GRB 251001A

Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of GRB 251001A which was also detected by Fermi/GBM (GCN Circ. 41993
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 ), Insight-HXMT (Wang et al., 42057). We detect only the second pulse as seen in the GBM lightcurve because the source is likely at a sky position where our effective area is low.

The source was detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-10-01 02:40:59.96 UTC. The measured peak count rate associated with the burst is 372 (+65, -75) counts/s above the background in the combined data of all four quadrants, with a total of 333 (+172, -206) counts. The local mean background count rate was 1380 (+11, -11) counts/s. Due to the intrinsic 1 s binning of veto data, we cannot reliably estimate a T90 from it.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

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

SVOM/ECLAIRs team: Frédéric Piron (LUPM), Olivier GODET (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a burst GRB 251001A (SVOM trigger reference: sb25100101) at 2025-10-01T02:40:58.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#42034
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 ) and Insight-HXMT (Wang et al., GCN#42056
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 ). 

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 7.2 +1.8/-0.4 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb251001A.png

In addition, the position of this burst, as determined by Fermi/GBM (RA= 224.4, DEC= -56.2, GCN#42034
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 ), is located at about 101 degrees from the SVOM optical axis, which is outside the field of view of ECLAIRs. Nevertheless, ECLAIRs detected the GRB through the shield above 50 keV. 

With this localization, the time-averaged spectrum from T0-2 to T0+7 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.73 +0.41/-0.32 and the cutoff energy, parameterized as Epeak, is 51 +/-7 keV. The event fluence (10-1000 keV) in this time interval is (7.92 +0.94/-0.92)E-06 erg/cm^2. 

The localization of GRB 251001A in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb251001A_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: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)

Chen-Wei Wang, Zheng-Hang Yu, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team:

At 2025-10-01T02:40:57.600 (T0), Insight-HXMT/HE was trigger by the burst GRB 251001A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN#42034
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 ).

The Insight-HXMT/HE light curve mainly consists of multi-pulses with a T90 of 8.2 +/-1.6 s. The 1s peak rate, measured from T0+5.300 s, is 759 cnts/sec. The total counts from this burst is 2439 counts.

The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb251001A.png

All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. 

Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.

M. Dafcikova (MUNI), S. Lesage (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 02:40:56.95 UT on 01 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251001A (trigger 780979261/251001112).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 224.44, Dec = -56.23 (J2000 degrees, equivalent to
J2000 14h 57m, -56d 13'), with a statistical uncertainty of 1.79 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a
systematic error which we have characterized as a mixture of two Gaussians,
one with a radius of 1.8 degrees (52% contribution) and one with a radius
of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).

The angle from the Fermi LAT boresight is 116 degrees.

The GBM light curve consists of two peaks with a duration (T90)
of about 7 s (50-300 keV). The time-averaged spectrum
from T0+0.003 to T0+8.640 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.63 +/- 0.08 and the cutoff energy,
parameterized as Epeak, is 138 +/- 7 keV.

The event fluence (10-1000 keV) in this time interval is
(4.4 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+6.0 s in the 10-1000 keV band is 24.8 +/- 0.6 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/"

T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:

The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
780979261 at 02:40:56 on 01 Oct. 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) = 227.4 deg
Decl.(2000.0) = -57.5 deg
The 1 sigma statistical error radius is 2.5 deg.
We estimate an additional systematic error of 1 deg.

Further details are available at:
https://grb.mpe.mpg.de/grb/GRB251001112/

The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB251001112/healpix

The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB251001112/json

                        

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

At 02:40:56 UT on 1 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251001A (trigger 780979261.951484 / 251001112).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 224.4, Dec = -56.2 (J2000 degrees, equivalent to J2000 14h 57m, -56d 12'), with a statistical uncertainty of 1.8 degrees.

The angle from the Fermi LAT boresight is 116.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251001112/quicklook/glg_skymap_all_bn251001112.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/bn251001112/quicklook/glg_healpix_all_bn251001112.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251001112/quicklook/glg_lc_medres34_bn251001112.gif