GRB 250926A
GCN Circular 42027
Subject
GRB 250926A: CALET Gamma-Ray Burst Monitor detection
Date
2025-09-29T15:19:54Z (13 days ago)
From
Yuta Kawakubo at Aoyama Gakuin University <kawakubo@phys.aoyama.ac.jp>
Via
Web form
S. Ricciarini (U of Florence), 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, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC),
M. L. Cherry (LSU), P. S. Marrocchesi (U of Siena),
and the CALET collaboration:
The GRB 250926A (Fermi GBM Final Real-time Localization:
Fermi GBM team, GCN Circ. 41993; AstroSat CZTI detection: Goyal
et al., GCN Circ. 42008; Insight-HXMT detection: Wang et al., GCN Circ.
42013; triangulation location with Insight-HXMT/HE and Fermi/GBM: Zhang
et al., GCN Circ. 42019) triggered the CALET Gamma-ray Burst Monitor (CGBM)
at 00:21:23.82 UTC on 26 September 2025
(https://cgbm.calet.jp/cgbm_trigger/flight/1442880852/index.html).
The burst signal was seen by only the SGM detector.
The burst light curve shows a single pulse that starts
at T+0.75 sec, peaks at T+1.08 sec, and ends at T+1.35 sec.
The T90 and T50 durations measured by the SGM data are 0.55 +/- 0.11 sec
and 0.25 +/- 0.10 sec (40-1000 keV), respectively.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1442880852/
The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.
GCN Circular 42019
Subject
GRB 250926A: triangulation location with Insight-HXMT/HE and Fermi/GBM
Date
2025-09-28T11:53:08Z (14 days ago)
From
zhangjinpeng@ihep.ac.cn
Via
Web form
Jin-Peng Zhang, Cheng-Kui Li, Chen-Wei Wang and Shao-Lin Xiong report on behalf of the Insight-HXMT team:
GRB 250926A was detected by Fermi/GBM (the Fermi GBM team, GCN 41993), Insight-HXMT (Chen-Wei Wang et al., GCN 42013) and AstroSat CZTI (A. Goyal et al., GCN 42008). A possible association with Orion Nebula was suggested according to the Fermi/GBM localization result of this burst (Rosa L. Becerra et al., GCN 42000).
To evaluate this association, we triangulate GRB 250926A with Insight-HXMT/HE and Fermi/GBM event-by-event data using the Li-CCF method [1]. The resulted localization annulus with 3-sigma error (statistical only) is as follows:
| Annulus Center RA (J2000, deg) | Annulus Center Dec (J2000, deg) | Annulus Radius (deg) |
| ------------------------------ | ------------------------------- | -------------------- |
| 356.91 | 27.35 | 113 +11/-10 |
According to this result, the Orion Nebula falls well out of the 3-sigma region of the triangulation annulus of GRB 250926A, thus they are very likely unrelated. Further improvement in the localization of this GRB is possible with more data from other instruments.
[1] Xiao, S. et al. ApJ 920, 43 (2021). https://doi.org/10.3847/1538-4357/ac1420
GCN Circular 42013
Subject
GRB 250926A: Insight-HXMT detection
Date
2025-09-27T15:01:50Z (15 days ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team:
At 2025-09-26T00:21:24.600 (T0), Insight-HXMT/HE was trigger by the burst GRB 250926A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN#41993) and AstroSat (A. Goyal et al., GCN#42008).
The Insight-HXMT/HE light curve mainly consists of a precursor followed by a main pulse with a T90 of 2.1 +2.3/-0.7 s. The 1s peak rate, measured from T0-0.050 s, is 3838 cnts/sec. The total counts from this burst is 4746 counts.
The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb250926A.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.
GCN Circular 42008
Subject
GRB 250926A: AstroSat CZTI detection
Date
2025-09-26T18:53:22Z (15 days ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
A. Goyal (IITB), A. Arya (IITB), S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), 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 250926A which was also detected by Fermi/GBM (GCN Circ. 41993).
The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-09-26 00:21:21.75 UTC. The measured peak count rate associated with the burst is 739 (+163, -167) counts/s above the background in the combined data of all four quadrants, with a total of 396 (+100, -119) counts. The local mean background count rate was 265 (+15, -17) counts/s. Using cumulative rates, we measure a T90 of 1.5 (+0.3, -0.7) s.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-09-26 00:21:21.04 UTC. The measured peak count rate associated with the burst is 763 (+72, -74) counts/s above the background in the combined data of all quadrants, with a total of 891 (+229, -239) counts. The local mean background count rate was 1162 (+8, -9) 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
GCN Circular 42000
Subject
GRB 250926A: DDOTI Optical Observations and Spatial Coincidence with the Orion Nebula
Date
2025-09-26T12:19:19Z (16 days ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Sahil Atri (U Roma), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report:
We observed the field of GRB 250926A detected by Fermi/GBM (Fermi GBM Team GCN Circ. 41993) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the night of 2025-09-26 UTC.
DDOTI imaged the GBM error region (Fermi GBM Team, GCN Circ. 41993) from 09:06 to 11:53 UTC (T+8.6 to T+11.5 hours after the trigger), covering the entire region and obtaining a total exposure of 64 minutes.
Comparing our observations to the USNO-B1 and Pan-STARRS PS1 DR2 catalogues, we detect no uncatalogued fading sources within the observed field down to a 10-sigma limiting AB magnitude of:
w > 20.3
This value is not corrected for Galactic extinction.
We note that the Orion Nebula (d ~400 pc) lies within the error region reported by the Fermi/GBM Team. Given the possible association with this nearby star-forming region and the moderate duration of the signal (T90 ~ 5 s), we strongly encourage follow-up observations of this event.
Further analysis is ongoing.
We thank the staff of the Observatorio Astronómico Nacional at the Sierra de San Pedro Mártir, especially María H. Riesgo Tirado, for their invaluable assistance during the night.
GCN Circular 41994
Subject
Fermi GRB 250926A: Global MASTER-Net observations report
Date
2025-09-26T01:31:05Z (16 days 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 250926A ( Fermi GBM team, GCN 41993) errorbox 71 sec after notice time and 114 sec after trigger time at 2025-09-26 00:23:18 UT, with upper limit up to 19.5 mag. The observations began at zenith distance = 53 deg. The sun altitude is -47.4 deg.
The galactic latitude b = -20 deg., longitude l = 210 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3000001
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
124 | 2025-09-26 00:23:18 | MASTER-SAAO | (05h 28m 42.38s , -08d 02m 54.5s) | C | 20 | 18.4 |
159 | 2025-09-26 00:23:18 | MASTER-SAAO | (05h 28m 42.37s , -08d 02m 54.4s) | C | 90 | 19.4 | Coadd
124 | 2025-09-26 00:23:18 | MASTER-SAAO | (05h 30m 34.49s , -08d 18m 14.3s) | C | 20 | 18.4 |
159 | 2025-09-26 00:23:18 | MASTER-SAAO | (05h 30m 34.49s , -08d 18m 14.2s) | C | 90 | 19.5 | Coadd
168 | 2025-09-26 00:23:57 | MASTER-SAAO | (05h 28m 42.29s , -08d 02m 53.3s) | C | 30 | 18.6 |
168 | 2025-09-26 00:23:57 | MASTER-SAAO | (05h 30m 34.43s , -08d 18m 12.4s) | C | 30 | 18.7 |
222 | 2025-09-26 00:24:46 | MASTER-SAAO | (05h 28m 42.23s , -08d 02m 51.9s) | C | 40 | 18.8 |
222 | 2025-09-26 00:24:46 | MASTER-SAAO | (05h 30m 34.40s , -08d 18m 10.2s) | C | 40 | 18.8 |
286 | 2025-09-26 00:25:45 | MASTER-SAAO | (05h 28m 42.20s , -08d 02m 50.5s) | C | 50 | 18.9 |
286 | 2025-09-26 00:25:45 | MASTER-SAAO | (05h 30m 34.36s , -08d 18m 08.0s) | C | 50 | 19.0 |
360 | 2025-09-26 00:26:54 | MASTER-SAAO | (05h 28m 42.14s , -08d 02m 49.6s) | C | 60 | 19.1 |
360 | 2025-09-26 00:26:54 | MASTER-SAAO | (05h 30m 34.29s , -08d 18m 06.4s) | C | 60 | 19.2 |
651 | 2025-09-26 00:31:44 | MASTER-SAAO | (05h 28m 05.28s , -07d 01m 44.0s) | C | 60 | 18.7 |
711 | 2025-09-26 00:31:44 | MASTER-SAAO | (05h 28m 05.28s , -07d 01m 44.0s) | C | 180 | 19.4 | Coadd
651 | 2025-09-26 00:31:44 | MASTER-SAAO | (05h 29m 57.17s , -07d 16m 59.6s) | C | 60 | 18.8 |
711 | 2025-09-26 00:31:44 | MASTER-SAAO | (05h 29m 57.17s , -07d 16m 59.6s) | C | 180 | 19.5 | Coadd
731 | 2025-09-26 00:33:04 | MASTER-SAAO | (05h 28m 05.23s , -07d 01m 43.2s) | C | 60 | 18.7 |
731 | 2025-09-26 00:33:04 | MASTER-SAAO | (05h 29m 57.12s , -07d 16m 58.5s) | C | 60 | 18.8 |
810 | 2025-09-26 00:34:23 | MASTER-SAAO | (05h 28m 05.16s , -07d 01m 43.1s) | C | 60 | 18.7 |
810 | 2025-09-26 00:34:23 | MASTER-SAAO | (05h 29m 57.08s , -07d 16m 58.0s) | C | 60 | 18.8 |
890 | 2025-09-26 00:35:43 | MASTER-SAAO | (05h 28m 05.10s , -07d 01m 42.8s) | C | 60 | 18.6 |
890 | 2025-09-26 00:35:43 | MASTER-SAAO | (05h 29m 57.06s , -07d 16m 57.6s) | C | 60 | 18.8 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41993
Subject
GRB 250926A: Fermi GBM Final Real-time Localization
Date
2025-09-26T00:31:52Z (16 days 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 00:21:23 UT on 26 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250926A (trigger 780538888.96614 / 250926015).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 83.0, Dec = -7.0 (J2000 degrees, equivalent to J2000 05h 31m, -7d 00'), with a statistical uncertainty of 1.8 degrees.
The angle from the Fermi LAT boresight is 79.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926015/quicklook/glg_skymap_all_bn250926015.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/bn250926015/quicklook/glg_healpix_all_bn250926015.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250926015/quicklook/glg_lc_medres34_bn250926015.gif