GRB 250918A
GCN Circular 41889
Subject
Fermi GRB 250918A: Global MASTER-Net observations report
Date
2025-09-19T18:30:47Z (2 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-SAAO robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 250918A ( Fermi GBM team, GCN 41865) errorbox 1 days 18846 sec after notice time and 1 days 18882 sec after trigger time at 2025-09-19 17:47:00 UT, with upper limit up to 15.6 mag. Observations started at twilight. The observations began at zenith distance = 60 deg. The sun altitude is -16.5 deg.
The galactic latitude b = 38 deg., longitude l = 340 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2994371
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
105313 | 2025-09-19 17:47:00 | MASTER-SAAO | (14h 44m 17.11s , -15d 37m 24.9s) | C | 60 | 15.6 |
105313 | 2025-09-19 17:47:00 | MASTER-SAAO | (14h 46m 06.96s , -15d 52m 16.6s) | C | 60 | 14.9 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41887
Subject
GRB 250918A: SVOM/GRM observation
Date
2025-09-19T16:42:33Z (2 months ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
SVOM/GRM team: Chen-Wei Wang, Chao Zheng, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Feliu Lacreu (IAP), Stéphane Schanne (CEA)
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by a burst GRB 250918A (SVOM trigger reference: sb25091803) at 2025-09-18T12:32:24.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#41865).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 34.0 +8.0/-4.0 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250918A.png
In addition, the position of this burst, as determined by Fermi/GBM (RA= 221.8, DEC= -15.9, GCN#41865), is located at about 114 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.
With this localization, the time-averaged spectrum from T0-12 to T0+30 s is best fitted by a power law function. The power law index is 2.64 +/-0.15. The event fluence (10-1000 keV) in this time interval is (3.28 +/-0.05)E-05 erg/cm^2.
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: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)
GCN Circular 41866
Subject
GRB 250918A: BALROG localization (Fermi Trigger 779891543 / GRB 250918522)
Date
2025-09-18T13:33:35Z (2 months ago)
From
Jochen Greiner at MPE <jcgrog@mpe.mpg.de>
Via
email
T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:
The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
779891543 at 12:32:18 on 18 Sept. 2025 were automatically fitted for spectrum
and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427;
Berlato et al. 2019, ApJ 873, 60).
The best-fit position is:
RA(2000.0) = 225.6 deg
Decl.(2000.0) = -16.2 deg
The 1 sigma statistical error radius is 4.1 deg.
We estimate an additional systematic error of 2 deg.
Further details are available at:
https://grb.mpe.mpg.de/grb/GRB250918522/
The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB250918522/healpix
The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB250918522/json
GCN Circular 41865
Subject
GRB 250918A: Fermi GBM Final Real-time Localization
Date
2025-09-18T12:42:57Z (2 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 12:32:18 UT on 18 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250918A (trigger 779891543.0931 / 250918522).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 221.8, Dec = -15.9 (J2000 degrees, equivalent to J2000 14h 47m, -15d 54'), with a statistical uncertainty of 3.4 degrees.
The angle from the Fermi LAT boresight is 93.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250918522/quicklook/glg_skymap_all_bn250918522.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250918522/quicklook/glg_healpix_all_bn250918522.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250918522/quicklook/glg_lc_medres34_bn250918522.gif