCN 40669) and its multi-wavelength follow-up observations (Schneider et al., GCN 40661, Evans et al., GCN 40662, Brivio et al., GCN 40665, Yao et al., GCN 40666, Lipunov et al., GCN 40673, Eyles-Ferris et al., GCN 40686, Brivio et al., GCN 40688, Siegel et al., GCN 40690), we performed two target-of-opportunity (ToO) observations with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The X-ray counterpart detected in the autonomous follow-up observation (Lian et al. GCN 40669) as well as the Swift follow-up observation (Evans et al., GCN 40662) was detected in both epochs.
The first ToO observation began at 2025-06-10 15:36:01 (UTC), about 11 hours after the EP-WXT detection. The exposure time is 2975 seconds. Preliminary analysis shows that the 0.5-10 keV FXT spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 9.5e19 cm^-2 and a photon index of 1.97(-0.77, +0.86). The unabsorbed 0.5-10 keV flux is 1.13 (-0.53, +1.24) e-13 erg/s/cm^2 (90% C.L.), about one order of magnitude lower than that measured in the autonomous follow-up observation (Lian et al. GCN 40669).
The second ToO observation began at 2025-06-11 15:35:15 (UTC), about 35 hours after the EP-WXT detection. The exposure time is 5950 seconds. Preliminary analysis shows that the 0.5-10 keV spectrum of this epoch can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 9.5e19 cm^-2 and a photon index of 1.39(-0.94, +0.83), yielding an unabsorbed 0.5-10 keV flux of 6.91(-4.42, +14.50)e-14 erg/s/cm^2 (90% C.L.). The uncertainties are given at 90 percent confidence level for all the above parameters.
EP-FXT will continue monitoring the source in the forthcoming days. Multi-band follow-up observations are encouraged. The contact TAs of EP250610a are Tianying Lian and Huaqing Cheng, please contact them via the email tylian@nao.cas.cn and hqcheng@nao.cas.cn if needed.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 40690
Subject
EP250610A: Swift/UVOT Upper limits
Date
2025-06-11T14:39:47Z (4 months ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M. H. Siegel (PSU) reports on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of EP250610A 3 ks after the EP trigger (Zhang et al., GCN Circ. 40660). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 40662) is seen in the initial UVOT exposures.
Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 3087 9399 2103 >21.44
u 35617 41300 1846 >21.33
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 40688
Subject
EP250610a: TNG NIR upper limit
Date
2025-06-11T11:56:27Z (4 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, P. D'Avanzo, M. Ferro, S. Campana (INAF-OAB), L. Izzo (INAF - OACn), D. B. Malesani (DAWN/NBI & Radboud Univ.), R. Salvaterra (INAF-IASF Milan), A. Melandri (INAF-OAR), V. Lorenzi (INAF-TNG) report on behalf of the CIBO collaboration:
We observed the field of EP250610a detected by EP/WXT (Zhang et al., GCN 40660) with the Italian 3.6m TNG telescope, located in Canary Islands (Spain), equipped with the near-infrared camera NICS in imaging mode. A series of images were obtained with the J filter on 2025-06-11 at a mid-time of about 18.1 hours after the burst.
No clear afterglow candidate is detected within the XRT position (source 1, Evans et al., GCN 40662) down to a 3-sigma limiting magnitude of J ~ 22 (Vega; calibrated against the 2MASS catalogue).
GCN Circular 40686
Subject
EP250610a: Liverpool Telescope optical upper limits
Date
2025-06-11T10:13:31Z (4 months ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris, P. T. O’Brien and R. L. C. Starling (U of Leicester) report:
We observed the field of the X-ray transient EP250610a (Zhang et al., GCN 40660; Lian et al., GCN 40669) with the 2m Liverpool Telescope using the IO:O instrument. We obtained 6x150s exposures in each of the SDSS r’ and SDSS g’ filters starting at 2025-05-11 00:16:50 UT, approximately 19.9 hours after the X-ray detection.
We performed image subtraction on the stacked images using reference images from Pan-STARRS and also compared the stacked and reference images manually. In agreement with Schneider et al. (GCN 40661), Brivio et al. (GCN 40665), Yao et al. (GCN 40666) and Lipunov et al. (GCN 40673), we identify no new sources within the positional uncertainty of the X-ray source identified by Evans et al. (GCN 40662) and Lian et al. (GCN 40669).
At the position of the X-ray source, we derive 3-sigma upper limits of r’ > 22.1 and g’ > 22.1 with photometry calibrated to Pan-STARRS and not corrected for Galactic extinction.
GCN Circular 40673
Subject
EP250610a: Global MASTER-Net observations report
Date
2025-06-10T19:36:20Z (4 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250610a ( EP Team et al., GCN 40660) errorbox 972 sec after notice time and 9094 sec after trigger time at 2025-06-10 06:57:24 UT, with upper limit up to 17.6 mag. The observations began at zenith distance = 82 deg. The sun altitude is -57.2 deg.
MASTER-Kislovodsk robotic telescope located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP250610a errorbox 44872 sec after notice time and 52993 sec after trigger time at 2025-06-10 19:09:03 UT, with upper limit up to 17.4 mag. The observations began at zenith distance = 15 deg. The sun altitude is -18.3 deg.
The galactic latitude b = 42 deg., longitude l = 61 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2898181
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
9229 | 2025-06-10 06:57:24 | MASTER-OAFA | (16h 33m 32.66s , +38d 48m 08.6s) | C | 270 | 17.6 | Coadd
53024 | 2025-06-10 19:09:03 | MASTER-Kislovodsk | (16h 31m 32.68s , +38d 50m 44.6s) | C | 60 | 17.0 |
53210 | 2025-06-10 19:12:09 | MASTER-Kislovodsk | (16h 31m 34.53s , +38d 48m 49.2s) | C | 60 | 17.4 |
53547 | 2025-06-10 19:17:46 | MASTER-Kislovodsk | (16h 31m 41.01s , +38d 50m 51.7s) | C | 60 | 17.1 |
53720 | 2025-06-10 19:20:40 | MASTER-Kislovodsk | (16h 31m 35.46s , +38d 50m 56.6s) | C | 60 | 16.0 |
53893 | 2025-06-10 19:23:32 | MASTER-Kislovodsk | (16h 31m 32.13s , +38d 48m 59.5s) | C | 60 | 17.0 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 40669
Subject
EP250610a: refined analysis of the EP-WXT and EP-FXT observations
Date
2025-06-10T14:16:44Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
T.Y. Lian, H. Q. Cheng (NAO, CAS), Y. J. Zhang (THU) and C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:
The fast X-ray transient EP250610a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Zhang et al., GCN 40660), and followed up by several optical and X-ray telescopes (Schneider et al., GCN 40661, Brivio et al., GCN 40665, Evans et al., GCN 40662). Refined analysis of the WXT data shows that the event started at T0=2025-06-10T04:23:24 (UTC) and lasted for about 140s before the interruption by the autonomous follow-up observation. The peak flux (0.5-4 keV) is estimated to be 2.2 x 10^-9 erg/s/cm^2. The averaged 0.5-4 keV spectrum can be fitted with an absorbed power-law with a Galactic hydrogen column density of 9.5 x 10^19 cm^-2 and a photon index of 0.7 (-/+0.8). The derived average unabsorbed 0.5-4 keV flux is 5.2 (-2.3, +4.2) x 10^-10 erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2025-06-10T04:26:32, about 3 minutes after T0, with an exposure time of 3946 seconds. Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 248.3567, DEC = 38.5362 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is spatially consistent with the candidate X-ray counterpart detected by Swift/XRT (Source 1 in Evans et al., GCN 40662). The averaged 0.5-10 keV FXT spectrum can be fitted with an absorbed power-law with a Galactic hydrogen column density of 9.5 x 10^19 cm^-2 and a photon index of 2.34 (-0.12, +0.12). The derived unabsorbed 0.5-10 keV flux is 1.87 (-0.17, +0.19) x 10^-12 erg/s/cm^2. The uncertainties are at the 90% confidence level for all the above parameters.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
The contact TAs of this source are Tianying Lian and Huaqing Cheng, please contact them via the email tylian@nao.cas.cn and hqcheng@nao.cas.cn if needed.
GCN Circular 40666
Subject
EP250610a: SVOM/VT optical upper limit
Date
2025-06-10T12:55:24Z (4 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
Z. H. Yao, L. P. Xin, Y. L. Qiu, H. L. Li, C. Wu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, Y. N. Ma, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA),Y. J. Zhang (THU), T. Y. Lian, H. Q. Cheng, C. C. Jin (NAO, CAS), Lin Lan (NAOC), Run-Chao Chen (NJU), Chen-Wei Wang, Wen-Jun Tan (IHEP) report on behalf of the SVOM and EP mission team:
SVOM performed a Target of Opportunity observation of EP250610a detected by EP/WXT (Zhang et al., GCN 40660). SVOM/VT began observing the field at 2025-06-10 06:16:38 UTC, 1.85 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
No credible candidate was detected in our single or stacked images within the errorbox of EP/FXT (Zhang et al., GCN 40660) or Swift/XRT (Evans et al., GCN 40662 ), the three sigma limits are:
[date-obs|mid-time] | exposure time (s) | band | upper limit (AB)
----------- --------|-------------------|------|-----------------
2025-06-10T07:29:10 | 76×70 | VT_B | 23.93
2025-06-10T07:29:10 | 76×70 | VT_R | 23.45
The upper limit is consistent with reports (Schneider et al., GCN 40661, Brivio et al., GCN 40665).
Deeper or redder follow-ups are encouraged to investigate the nature of the transient。
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Centre for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN Circular 40665
Subject
EP250610a: REM optical/NIR observations
Date
2025-06-10T12:20:35Z (4 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:
We observed the field of EP 250610a detected by EP/WXT (Zhang et al., GCN 40660) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, and H bands, started on 2025 June 10 at 05:02:09 UT (i.e. 36 min after the burst), and lasted for about 1 hour.
From preliminary inspection, we do not detect any possible counterpart at the position of the candidate detected by Swift/XRT (source 1, Evans et al., GCN 40662) down to the following 3sigma limits:
r > 18.7 (AB; calibrated against the Pan-STARRS catalogue),
at a mid-time of 66 min after the trigger;
H > 16.6 (Vega; calibrated against the 2MASS catalogue),
at a mid-time of 39 min after the trigger.
GCN Circular 40662
Subject
EP250610a: Swift-XRT counterpart detection
Date
2025-06-10T08:55:22Z (4 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P.
Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N.
Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro
(INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato
(INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the
Swift-XRT team:
Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected
source EP250610a, collecting 2.1 ks of Photon Counting (PC) mode data between T0+2.8
ks and T0+9.4 ks after the trigger. A candidate counterpart has been found. The
details of this source are:
Source 1 (SWIFT J163325.5+383159):
==================================
RA (J2000.0): 248.3564 = 16h 33m 25.54s
Dec (J2000.0): +38.5332 = +38d 31' 59.5"
Error: 4.6 (arcsec, radius, 90% confidence).
Detect flag: GOOD
Distance: 2.2 arcmin from the Einstein Probe/WXT position.
Mean rate: 0.0111 +/- 0.0027 ct s^-1
Mean flux: (4.4 +/- 1.1)e-13 erg cm^-2 s^-1
Peak rate: 0.0111 +/- 0.0027 ct s^-1
Peak flux: (4.4 +/- 1.1)e-13 erg cm^-2 s^-1
ECF: 3.96e-11 erg cm^-2 ct^-1, assuming NH=1.07e+20 cm^-2,
gamma=1.73; determined from a spectral fit.
LSXPS UL: 2.4e-03 ct/sec, (0.3-10 keV)
so the source is 3.1-sigma above this 3-sigma upper limit.
There is no evidence for fading.
We have detected a total of 2 sources. These have been automatically classified as
follows:
* 0 likely counterparts
* 1 candidate counterpart
* 1 uncatalogued X-ray source
* 0 known X-ray sources
Uncatalogued X-ray sources
--------------------------
Source 2 (SWIFT J163309.1+383226):
==================================
RA (J2000.0): 248.2883 = 16h 33m 09.19s
Dec (J2000.0): +38.5406 = +38d 32' 26.2"
Error: 7.1 (arcsec, radius, 90% confidence).
Detect flag: REASONABLE
Distance: 78 arcsec from the Einstein Probe/WXT position.
Mean rate: (2.4 [+1.6, -1.1])e-3 ct s^-1
Mean flux: (1.02 [+0.70, -0.49])e-13 erg cm^-2 s^-1
Peak rate: (2.4 [+1.6, -1.1])e-3 ct s^-1
Peak flux: (1.02 [+0.70, -0.49])e-13 erg cm^-2 s^-1
LSXPS UL: 3.1e-03 ct/sec, (0.3-10 keV)
so the source is not above this 3-sigma upper limit.
There is no evidence for fading.
There is 1 2MASS object within the source's 3-sigma error radius.
All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with
catalogues and upper limits from other missions, we assumed a power-law spectrum
with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
The results of the XRT-team automatic analysis of the XRT observations, including a
position-specific upper limit calculator, are available at
https://www.swift.ac.uk/EP.
This circular is an officicial product of the Swift-XRT team.
GCN Circular 40661
Subject
EP250610a: COLIBRÍ optical upper limit
Date
2025-06-10T07:27:07Z (4 months ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
Benjamin Schneider (LAM), Ramandeep Gill (UNAM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Antonio de Ugarte Postigo (LAM):
We imaged the field of the EP250610a (Zhang et al., GCN Circ. 40660) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-06-10T05:04:33 to 05:23:35 UTC (from 38.7 to 57.8 minutes after the trigger, 3 minutes after the notice) and obtained 16 minutes of exposure in the i filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the FXT position (Zhang et al., GCN Circ. 40660) down to the following 3-sigma limit:
i > 22.9
Further observations and analysis are ongoing.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular 40660
Subject
EP250610a: Einstein Probe detection of an X-ray transient
Date
2025-06-10T06:40:33Z (4 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. J. Zhang (THU), T. Y. Lian, H. Q. Cheng, C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250610a. The transient triggered EP-WXT (ID: 01709178308) at 2025-06-10T04:25:50 (UTC). The WXT position of the source is R.A. = 248.310 deg, DEC = 38.527 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 248.3573 deg, DEC = 38.5387 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
Further information will be updated when the telemetry data is received.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).