GRB 250818B
GCN Circular 41516
Subject
GRB 250818B: MeerKAT Detection
Date
2025-08-24T01:11:07Z (a year ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form
G. Schroeder (Cornell), W. Fong (Northwestern), T. Laskar (Utah), L. Rhodes (TSI/McGill) report:
We observed the location of the SVOM short-duration GRB 250818B (Wang et al., GCN 41405) with a Swift/XRT counterpart (Ferro et al., GCN 41407) with the MeerKAT radio telescope under Director’s Discretionary program DDT-20250822-GS-01 (PI Schroeder) at 1.3 GHz and 3.1 GHz on 2025 August 23 (~5 days post burst).
In preliminary analysis, we detect the radio afterglow (Ricci et al., GCN 41455) in the 3.1 GHz image, but not in 1.3 GHz (rms of 7 microJy/beam). We measure a flux density of ~30 microJy at 3.1 GHz. Further observations are planned.
We thank Fernando Camilo for the rapid approval of the DDT proposal, and the staff at the South African Radio Astronomy Observatory for scheduling these observations.
GCN Circular 41455
Subject
GRB 250818B: 10 GHz VLA detection
Date
2025-08-20T18:16:07Z (a year ago)
From
Roberto Ricci at INAF-IRA <ricci@ira.inaf.it>
Via
Web form
R. Ricci, M. Yadav, E. Troja (U Rome) on behalf of a larger collaboration:
We observed the short-duration GRB 250818B discovered by SVOM (GCN 41405) with Very Large Array in X-band at the centre frequency of 10 GHz with a bandwidth of 4 GHz on Aug 20th 2025, 2.3 days after the burst.
After reducing the data in latest CASA version (v6.6.1) with standard procedures a detection was found at a position compatible with GOTO (GCN 41406) and NOT (GCN 41426) optical transient. We estimate a flux density of approximately 120 microJy.
We thank the VLA staff for promptly executing the observations.
GCN Circular 41445
Subject
GRB 250818B: FTW optical and NIR observations show reddening of the counterpart
Date
2025-08-20T14:06:15Z (a year ago)
Edited On
2025-08-20T14:40:37Z (a year ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Jennifer Fabà (LMU), Vani Jain (LMU), Xiaoxiong Zuo (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:
We observed the counterpart of the short GRB 250818B (Wang et al., GCN 41405; Kumar et al., GCN 41406; Ferro et al., GCN 41407; Yao et al., GCN 41409; Yheng et al., GCN 41417; Fong et al., GCN 41419; Li et al., GCN 41424; Bendtsen et al., GCN 41426; Moskvitin et al., GCN 41428; An et al., GCN 41430; Kumar et al., GCN 41431; Siegel et al., GCN 41435; Dimple et al., GCN 41442) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J-band simultaneously for 35x 180 s starting at 2025-08-19T01:13:45 UT (0.91 days after the trigger) and again for 30x 180 s starting at 2025-08-20T01:27:02 (1.9 days after the trigger). In the two epochs, we detect the counterpart at
r = (21.12 +/- 0.02) mag (1st epoch),
r = (22.26 +/- 0.08) mag (2nd epoch).
We note a significant reddening between the two epochs, where the r-J color increases by nearly 0.6 mag. The cause of the color evolution is unclear, but it is potentially consistent with either a red host galaxy or an intrinsic color evolution of the counterpart. Further observations are encouraged.
The r-band magnitudes are calibrated against the PS1 catalog, and the J-band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.
We thank Michael Schmidt from the Wendelstein Observatory for obtaining these observations.
GCN Circular 41444
Subject
GRB 250818B: Kilonova-Catcher optical afterglow detection
Date
2025-08-20T13:19:56Z (a year ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
R. Hellot, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), D. Akl (NYUAD), M. Molham (NRIAG), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 250818B detected by SVOM/ECLAIRs and SVOM/GRM (Wang et al., GCN 41405) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a TEC160FL telescope operated by M. Freeberg and the CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at T0+5hr.
In our stacked frames, we detect an uncatalogued optical source consistent with the afterglow candidate reported by several teams: GOTO (Kumar et al., GCN 41406), Swift/XRT (Ferro et al., GCN 41407), SVOM/VT (Yao et al., GCN 41409), KAIT (Zheng et al., GCN 41417), Keck (Fong et al., GCN 41419), EP/FXT (Li et al.; GCN 41424), NOT (Bentdsen et al., GCN 41426), SAO/RAS (Moskvitin et al., GCN 41428), TRT (An et al., GCN 414430), the 1m Ledesi telescope (Kumar et al., GCN 41431), Swift/UVOT (Siegel et al., GCN 41435) and the Liverpool telescope (Dimple et al., GCN 41442).
We report some of our follow-up results in the table below:
+---------------+-----------+------------+----------------+--------------+
| Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument |
+===============+===========+============+================+==============+
| 4.97 | 15 x 300s | sdssr (AB) | 19.47 +/- 0.07 | CDK17-AITP |
| 7.53 | 11 x 300s | sdssr (AB) | 20.17 +/- 0.16 | TEC160FL |
+---------------+-----------+------------+----------------+--------------+
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 41442
Subject
GRB 250818B: Liverpool Telescope optical detections
Date
2025-08-20T11:30:44Z (a year ago)
From
Dimple at University of Birmingham <dimplepanchal96@gmail.com>
Via
Web form
Dimple (U. Birmingham), B. P. Gompertz (U. Birmingham), and A. Kumar (RHUL) report on behalf of a larger collaboration:
We conducted follow-up observations of GRB 250818B (Wang et al., GCN Circ. 41405) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 03:46:39 UT on 2025-08-20, ~2 days after the SVOM/ECLAIRs trigger, and consisted of 5 x 240 s exposures in each of the SDSS g, r, i, and z filters.
We detect the optical afterglow (Kumar et al., GCN Circ. 41406; Yao et al., GCN Circ. 41409; Zheng et al., GCN Circ. 41417; Bendtsen et al., GCN Circ. 41426; Moskvitin et al., GCN Circ. 41428; An et al., GCN Circ. 41430; Kumar et al., GCN Circ. 41431) in all filters, and measure an AB magnitude of r = 22.14 ± 0.10 (mid-time t0+2.03 days).
Magnitudes are calibrated against nearby Pan-STARRS stars and are not corrected for Galactic extinction.
GCN Circular 41435
Subject
GRB 250818B: Swift/UVOT Detection
Date
2025-08-20T04:23:17Z (a year 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 250818B 1.7 ks after the SVOM/ECLAIRs trigger (Wang et al., GCN Circ. 41405). An uncatalogued source consistent with the XRT position (Farro et al., GCN Circ. 41407) and the optical transient (Kumar et al., GCN Circ. 41406; Yao et al., GCN Circ. 41409; Zheng et al., GCN Circ. 41417; Bendtsen et al., GCN Circ. 41426; Moskvitin et al., GCN Circ. 41428; An et al., GCN Circ. 41430; Kumar et al., GCN Circ. 41431) is detected in the UVOT exposures.
The preliminary UVOT position is:
RA (J2000) = 03:04:13.45 = 46.05602 (deg.)
Dec (J2000) = -03:07:31.2 = -3.12532 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).
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 1723 6610 1544 18.25+/-0.06
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.063 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 41431
Subject
GRB 250818B: optical afterglow detection with Lesedi (GOTO25fzq)
Date
2025-08-19T14:54:41Z (a year ago)
From
Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar (RHUL), D. O’Neill (Birmingham), N. Rawat (SAAO), R. A. J. Eyles-Ferris (U. Leicester), R. Dastidar (UNAB), D. Buckley (SAAO), D. Steeghs (Warwick), J. R. Maund (RHUL), R. L. C. Starling (U. Leicester) and G. Ramsay (Armagh) report:
We observed the optical afterglow GOTO25fzq (Kumar et al. GCN 41406) associated with GRB 250818B (Wang et al., GCN 41405), also reported by Yao et al. (GCN 41409), Zheng et al. (GCN 41417), Bendtsen et al. (GCN 41426), Moskvitin et al. (GCN 41428), and An et al. (GCN 41430). The observations are conducted using the 1-m Lesedi telescope at the South African Astronomical Observatory (SAAO), Sutherland, South Africa, between 2025-08-19 00:51:24 UT and 2025-08-19 04:05:00 UT (between 21.37 and 24.60 hours post-trigger) in g’, r’, i’, and z’ bands (6×450s in g and z, 6×400s in r and i bands).
OT was clearly detected in the stacked g’- and r’-band images, whereas i’ and z’ images are severely affected by trailing. Preliminary aperture photometry yields the following magnitudes in the g’ and r’ bands:
DATE-OBS (Start_time) T-T0 (hrs) Filter Exp (s) Magnitude (AB)
--------------------------------------------------------------------------------------
2025-08-19 00:51:24 UT 21.37 g’ 6x450s 21.39 ± 0.09
2025-08-19 01:36:43 UT 22.13 r’ 6x400s 21.58 ± 0.47
Although our r'-band image is also significantly affected by trailing, aperture photometry confirms a low-significance detection, yielding the reported magnitude.
Photometric calibration was performed using reference stars from the SDSS catalogue. Magnitudes are not corrected for Galactic extinction. Follow-up observations are ongoing, and further analysis is in progress.
Data were obtained using the 1-m Lesedi robotic telescope at the SAAO, equipped with the Mookodi low-resolution spectrograph and imager operating in fully robotic imaging mode. We thank the SAAO Instrumentation and Operations (IO) team for their continued support.
GCN Circular 41430
Subject
GRB 250818B: TRT Optical Observation
Date
2025-08-19T13:29:47Z (a year ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Via
Web form
J. An (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), X. Liu, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC) report on behalf of a large collaboration:
We carried out two-epoch observations of GRB 250818B (Wang et al., GCN 41405), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Fresno, California, U.S.A (SRO) and New South Wales, Australia (SBO), respectively. And we obtained several frames in the B, V, R & I band.
The optical afterglow (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al., GCN 41417; Bendtsen et al., GCN 41426; Moskvitin et al., GCN 41428) was clearly detected in our stack frames. The photometric results are presented in the following table:
|mid time(h)|filter|mag(Vega)|site|
|--|--|--|--|
|7.75|R|19.98+/-0.06|SRO|
|12.64|R|20.23+/-0.06|SBO|
The presented magnitudes are calibrated with Pan-STARRS DR2 catalog and not corrected for Galactic extinction.
GCN Circular 41428
Subject
GRB 250818B: SAO RAS optical observations
Date
2025-08-19T08:13:33Z (a year ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova, V. Vlasyuk, Yu. Sotnikova (SAO RAS),
Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) report
on behalf of a larger collaboration.
We observed the field of the SVOM short GRB 250818B (Wang et al.,
GCN 41405; also detected by Swift/XRT, Ferro et al., GCN 41407
and EP-FXT, Li et al., GCN 41424) with the 1-m SAO RAS telescope
Zeiss-1000 equipped with CCD-photometer. We obtained 13 x 300 sec.
images in Rc band on August 18, 23:15:02 -- August 19, 00:33:37 UT
(t_mid - T0 = 20.4196 hours = 0.85082 days).
The OT (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al.,
GCN 41417; Fong et al., GCN 41419; Bendtsen et al., GCN 41426)
is clearly detected in the stacked image with the brightness of
R = 20.88 +/- 0.04 (R_lim = 23.0).
This preliminary photometry is based on nearby Pan-STARRS objects
(magnitudes were converted with the Lupton 2005 equations)
and not corrected for Galactic extinction.
GCN Circular 41426
Subject
GRB 250818B: NOT optical observations
Date
2025-08-19T04:07:05Z (a year ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
Jonas Broe Bendtsen (SDU), Johanne Hein Pedersen (SDU), Isabella Alexandra Koch (SDU), Sophie Lund Wagner (SDU), Julie Magaard Knudsen (SDU), Roar Holmberg Rasmussen (NOT and Aarhus), Kostas Valeckas (NOT and NBI), Johan P. U. Fynbo (DAWN/NBI), Daniele B. Malesani (DAWN/NBI and Radboud Univ.), Antonio de Ugarte Postigo (LAM), report on behalf of a larger collaboration:
We observed the optical counterpart (Kumar et al., GCN 41406; Yao et al., GCN 41409; Zheng et al., GCN 41417; Fong et al., GCN 41419) of the short SVOM GRB 250818B (Wang et al., GCN 41405) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried in the SDSS r band out using the ALFOSC camera, for a total exposure time of 3x300 s.
The optical afterglow is clearly detected in our images. Its coordinates, calibrated against the Gaia catalog, are:
RA(J2000) = 03:04:13.483
Dec(J2000) = -03:07:30.74
with an error of 0.1".
At a mean epoch 2025 Aug 19.128 UT (23.60 hr after the trigger), we measure r = 21.15 +- 0.04 (AB), calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction.
GCN Circular 41424
Subject
GRB 250818B: EP-FXT counterpart detection
Date
2025-08-19T02:25:18Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), Y.-C. Fu (BNU), W.-D. Zhang and Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250818B (SVOM/sb25081801, Wang et al. GCN 41405) at 2025-08-18 06:00:27 (UTC), about 2.52 hours after the SVOM/ECLAIRs trigger, with an exposure time of ~2 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 (Kumar et al. GCN 41406, Ferro et al. GCN 41407, Yao et al. GCN 41409, Zheng et al. GCN 41417, Fong et al. GCN 41419). Preliminary analysis on this source are conducted, with the details listed as follows.
Source 1: EPF_J030413.4-030730
RA (J2000): 46.056
Dec (J2000): -3.1247
Flux: 7.5432e-12 erg/s/cm2 (observed, 0.5-10 kev)
Flux_err: 6.1924e-13 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 41419
Subject
GRB 250818B: Keck redshift of the optical afterglow
Date
2025-08-18T22:13:52Z (a year ago)
From
Wen-fai Fong at Northwestern University <wfong@northwestern.edu>
Via
Web form
W. Fong, A. C. Gordon (Northwestern), A. J. Levan (Radboud), N. R. Tanvir (Leicester), Y. Dong, A. Suresh, C. Liu (Northwestern) report:
We observed the position of the SVOM short-duration GRB 250818B (Wang et al., GCN 41405) with a Swift/XRT counterpart (Ferro et al., GCN 41407). We obtained spectroscopy of the optical afterglow candidate detected by GOTO (Kumar et al., GCN 41406) and SVOM/VT (Yao et al., GCN 41409) using the Low-Resolution Imaging Spectrograph (LRIS) mounted on the Keck I telescope atop Mauna Kea in Hawaii (PI: Chang Liu; Program O397). We obtained 3x300-sec exposures using the 400/3400 grism on the blue side and the 400/8500 grating on the red side at an airmass of 1.2 in 0.9” seeing and clear conditions, starting on 2025 Aug 18 at 14:10 UT (approximately 10.68 hr post-burst). The spectrum spans approximately 3500 to 9500 Ang.
The continuum is well-detected along with several prominent absorption features. In particular, we identify absorption features of FeII (2344, 2374, 2382), MgII (2796, 2803), MgI (2852), CaII H&K, and possibly AlII (1670), at a common redshift of z=1.216 which we consider to be the redshift of the GRB. This implies an optical luminosity of ~5e45 erg/s at 0.67 hr after the burst (using Kumar et al., GCN 41406), which is on the bright side compared to the handful of short GRBs observed at these epochs.
We thank Keck Observatory staff Rita Morris and Josh Walawender for their assistance with our observing night.
GCN Circular 41417
Subject
GRB 250818B: KAIT optical observations
Date
2025-08-18T21:38:07Z (a year ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and
Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of GRB 250818B (Wang et al.,
GCN 41405) with a set of 90x60s images in the clear (roughly R)
filters, at a mid time of 8.0 hours after the trigger. We clearly
detected the optical afterglow (Kumar et al., GCN 41406; Yao et
al, GCN 41409) in the coadd image with a brightness of 19.9 +/-
0.2 mag (Vega).
GCN Circular 41409
Subject
GRB 250818B: SVOM/VT optical observation
Date
2025-08-18T07:13:51Z (a year ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Z. H. Yao, L. P. Xin, H. L. Li, Y. L. Qiu, C. Wu, Y. N. Ma, 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), Z. Q. Wang (GXU), Y. F. Liang (PMOC) report on behalf of the SVOM/VT team.
SVOM performed an automatic slew on the burst GRB 250818B triggered by SVOM/ECLAIRs (Wang et al., GCN 41405). SVOM/VT start the observations at 2025-08-18T03:32:27.5, 198.5 seconds after the burst, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data availible, the optical counterpart (GOTO25fzq, Kumar et al., GCN 41406) at the position consistent with the locations of Swift/XRT (Ferro et al., GCN 41407), was clearly detected in VT_B. The magnitudes are:
T-T0 (s) | exposure time (s) | band | mag (AB) | mag err
------------------|-------------------|------|----------|--------
198.5 | 50 | VT_B | 17.61 | 0.03
3963.5 | 50 | VT_B | 19.02 | 0.03
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 Ziqi Wang (ziqi.wang@st.gxu.edu.cn)
Please contact the BA by email if you require additional information regarding the SVOM follow-up of this burst.
GCN Circular 41407
Subject
GRB 250818B: Swift-XRT afterglow detection
Date
2025-08-18T05:50:58Z (a year ago)
Edited On
2025-08-20T15:14:34Z (a year ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
M. Ferro (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU), M.A.
Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of GRB 250818B. We
searched for X-ray sources in 604 s of Photon Counting (PC) mode data.
The total exposure at the position of the afterglow (see below) is 604
s, obtained between T0+1.7 ks and T0+2.3 ks.
An uncatalogued X-ray source is detected and is above the RASS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow. The position of this source is RA, Dec=46.0552, -3.1251
which is equivalent to:
RA (J2000): 03h 04m 13.24s
Dec(J2000): -03d 07' 30.24"
with an uncertainty of 3.6 arcsec (radius, 90% confidence). The light
curve is consistent with a constant source of mean count rate 7.1e-01
ct/sec, though only covers a short interval of time. The position is
also consistent with the fading optical counterpart candidate
GOTO25fzq/AT 2025ukm reported by GOTO (GCN Circ. 41406).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.67 (+0.26, -0.20). The
best-fitting absorption column is 8.6 (+8.1, -2.1) x 10^20 cm^-2,
consistent with the Galactic value of 6.5 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 4.0 x 10^-11 (4.5 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 8.6 (+8.1, -2.1) x 10^20 cm^-2
Galactic foreground: 6.5 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.67 (+0.26, -0.20)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/03000030.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00031.
This circular is an official product of the Swift-XRT team.
GCN Circular 41406
Subject
GRB 250818B: GOTO optical counterpart candidate
Date
2025-08-18T05:44:49Z (a year ago)
From
Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar, S. Belkin, B. P. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, D. O'Neill, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to SVOM/ECLAIRs GRB 250818B (sb25081801, Wang et al. GCN 41405). Targeted observations were performed at 2025-08-18 from 04:01:34 UT to 05:09:40 UT (from +0.54 to 1.67 h post-trigger, respectively). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm).
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks.
We identify GOTO25fzq/AT 2025ukm as a new candidate optical counterpart within the SVOM/ECLAIRs 90% localisation region. We find no evidence of this source prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). The GOTO observations, obtained 9.23 hours before the trigger, show no source at this position, with a 3-sigma limiting magnitude of L > 20.3 (AB). The source shows a clear decay and is a strong candidate optical counterpart to GRB 250818B.
Name | RA(J2000) | Dec(J2000) | T-T0 (h) | Filter | Mag (AB)
GOTO25fzq | 03:04:13.52 | -03:07:30.82 | 0.54 | L | 18.71 ± 0.14
| 1.67 | L | 19.49 ± 0.18
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica
de Canarias (IAC).
GCN Circular 41405
Subject
GRB 250818B: SVOM detection of a short burst
Date
2025-08-18T04:00:21Z (a year ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Z. Q. Wang (GXU), Y. F. Liang (PMOC), W. J. Xie, D. H.Zhao (NAOC), F. Robinet (IJCLab), L. Zhang (IHEP), report on behalf of the SVOM mission team:
At 2025-08-18T03:29:09 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250818B (SVOM burst-id sb25081801).
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 8 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 10.74 in the [5-20] keV energy band over a time window of 2.50 seconds starting at 2025-08-18T03:29:08.
The localization of the best alert is R.A., Dec. 46.0666, -3.1784 degrees:
R.A. (J2000) = 3h04m15.99s
Dec. (J2000) = -3d10m42.10s
with a 90% confidence level (C.L.) radius of 7.44 arcmin (including systematic error of 2 arcmin added in quadrature).
This burst also triggered SVOM/GRM at 2025-08-18T03:29:08 on a timescale of 1 seconds with an SNR of 6.80.
https://www.bursthub.cn//admin/static/svgrb250818B.png
SVOM slewed to the burst. However, the MXT was occulted by the Earth. The burst localization results will be published later in another GCN circular.
A SVOM ToO has been programmed for follow-up.
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 Ziqi.Wang: ziqi.wang@st.gxu.edu.cn.
Please contact the BA by email if you require additional information.