GRB 250902A

T. Tamura (Kanagawa U), A. Yoshida, T. Sakamoto, S. Sugita,
Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA),
Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), 
Y. Shimizu (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU),
S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The long GRB 250902A (Fermi GBM Final Real-time Localization: 
Fermi GBM team, GCN Circ. 41648
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 ;  SVOM/GRM observation: Wang et al.,
GCN Circ. 41653
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 ; Fermi GBM Detection: Bala et al., GCN Circ. 41662
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 ;
GECAM-B detection: Zheng et al., GCN Circ. 41687
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 ; VZLUSAT-2 detection:
Ripa et al., GCN Circ. 41701
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 ) triggered the CALET Gamma-ray Burst 
Monitor (CGBM) at 01:30:01.56 UTC on 2 September 2025
(https://cgbm.calet.jp/cgbm_trigger/flight/1440811415/index.html).
The burst signal was seen by all CGBM detectors.  Because of a problem with the ground alert processing script, the GCN notice was not distributed automatically for this event.

The burst light curve shows a multi-peaked structure that starts
at T-0.6 sec, peaks at T+2.9 sec, and ends at T+6.8 sec.
The T90 and T50 durations measured by the SGM data are 6.1 +/- 0.3 sec
and 2.5 +/- 0.3 sec (40-1000 keV), respectively.

The ground-processed light curve is available at

https://cgbm.calet.jp/cgbm_trigger/ground/1440811415/

The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.

J. Ripa, M. Dafcikova (Masaryk U.), A. Pal (Konkoly Observatory), N. Werner  (Masaryk U.), M. Ohno (Hiroshima U.),  L. Meszaros, B. Csak (Konkoly Observatory), H. Takahashi (Hiroshima U.), F. Munz , M. Topinka, F. Hroch, N. Husarikova, J.-P. Breuer (Masaryk U.), J. Hudec, J. Kapus, M. Frajt, M. Rezenov (Spacemanic s.r.o), R. Laszlo (Needronix), G. Galgoczi (Wigner Research Center/Eotvos U.), N. Uchida (ISAS/JAXA), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), Y. Fukazawa, K. Hirose, H. Matake (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), L. L. Kiss (Konkoly Observatory),  T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), K. Torigoe (Hiroshima U.), P. Svoboda, V. Daniel, J. Dudas, M. Junas, J. Gromes (VZLU), I. Vertat (FEL ZCU)  -- the VZLUSAT-2/GRB payload collaboration.

The long-duration GRB 250902A (Fermi/GBM detection GCN 41662
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 ; NuSTAR detection trigger time 2025-09-02 01:29:54 UTC; SVOM detection GCN 41653
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 ; GECAM-B detection GCN 41687
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 ) was detected by the GRB detectors on board the VZLUSAT-2 3U CubeSat (https://www.vzlusat2.cz/en/).

The data acquisition was performed by the GRB detector unit no. 1. The detection was confirmed at the peak time 2025-09-02 01:30:02 UTC. The T90 duration is 8 s and the significance during T90 reaches 29 sigma.

The light curve obtained by VZLUSAT-2 is available here:
https://vzlusat2.konkoly.hu/static/share/GRB250902A_GCN_VZLUSAT2.pdf

All VZLUSAT-2 detections are listed at: https://monoceros.physics.muni.cz/hea/VZLUSAT-2/
The GRB detectors on VZLUSAT-2 are a demonstration payload for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). Two GRB modules of VZLUSAT-2 are placed in a perpendicular manner and each consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~30 keV to ~1000 keV. VZLUSAT-2 was launched on 2022 January 13 from Cape Canaveral.

Chao Zheng, Chen-Wei Wang, Shao-Lin Xiong and Yue Huang (IHEP) report on behalf of GECAM team:

GECAM-B was triggered on-ground by GRB 250902A at 2025-09-02T01:30:00.550 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #41648
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 ) and SVOM/GRM (Wang et al., GCN #41653
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 ). According to the GECAM-B light curves in about 40-6000 keV, this burst mainly consists of a multi-pulses with a duration (T90) of 10.5 +1.0/-1.5 s.

The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb250902A.png

GECAM-B localized this burst to the following position (J2000): 
Ra: 166.6 deg 
Dec: 20.5 deg
Err: 0.7 deg (1-sigma, statistical only)

The time-averaged spectrum from T0-2 s to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.15 +0.14/-0.14 and the cutoff energy, parameterized as Epeak, is 185 +23/-16 keV. The event fluence (10-1000 keV) in this time interval is (2.69 +/-0.19)E-05 erg/cm^2.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

S. Bala (USRA) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 01:30:00.49 UT on 02 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250902A (trigger 778469405/250902062).
The Fermi-GBM position was reported in GCN 41648
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  (Fermi GBM Team 2025).

The angle from the Fermi LAT boresight is 78 degrees.

The GBM light curve consists of a bright pulse with a duration (T90)
of about 8.4 s (50-300 keV). The time-averaged spectrum
from T0-1.0 to T0+17.4 s is best fit by Band function with
a Band function with Epeak = 257 +/- 13 keV,
alpha = -1.25 +/- 0.02, and beta = -2.31 +/- 0.1.

A power law function with an exponential high-energy cutoff fits
the spectrum equally well with a with power law index = -1.28 +/- 0.01
and Epeak = 294 +/- 10 keV.

The event fluence (10-1000 keV) in this time interval is
(3.06 +/- 0.03)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+3.5 s in the 10-1000 keV band is 43.5 +/- 0.5 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/"

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

SVOM/ECLAIRs team: Tais Maiolino (LUPM)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a burst GRB 250902A (SVOM trigger reference: sb25090201) at 2025-09-02T01:30:01.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #41648
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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 6.9 +0.5/-0.3 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250902A.png

In addition, the position of this burst, as determined by Fermi/GBM (RA = 159.3, Dec = 16.3, GCN #41648
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 ), is located at about 164 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. ECLAIRs was not collecting data at the time of this burst.

With the localization from Fermi/GBM, the time-averaged spectrum from T0-1 to T0+15 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.66 +0.06/-0.06 and the cutoff energy, parameterized as Epeak, is 431 +128/-76 keV. The event fluence (10-1000 keV) in this time interval is (4.47 +0.10/-0.10)E-05 erg/cm^2. 

The 'Amati' relation diagram of GRB 250902A is shown at:  
https://www.bursthub.cn//admin/static/grb250902A_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)

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

At 01:30:00 UT on 2 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250902A (trigger 778469405.491528 / 250902062).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 159.3, Dec = 16.3 (J2000 degrees, equivalent to J2000 10h 37m, 16d 18'), with a statistical uncertainty of 1.0 degrees.

The angle from the Fermi LAT boresight is 78.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902062/quicklook/glg_skymap_all_bn250902062.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/bn250902062/quicklook/glg_healpix_all_bn250902062.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250902062/quicklook/glg_lc_medres34_bn250902062.gif