GRB 250813B
GCN Circular 41449
Subject
GRB 250813B: Swift/UVOT Upper Limit
Date
2025-08-20T15:17:24Z (3 days 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 GRB 250813B 9.6 ks after the SVOM/ECLAIRs trigger (Xie et al., GCN Circ. 41352). We do not detect any afterglow consistent with the XRT position (Page et al., GCN Circ. 41359) or optical position (Jelinek et al., GCN Circ. 41355).
Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 9599 16347 1576 >21.07
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.072 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 41412
Subject
GRB 250813B: SVOM/ECLAIRs refined analysis
Date
2025-08-18T15:48:24Z (5 days ago)
Edited On
2025-08-18T17:45:24Z (5 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. Brunet, O. Godet, H. Yang (IRAP), W.J. Xie (NAOC), B.T. Wang (YNAO, CAS)
report on behalf of the SVOM/ECLAIRs team:
Using the event-by-event data downloaded through the X-band ground station, we report further analysis of ECLAIRs observations of GRB 250813B (SVOM burst-id sb25081303).
The burst that triggered ECLAIRs onboard (Xie et al. GCN 41352/[41354](https://gcn.nasa.gov/circulars/41354?view=index&query=250813B&startDate=&endDate=&sort=circularID)) consists of multiple peaks. By considering only the data before the slew of the platform, the T90-value is 26.8 +0.4/-0.5 s in the 4-120 keV energy band. We note that ECLAIRs detected some emission up to at least T0+90s (T0 = 2025-08-13T22:51:12 UTC).
The time-averaged spectrum from T0-5.5 s to T0+36 s in the energy range 4-120 keV is best fitted by a cutoff power law. The powerlaw index is -0.936 +0.031/-0.032 and the cutoff energy, parameterized as Epeak, is 104 +12/-10 keV. With this model, the total fluence in 4-120 keV is (1.10 +0.01/-0.03)e-5 erg/cm^2.
With a redshift of z = 1.752 (Schneider et al., GCN 41363), the burst isotropic energy Eiso (from 1 keV to 10 MeV in its rest frame) is (1.34 +0.07/-0.08)e53 erg, which makes this GRB consistent with type II GRBs in the 'Amati' relation diagram (Amati et al. 2002).
We note that the ECLAIRs results are consistent within the errors with those provided by both SVOM/GRM (Tan et al., GCN 41383) and Fermi/GBM (Smith et al., GCN 41376).
All quoted errors are at the 68% confidence level.
We note that the calibration of SVOM/ECLAIRs is ongoing thus these results are preliminary.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.
The SVOM/ECLAIRs point of contact for this GCN circular is Marius Brunet: (marius.brunet AT irap.omp.eu).
GCN Circular 41394
Subject
GRB 250813B: Liverpool Telescope optical upper limits
Date
2025-08-15T14:54:03Z (8 days ago)
From
Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar, J. R. Maund (RHUL), N. C. Sun (UCAS), W. X. Li, Y. N. Wang (NAOC), and K. Wiersema (Herts) report:
Using the IO:O imager on the 2.0 m Liverpool Telescope, we observed the field of the SVOM/ECLAIRs–triggered GRB 250813B (Xie et al., GCNs 41352, 41354), also detected by EP–FXT (Wu et al., GCN 41361), SVOM/MXT (Götz et al., GCN 41368), Fermi/GBM (Smith et al., GCN 41376), and SVOM/GRM (Tan et al., GCN 41383). Observations in the r and z bands were obtained on 2025-08-15 at 02:42:32.1 UT and 03:13:37.8 UT, corresponding to ~1.16 and ~1.18 days post-trigger, respectively. Each band consisted of 3 × 600 s exposures.
We do not detect the optical afterglow of GRB 250813B (Jelinek et al., GCN 41355; de Ugarte Postigo et al., GCN 41356; Schneider et al., GCN 41363; Xin et al., GCN 41369; Corcoran et al., GCN 41370) down to limiting magnitudes of r >22.0 and z >20.6.
Photometric calibration was performed using Pan-STARRS stars. The quoted magnitudes are not corrected for Galactic extinction in the direction of the GRB.
GCN Circular 41388
Subject
GRB 250813B: Calapai Observatory, Massa S. Giorgio (Messina), upper limit
Date
2025-08-15T10:13:58Z (9 days ago)
From
Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, Messina, Italy <giovannicalapai@tiscali.it>
Via
Web form
Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, (Messina) Italy
Member of: GRB/UAI Gamma Ray Burst Section of Unione Astrofili Italiani.
Report:
We imaged the field of GRB 250813B detected by SVOM/ECLAIRs telescope (Xie et al. GCN 41352 - 41354), with the 11 inches Schmidt-Cassegrain (Celestron 11) telescope F/D=6,3.
The observations were started at 2025-08-14 00:02 UT (approximately 1.19 hours after burst) stacking a set of unfiltered CCD image. The observations were carried out with partially clear skies and Moon (illumination: 74%, distance: 42°).
We co-added 93 exposures of 60 sec each.
Start T0+ End T0+ CR lim
1.19 hour 4.44 hour 20.1
We did not found any optical counterpart in the error box of the XRTcandidate (Page et al. GCN 41359).
Magnitudes were estimated with the PanSTARRS cat. and are not corrected for galactic dust extinction.
Our observations are consistent with other already reported Jelinek et al. (GCN 41355), Gill et al. (GCN 41356), Corcoran et al. (GCN 41370).
The message may be cited.
GCN Circular 41383
Subject
GRB 250813B: SVOM/GRM observation
Date
2025-08-15T07:23:53Z (9 days ago)
From
tanwj@ihep.ac.cn
Via
Web form
SVOM/GRM team: Wen-Jun Tan, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Stéphane Schanne
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by GRB 250813B (SVOM burst-id sb25081303) at 2025-08-13T22:51:12UTC (T0), which is also detected by SVOM/ECLAIRs (W.J. XIE et al., GCN #41354), Swift-XRT (K.L. Page et al., GCN #41359), EP-FXT (Q. Y. Wu et al., GCN #41361), SVOM/MXT (D. Götz et al., GCN #41368) and FERMI/GBM (Jacob Smith et al., GCN #41376)
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multiple pulses with a T90 of 24.9 +/-0.76 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250813B.png
With this localization given by SVOM/ECLAIRs (RA= 336.728, DEC= 12.498, GCN #41354), the time-averaged spectrum from T0-6 to T0+35 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.84 +/-0.07 and the cutoff energy, parameterized as Epeak, is 109 +6/-5 keV. The event fluence (10-1000 keV) in this time interval is (1.35 +/-0.20)E-05 erg/cm^2.
With the redshift z = 1.752 by VLT/X-shooter (B. Schneider et al., GCN#41363), the Eiso (from 1 keV to 10 MeV in rest frame) of this burst is (1.16 +0.20/-0.18)E53 erg. GRB 250813B is consistent with Type II GRBs in the 'Amati' relation diagram, as shown at:
https://www.bursthub.cn//admin/static/grb250813B_amati.png
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center 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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.
The SVOM point of contact for this burst is: Wen-Jun Tan (IHEP)(tanwj@ihep.ac.cn)
GCN Circular 41376
Subject
GRB 250813B: Fermi GBM Observation
Date
2025-08-14T19:17:11Z (9 days ago)
From
Jacob Smith at Fermi-GBM Team <jrs0118@uah.edu>
Via
Web form
Jacob Smith (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
At 22:51:11.24 UT on 13 August 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250813B (trigger 776818276/250813952), which was also detected by SVOM/ECLAIRs (W.J. Xie, et al. 2025, GCN 41354).
The Fermi GBM on-ground location is consistent with the SVOM/ECLAIRs position.
An optical afterglow was first reported by Ondrejov D50 (M. Jelinek, et al. 2025, GCN 41353) which also reports an extremely rapid decay (M. Jelinek, et al. 2025, GCN 41355).
A redshift of z=1.752 was report by the VLT (B. Schneider, et al. 2025, GCN 41363).
The angle from the Fermi LAT boresight is 99 degrees.
The GBM light curve consists of multiple emission episodes with a duration (T90)
of about 24.1 s (50-300 keV). The time-averaged spectrum
from T0-3.8 to T0+31.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.82 +/- 0.04 and the cutoff energy,
parameterized as Epeak, is 114 +/- 3 keV. A Band function fits equally well with Epeak = 112 +/- 4, alpha = -0.81 +/- 0.04, and beta = -3.67 +/- 0.79.
The event fluence (10-1000 keV) in this time interval is
(1.30 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+11 s in the 10-1000 keV band is 8.1 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/
GCN Circular 41370
Subject
GRB 250813B: NOT observations of the optical afterglow
Date
2025-08-14T12:18:54Z (9 days ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
G. Corcoran (UCD), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), B. Schneider (LAM), B. P. Gompertz (Birmingham), V. Vuolteenaho (Univ. of Oulu), report on behalf of a larger collaboration:
We observed the field of the SVOM/ECLAIRs GRB 250813B (Xie et al., GCN 41352; Xie et al., GCN 42354), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 6x150 s exposures in the r-band and 5x200 s in the z-band, starting at 2025-08-14 00:05:04 UT (1.231 hr after the trigger).
The optical afterglow reported by Jelinek et al. (GCN 41355), de Ugarte Postigo et al. (GCN 41356), Schneider et al. (GCN 41363) and Xin et al. (GCN 41369) is well detected with a preliminary magnitude of
r = 21.00 +/- 0.08 (AB, mid-time 1.379 hr after trigger);
z = 20.04 +/- 0.09 (AB, mid-time 1.689 hr after trigger).
The photometric calibration was performed using nearby stars from the Pan-STARRS catalog and the magnitudes are not corrected for Galactic extinction.
GCN Circular 41369
Subject
GRB 250813B: SVOM/VT optical observations
Date
2025-08-14T12:16:34Z (9 days ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), B. T. Wang (YNAO) report on behalf of the SVOM/VT team.
SVOM performed an automatic slew on the burst GRB 250813B triggered by SVOM/ECLAIRs (Xie et al., GCN 41354). SVOM/VT start the observations at 278 seconds after the burst, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The optical counterpart (Jelinek et al., GCN 41353 & GCN 41355; Schneider et al., GCN 41363; de Ugarte Postigo et al., GCN 41356) at the position consistent with the locations of Swift/XRT (Page et al., GCN 41359), EP/FXT (Wu et al., GCN 41361) and SVOM/MXT(Xie et al., GCN 41354, Gotz et al., GCN 41368), was clearly detected in both VT_R and VT_B.
During the X-band data available, the brightness was fading from VT_R=16.61+/0.02 mag and VT_B=17.73+/-0.03 mag at 303 seconds post the burst to VT_R=20.9+/-0.1 mag and VT_B=22.1+/-0.2 mag at 4.27 hours after the burst.
Our photometry was not corrected for Galactic extinction.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center 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.
The SVOM point of contact for this burst is Wenjin Xie (xiewj@bao.ac.cn).
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular 41368
Subject
Subject GRB 250813B: SVOM/MXT refined analysis
Date
2025-08-14T11:45:54Z (9 days ago)
From
Diego Gotz at CEA <diego.gotz@cea.fr>
Via
Web form
D. Götz (CEA), F. Robinet (IJCLab), P. Maggi (ObAS), H. Goto (Kanazawa Univ./CEA), M. Moita (CEA), C. Plasse (CEA), , C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team:
GRB 250813B (Xie et al. GCN 41352/41354) was observed by SVOM/MXT after an automatic SVOM slew, starting at T0 = 2025-08-13T22:54:01, 169 s after trigger time Tb. MXT observed for the remainder of the orbit, cumulating 188 s of useful data before entering Earth occultation.
Using the full X-band dataset of the first orbit, the position of the MXT afterglow is refined to:
R.A. (J2000) = 22h26m58.6s
Dec (J2000) = +12d27m08s
with a 90% C.L. radius of 54 arc sec (including 25 arcseconds systematic error added in quadrature). This position is at 47 arc sec from the optical counterpart reported in GCN 41353.
The light curve exhibits an initial fast fading with temporal decay of -2.3+/-0.5.
We analysed the early time (first orbit) spectrum and modelled it with a redshifted (z=1.75 GCN 41363) absorbed power-law, a photon index Gamma 1.4+-/0.3 at 90% C.L and a redshifted absorbing column of NH = <0.8x1e22 cm^-2 on top of a Galactic NH = 4.86x10^20 cm^-2.
The source is still detected on the subsequent orbit between Tb+3900 and Tb+6100 s but not after.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. MXT was developed jointly by CEA, CNES, University of Leicester, IJCLab and MPE.
The SVOM point of contact for this burst is Wenjin Xie (xiewj@bao.ac.cn).
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular 41363
Subject
GRB 250813B: VLT X-shooter spectroscopic redshift z = 1.752
Date
2025-08-14T10:29:14Z (10 days ago)
From
Gregory Corcoran at University College Dublin <gregory.corcoran@ucdconnect.ie>
Via
Web form
B. Schneider (LAM), G. Corcoran (UCD), E. Le Floc’h (CEA), A. de Ugarte Postigo (LAM), J. An (NAOC), A. L. Thakur (INAF-IAPS), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (U. Leicester) report on behalf of the Stargate collaboration:
We observed the optical afterglow (Jelinek et al., GCN 41355; de Ugarte Postigo et al., GCN 41356) of GRB 250813B detected by SVOM/ECLAIRs (Xie et al., GCN 41352; Xie et al., GCN 42354) with ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.
In images taken with the acquisition camera on Aug 14 05:40 UT (6.82 hr after the ECLAIRs trigger), the optical afterglow is clearly detected in g, r, and z bands. We measured a preliminary magnitude r = 22.24 +/- 0.06 AB (calibrated against the Pan-STARRS PS1 catalog).
Our spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. Observations started on 2025-08-14 at approximately 05:50 UT (6.98 hr after the SVOM trigger). In a preliminary reduction of the spectra, we detect a faint continuum down to ~3590 AA at the blue end. From the detection of a multitude of absorption lines, which we interpret as due to C II, Si IV, Si II, C IV, C I, Al II, Al III, Mn I, Fe II, Fe II*, Mn II, Mg II, Mg I, Ca II, we infer a redshift of z = 1.752.
We acknowledge excellent support from the ESO observing staff in Paranal, in particular Celia Desgrange, Rodrigo Romero, and Akke Corporaal. The analysis of this spectrum was carried out with the help of the zHunter tool (https://doi.org/10.5281/zenodo.15189495).
GCN Circular 41361
Subject
GRB 250813B: EP-FXT counterpart detection
Date
2025-08-14T09:01:39Z (10 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q. Y. Wu, H. Q. Cheng, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250813B (SVOM/sb25081303, Xie et al. GCN #41352, #41354) at 2025-08-14 00:58:26(UTC), about 2 hours after the SVOM/ECLAIRs trigger, with an exposure time of ~1.3 ks. One uncatalogued source is detected within the ECLAIRs error circle, and the source is spatially consistent with the counterpart reported in optical and X-ray bands (Xie et al. GCN #41352, #41354, Jelinek et al. GCN #41353, #41355, Gill et al. GCN #41356, Page et al. GCN #41359). Preliminary analysis on this source are conducted (note that the exposure time for the analysed data is not complete), with the details listed as follows.
Source 1: EPF_J222700.2+122741
RA (J2000): 336.7507
Dec (J2000): 12.4617
Flux: 1.00 x 10^-11 erg/s/cm2 (observed, 0.5-10 kev)
Flux_err: 1.2 x 10^-12 erg/s/cm2 (90% C. L.)
The position uncertainty of the source is about 10 arcsec in radius (90% C.L. statistical and systematic).
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 41359
Subject
GRB 250813B: Swift-XRT afterglow detection
Date
2025-08-14T08:09:11Z (10 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
K.L. Page, A.P. Beardmore and P.A. Evans (U. Leicester) report on behalf
of the Swift-XRT team:
We have analysed 1.6 ks of XRT data for GRB 250813B, from 9.6 to 16.3
ks after the SVOM/ECLAIRs trigger (GCN Circ. 41354). The data are
entirely in Photon Counting (PC) mode. We find an uncatalogued X-ray
source at RA, Dec = 336.7505, +12.4613 which is equivalent to:
RA (J2000): 22 27 00.13
Dec(J2000): +12 27 40.5
with an uncertainty of 3.7 arcsec (radius, 90% confidence).
There is no evidence for fading during this interval, with the X-ray
source showing a mean count rate of 0.17 +/- 0.02 count s^-1. However,
the localisation of this source is consistent with the fading optical
afterglow found by Ondrejov D50 (GCN Circ. 41353, 41355) and
SVOM/COLIBRI (GCN Circ. 41356), and so we consider it the X-ray
afterglow.
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.1 (+0.4, -0.3). The
best-fitting absorption column is 2.5 (+1.3, -1.1) × 10^21 cm^-2, in
excess of the Galactic value of 6.2 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 6.2 x 10^-12 (9.3 x
10^-12) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 2.5 (+1.3, -1.1) x 10^21 cm^-2
Galactic foreground: 6.2 x 10^20 cm^-2
Photon index: 2.1 (+0.4, -0.3)
This circular is an official product of the Swift-XRT team.
GCN Circular 41356
Subject
GRB 250813B: SVOM/COLIBRÍ (FM-GFT) optical observations
Date
2025-08-14T05:51:16Z (10 days ago)
Edited On
2025-08-14T17:14:40Z (9 days ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
Ramandeep Gill (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), 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), Benjamin Schneider (LAM), W.J. Xie (NAOC), and B.T. Wang (YNAO, CAS):
We imaged the field of the SVOM GRB 250813B (W.J. Xie et al., GCN Circ. 41352) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-08-14 04:05:30 to 04:44:19 UTC (mean epoch 5.5618 hours after the trigger) and obtained 32x60 s of exposure in the i filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
We detected the optical counterpart reported by Jelinek et al., GCN Circs. 41353 and 41355, at a preliminary magnitude of:
i = 21.38 +/- 0.09
Further observations 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 41355
Subject
GRB 250813B: Ondrejov D50 fast decay
Date
2025-08-14T00:46:58Z (10 days ago)
From
Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>
Via
email
M. Jelinek, J. Strobl, F. Novotny, R. Hudec (ASU CAS Ondrejov)
report:
Further to our earlier report (GCN 41353), we provide refined photometric
analysis of the optical afterglow of GRB 250813B (Xie et al., GCN 41352).
Our photometry reveals an extremely rapid decay with the afterglow fading
from the initial magnitude of ~14.4 at ~100s post-trigger to near magnitude
20 at 0.03 days (43 minutes) after the trigger. This represents a fade of
approximately 5.6 magnitudes in less than one hour and an average decay
rate of ~1.8.
Potential observers should note the rapid evolution when planning follow-up
observations.
GCN Circular 41354
Subject
GRB 250813B: SVOM detection of a bright long burst (Correction for GCN 41352).
Date
2025-08-14T00:40:48Z (10 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
W.J. XIE (NAOC), B.T. WANG (YNAO, CAS), O. Godet, H. Yang (IRAP), H. Goto (CEA, Kanazawa univ.), F. Robinet (IJCLAB), L. Zhang (IHEP)
This GCN circular is the correction version for the GCN 41352. We sorry for the mistake in the GCN 41352, where the GRB name shall be GRB 250813B but not GRB 250813A.
At 2025-08-13T22:51:12 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250813B (SVOM burst-id sb25081303). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 16 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 92.23 in the [8-120] keV energy band over a time window of 20.4 seconds starting at 2025-08-13T22:51:15.
The localization of the best alert is R.A., Dec. 336.728, 12.498 degrees (J2000) with a 90% confidence level (C.L.) radius of 2.17 arcmin (including systematic error of 2 arcmin added in quadrature).
This burst also triggered SVOM/GRM at 2025-08-13T22:51:11 on a timescale of 1 second with an SNR of 8.
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2025-08-13T22:54:00 UTC, 168 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 336.7591, 12.4479 degrees:
R.A. (J2000) = 22h27m02.19s
Dec. (J2000) = 12d26m52.70s
with a 90% C.L. radius of 71 arcseconds including systematics.
This location is 3.51 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.
VT began observing the field after the slew. The analysis of the data will be published in a future circular.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The Burst Advocate (BA) on shift for this alert is Wenjin XIE (xiewj@bao.ac.cn).
Please contact the BA by email if you require additional information.
GCN Circular 41353
Subject
GRB 250813B: Ondrejov D50 optical afterglow detection
Date
2025-08-14T00:09:05Z (10 days ago)
From
Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>
Via
email
M. Jelinek, J. Strobl, F. Novotny, R. Hudec (ASU CAS Ondrejov)report:We observed the position of the SVOM-detected GRB 250813B (Xie et al., GCN 41352) with the D50 robotic telescope of the Astronomical Institute Ondrejov, near Prague, Czech Republic. We performed a series of 10s unfiltered exposuresstarting at 22:52:44.496 UT, i.e. 93.5s after the trigger.We detect the optical afterglow at 22:27:00.15 +12:27:44.5 (J2000), consistent with both the ECLAIRs localization (Xie et al., GCN 41352) and the refined MXTX-ray position. The source is clearly detected in single 10s unfiltered images.
GCN Circular 41352
Subject
GRB 250813B: SVOM detection of a bright long burst
Date
2025-08-13T23:38:17Z (10 days ago)
Edited On
2025-08-14T17:14:55Z (9 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
W.J. XIE (NAOC), B.T. WANG (YNAO, CAS), O. Godet, H. Yang (IRAP), H. Goto (CEA, Kanazawa univ.), F. Robinet (IJCLAB), L. Zhang (IHEP)
At 2025-08-13T22:51:12 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250813B (SVOM burst-id sb25081303). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 16 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 92.23 in the [8-120] keV energy band over a time window of 20.4 seconds starting at 2025-08-13T22:51:15.
The localization of the best alert is R.A., Dec. 336.728, 12.498 degrees (J2000) with a 90% confidence level (C.L.) radius of 2.17 arcmin (including systematic error of 2 arcmin added in quadrature).
This burst also triggered SVOM/GRM at 2025-08-13T22:51:11 on a timescale of 1 second with an SNR of 8. The lightcurve of GRM could be found below.
https://www.bursthub.cn//admin/static/svgrb250813A.png
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2025-08-13T22:54:00 UTC, 168 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 336.7591, 12.4479 degrees:
R.A. (J2000) = 22h27m02.19s
Dec. (J2000) = 12d26m52.70s
with a 90% C.L. radius of 71 arcseconds including systematics.
This location is 3.51 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.
VT began observing the field after the slew. The analysis of the data will be published in a future circular.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The Burst Advocate (BA) on shift for this alert is Wenjin XIE (xiewj@bao.ac.cn).
Please contact the BA by email if you require additional information.