GRB 250922A

SVOM/GRM team: Wen-Jun Tan, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Marius Brunet (IRAP) and Stéphane Schanne (CEA) 

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 250922A (SVOM burst-id sb25092201) at 2025-09-22T03:07:21.000 (T0), which is also detected by FERMI/GBM (GBM team, GCN # 41933)

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 15.89 +10.74/-1.52s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250922A.png

In addition, the position of this burst, as determined by Fermi/GBM (RA = 353.54, Dec = 0.42, GCN #41933
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 ), is located at about 50 degrees from the SVOM optical axis, which is just outside the ECLAIRs field of view.

With this localization given by FERMI/GBM, the time-averaged spectrum from T0-5 to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.61 +0.22/-0.40 and the cutoff energy, parameterized as Epeak, is 473 +165/-99 keV. The event fluence (10-1000 keV) in this time interval is (2.03 +0.33/-0.31)E-06 erg/cm^2. 

The localization of GRB 250922A in the 'Amati' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb250922A_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)

The Fermi GBM team reports the detection of a likely LONG GRB

"At 03:07:21.46 UT on 22 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250922A (trigger 780203246/250922130).
This trigger was initially classified as Below Horizon by the flight software,
but is in fact a GRB.

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 353.54, Dec = 0.42 (J2000 degrees, equivalent to J2000 23h 34m, +0d 25'),
with a statistical uncertainty of 6.13 degrees.

The angle from the Fermi LAT boresight is 86 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250922130/quicklook/glg_skymap_all_bn250922130.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/bn250922130/quicklook/glg_healpix_all_bn250922130.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250922130/quicklook/glg_lc_medres34_bn250922130.gif"