Gamma-ray Burst 251016B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #42316
The Fermi GBM team reports the detection of a likely LONG GRB
At 23:59:00 UT on 16 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251016B (trigger 782351945.722452 / 251016999).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 183.6, Dec = 59.6 (J2000 degrees, equivalent to J2000 12h 14m, 59d 36'), with a statistical uncertainty of 1.0 degrees.
The angle from the Fermi LAT boresight is 46.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251016999/quicklook/glg_skymap_all_bn251016999.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/bn251016999/quicklook/glg_healpix_all_bn251016999.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251016999/quicklook/glg_lc_medres34_bn251016999.gif
GCN Circular #42320
K. Yamaoka (Nagoya U), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU), 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), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration:
The long GRB 251016B (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 42316) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 23:58:57.45 UTC on 16 October 2025 (https://cgbm.calet.jp/cgbm_trigger/flight/1444693868/index.html). The burst signal was seen by all CGBM detectors.
The burst light curve shows a single pulse that starts at T+3.1 sec, peaks at T+5.6 sec, and ends at T+12.6 sec. The T90 and T50 durations measured by the SGM data are 5.9 +/- 0.5 sec and 2.0 +/- 0.1 sec (40-1000 keV), respectively.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1444693868/
The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University.
GCN Circular #42324
Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), A. Arya (IITB), U. Pathak (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 a bright,long GRB 251016B which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 42316).
The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-10-16 23:59:03.85 UTC. The measured peak count rate associated with the burst is 1394 (+217, -85) counts/s above the background in the combined data of three quadrants (out of four), with a total of 4295 (+210, -209) counts. The local mean background count rate was 214 (+6, -8) counts/s. Using cumulative rates, we measure a T90 of 5.4 (+0.8, -0.5) s. In the preliminary analysis, we find 498 Compton events associated with this event.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-10-16 23:59:02.60 UTC. The measured peak count rate associated with the burst is 1972 (+90, -100) counts/s above the background in the combined data of all quadrants, with a total of 6747 (+290, -334) counts. The local mean background count rate was 1255 (+6, -6) counts/s. We measure a T90 of 5.2 (+1.8, -0.7) s from the cumulative Veto light curve.
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 #42332
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-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 251016B ( Fermi GBM team, GCN 42316) errorbox 64428 sec after notice time and 64463 sec after trigger time at 2025-10-17 17:53:24 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 75 deg. The sun altitude is -38.0 deg.
The galactic latitude b = 57 deg., longitude l = 131 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3016439
We obtain a following upper limits.

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   64493 | 2025-10-17 17:53:24 |   MASTER-Kislovodsk | (12h 10m 29.71s , +59d 47m 44.4s) |   C |    60 | 17.9 |        
   64493 | 2025-10-17 17:53:24 |   MASTER-Kislovodsk | (12h 02m 34.87s , +60d 07m 45.3s) |   C |    60 | 18.2 |        
   64573 | 2025-10-17 17:54:44 |   MASTER-Kislovodsk | (11h 59m 57.83s , +57d 53m 49.9s) |   C |    60 | 17.7 |        
   64653 | 2025-10-17 17:56:03 |   MASTER-Kislovodsk | (12h 28m 38.77s , +57d 56m 06.7s) |   C |    60 | 17.7 |        
   64653 | 2025-10-17 17:56:03 |   MASTER-Kislovodsk | (12h 21m 10.62s , +58d 16m 11.0s) |   C |    60 | 17.9 |        
   64733 | 2025-10-17 17:57:23 |   MASTER-Kislovodsk | (12h 23m 18.88s , +61d 41m 55.2s) |   C |    60 | 17.6 |        
   64733 | 2025-10-17 17:57:23 |   MASTER-Kislovodsk | (12h 14m 53.21s , +62d 01m 54.8s) |   C |    60 | 18.2 |        
   64976 | 2025-10-17 18:01:26 |   MASTER-Kislovodsk | (12h 32m 54.86s , +60d 10m 17.7s) |   C |    60 | 18.2 |        
   65135 | 2025-10-17 18:04:06 |   MASTER-Kislovodsk | (12h 10m 31.27s , +59d 47m 15.5s) |   C |    60 | 17.9 |        
   65135 | 2025-10-17 18:04:06 |   MASTER-Kislovodsk | (12h 02m 38.29s , +60d 07m 24.6s) |   C |    60 | 18.4 |        
   65216 | 2025-10-17 18:05:26 |   MASTER-Kislovodsk | (12h 00m 00.77s , +57d 53m 08.9s) |   C |    60 | 17.6 |        
   65296 | 2025-10-17 18:06:46 |   MASTER-Kislovodsk | (12h 28m 41.30s , +57d 55m 38.0s) |   C |    60 | 17.6 |        
   65296 | 2025-10-17 18:06:46 |   MASTER-Kislovodsk | (12h 21m 14.36s , +58d 15m 48.0s) |   C |    60 | 18.0 |        
   65781 | 2025-10-17 18:14:51 |   MASTER-Kislovodsk | (12h 23m 20.47s , +61d 42m 21.0s) |   C |    60 | 17.6 |        
   65781 | 2025-10-17 18:14:52 |   MASTER-Kislovodsk | (12h 14m 56.78s , +62d 02m 32.5s) |   C |    60 | 18.1 |        
   66021 | 2025-10-17 18:18:52 |   MASTER-Kislovodsk | (12h 33m 02.17s , +60d 07m 57.4s) |   C |    60 | 18.1 |        
   66181 | 2025-10-17 18:21:31 |   MASTER-Kislovodsk | (12h 10m 40.13s , +59d 48m 37.0s) |   C |    60 | 17.9 |        
   66181 | 2025-10-17 18:21:31 |   MASTER-Kislovodsk | (12h 02m 49.68s , +60d 08m 56.5s) |   C |    60 | 18.3 |        
   66261 | 2025-10-17 18:22:51 |   MASTER-Kislovodsk | (12h 00m 02.22s , +57d 53m 04.0s) |   C |    60 | 17.6 |        
   66341 | 2025-10-17 18:24:11 |   MASTER-Kislovodsk | (12h 28m 46.58s , +57d 54m 54.8s) |   C |    60 | 17.6 |        
   66341 | 2025-10-17 18:24:12 |   MASTER-Kislovodsk | (12h 21m 22.73s , +58d 15m 14.6s) |   C |    60 | 17.9 |        
   66632 | 2025-10-17 18:29:02 |   MASTER-Kislovodsk | (12h 23m 25.23s , +61d 42m 58.9s) |   C |    60 | 17.7 |        
   66632 | 2025-10-17 18:29:02 |   MASTER-Kislovodsk | (12h 15m 03.72s , +62d 03m 15.8s) |   C |    60 | 18.1 |        
   67078 | 2025-10-17 18:36:28 |   MASTER-Kislovodsk | (12h 33m 12.10s , +60d 09m 22.3s) |   C |    60 | 18.1 |        
   67237 | 2025-10-17 18:39:07 |   MASTER-Kislovodsk | (12h 10m 48.06s , +59d 48m 19.7s) |   C |    60 | 17.9 |        
   67237 | 2025-10-17 18:39:08 |   MASTER-Kislovodsk | (12h 02m 58.20s , +60d 08m 43.5s) |   C |    60 | 18.1 |        
   67317 | 2025-10-17 18:40:27 |   MASTER-Kislovodsk | (12h 00m 05.23s , +57d 53m 23.6s) |   C |    60 | 17.6 |        
   67397 | 2025-10-17 18:41:47 |   MASTER-Kislovodsk | (12h 28m 47.12s , +57d 54m 50.5s) |   C |    60 | 17.6 |        
   67397 | 2025-10-17 18:41:47 |   MASTER-Kislovodsk | (12h 21m 23.71s , +58d 15m 15.1s) |   C |    60 | 18.0 |        
   67687 | 2025-10-17 18:46:37 |   MASTER-Kislovodsk | (12h 23m 27.68s , +61d 40m 36.8s) |   C |    60 | 17.6 |        
   67687 | 2025-10-17 18:46:37 |   MASTER-Kislovodsk | (12h 15m 10.60s , +62d 00m 56.4s) |   C |    60 | 18.1 |        
   67926 | 2025-10-17 18:50:37 |   MASTER-Kislovodsk | (12h 33m 18.29s , +60d 07m 34.3s) |   C |    60 | 18.1 |        
   68293 | 2025-10-17 18:56:43 |   MASTER-Kislovodsk | (12h 10m 54.87s , +59d 48m 17.3s) |   C |    60 | 17.9 |        
   68293 | 2025-10-17 18:56:43 |   MASTER-Kislovodsk | (12h 02m 59.73s , +60d 08m 46.1s) |   C |    60 | 18.2 |        
   68373 | 2025-10-17 18:58:03 |   MASTER-Kislovodsk | (12h 00m 24.20s , +57d 53m 11.7s) |   C |    60 | 17.6 |        
   68452 | 2025-10-17 18:59:23 |   MASTER-Kislovodsk | (12h 28m 56.85s , +57d 52m 31.8s) |   C |    60 | 17.6 |        
   68453 | 2025-10-17 18:59:23 |   MASTER-Kislovodsk | (12h 21m 29.31s , +58d 13m 01.5s) |   C |    60 | 17.9 |        
   68753 | 2025-10-17 19:04:23 |   MASTER-Kislovodsk | (12h 23m 37.61s , +61d 41m 55.7s) |   C |    60 | 17.6 |        
   68754 | 2025-10-17 19:04:24 |   MASTER-Kislovodsk | (12h 15m 21.05s , +62d 02m 17.0s) |   C |    60 | 18.1 |        
   68994 | 2025-10-17 19:08:25 |   MASTER-Kislovodsk | (12h 33m 21.64s , +60d 08m 01.0s) |   C |    60 | 18.1 |        
   69157 | 2025-10-17 19:11:08 |   MASTER-Kislovodsk | (12h 10m 49.35s , +59d 47m 14.2s) |   C |    60 | 17.6 |        
   69157 | 2025-10-17 19:11:08 |   MASTER-Kislovodsk | (12h 03m 11.06s , +60d 07m 42.8s) |   C |    60 | 18.2 |        
   69445 | 2025-10-17 19:15:56 |   MASTER-Kislovodsk | (12h 00m 15.93s , +57d 52m 23.4s) |   C |    60 | 17.6 |        
   69525 | 2025-10-17 19:17:15 |   MASTER-Kislovodsk | (12h 28m 54.84s , +57d 54m 20.9s) |   C |    60 | 17.5 |        
   69525 | 2025-10-17 19:17:15 |   MASTER-Kislovodsk | (12h 21m 39.39s , +58d 14m 53.4s) |   C |    60 | 18.1 |        
   69815 | 2025-10-17 19:22:05 |   MASTER-Kislovodsk | (12h 23m 48.31s , +61d 42m 05.7s) |   C |    60 | 17.6 |        
   69815 | 2025-10-17 19:22:05 |   MASTER-Kislovodsk | (12h 15m 32.13s , +62d 02m 32.5s) |   C |    60 | 18.0 |        
   69975 | 2025-10-17 19:24:45 |   MASTER-Kislovodsk | (11h 58m 36.13s , +61d 27m 27.6s) |   C |    60 | 17.9 |        
   70055 | 2025-10-17 19:26:05 |   MASTER-Kislovodsk | (12h 33m 35.77s , +60d 07m 51.8s) |   C |    60 | 17.9 |

Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.

GCN Circular #42383
Matt Godwin (UAH) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 23:59:00.72 UT on 16 October 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251016B (trigger 782351945/251016999).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 183.64, Dec = 59.60 (J2000 degrees, equivalent to J2000 12h 14m, +59d 36'), with a statistical uncertainty of 1.00 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 46 degrees.
The GBM light curve consists of a single bright emission episode with a duration (T90) of about 7.1 s (50-300 keV). The time-averaged spectrum from T0-0.8 to T0+13.5 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.07 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 310 +/- 1 keV.
The event fluence (10-1000 keV) in this time interval is (2.27 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+2.4 s in the 10-1000 keV band is 37.5 +/- 0.5 ph/s/cm^2.
A Band function fits the spectrum equally well with Epeak= 222 +/- 1 keV, alpha = -0.96 +/- 0 and beta = -2 +/- 0.01.
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/"
GCN Circular #42403
A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the MGNS/BepiColombo and HEND/Mars Odyssey teams,
J. Benkhoff on behalf of the BepiColombo team,
D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team,
A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi-GBM team,
S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, A. Tohuvavohu, and J. DeLaunay on behalf of the Swift-BAT team,
and
W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report:
The long-duration GRB 251016B (Ferm-GBM detection: The Fermi GBM team, GCN 42316; Godwin et al., GCN 42383; CALET-GBM detection: Yamaoka et al., GCN 42320; AstroSat-CZTI detection: Harsha et al., GCN 42324) was detected by Fermi (GBM trigger 782351945), CALET (GBM), AstroSat (CZTI), Konus-Wind, Swift (BAT), Mars-Odyssey (HEND), and BepiColombo (MGNS), at about 86340 s UT (23:59:00). The burst was outside the coded field of view of the BAT.
We have triangulated it to a preliminary, 3 sigma error box whose coordinates are:
```
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  180.582 (12h 02m 20s) +57.856 (+57d 51' 21")
 Corners:
  173.603 (11h 34m 25s) +54.048 (+54d 02' 53")
  182.331 (12h 09m 19s) +58.703 (+58d 42' 09")
  191.445 (12h 45m 47s) +61.827 (+61d 49' 37")
  178.988 (11h 55m 57s) +57.018 (+57d 01' 07")
 ---------------------------------------------
```
The error box area is 2295 sq. arcmin, and its maximum dimension is 12.186 deg (the minimum one is 5 arcmin). The Sun distance was 69 deg.
This localization may be improved.
A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB251016_T86340/IPN/ The HEALPix triangulation map is the multi-order HEALPix in units of probability density.
The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular.
GCN Circular #42448
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report:
The long-duration GRB 251016B (Ferm-GBM detection: The Fermi GBM team, GCN 42316; Godwin and Meegan, GCN 42383; CALET-GBM detection: Yamaoka et al., GCN 42320; AstroSat-CZTI detection: Harsha et al., GCN 42324; IPN triangulation: Kozyrev et al., GCN 42403) triggered Konus-Wind at T0=86340.495 s UT (23:59:00.495).
The burst light curve shows a single pulse which starts at ~T0-0.5 s and has a total duration of ~18.5 s. The emission is seen up to ~6 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB251016_T86340/
As observed by Konus-Wind, the burst had a fluence of 3.50(-0.18,+0.18)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+2.320 s, of 1.22(-0.11,+0.11)x10^-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst (measured from T0 to T0+11.520 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.98(-0.06,+0.06), the high energy photon index beta = -2.29(-0.10,+0.08), the peak energy Ep = 226(-18,+19) keV (chi2 = 93/85 dof).
The spectrum near the maximum count rate (measured from T0+0.256 to T0+3.328 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.85(-0.05,+0.06), the high energy photon index beta = -2.41(-0.11,+0.09), the peak energy Ep = 294(-21,+21) keV (chi2 = 67/67 dof).
All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.
GCN Circular #42602
Daniel Shy (a), C.C. Cheung (a), Bernard Phlips (a), Michael Streicher (b), James Mason (b), Douglas M. Groves (b), Feng Zhang (b), and Willy Kaye (b)
(a) U.S. Naval Research Laboratory, 4555 Overlook Ave SW, Washington, DC 20375 (b) H3D, Inc., 812 Avis Dr., Ann Arbor, MI 48108, USA
The cadmium zinc TElluride Radiation Imager (TERI) gamma-ray telescope [1], operating on the International Space Station, reports the detection of GRB 251016B, which was also detected by Fermi/GBM (GCN 42316, 42383), CALET (GCN 42320), AstroSat/CZTI (GCN 42324), IPN triangulation (GCN 42403), Konus-Wind (GCN 42448)
Using 1 Hz binning, and adopting T0 = 2025-10-16T23:59:00.72, we observe a single peak at T0+2.5s with an excess of 83 counts per second (cps) above the 39 cps baseline rate observed in nearby off-source intervals.
The analysis results presented here are preliminary and currently lack a detailed response function.
TERI is an Office of Naval Research funded technology demonstrator for large-volume pixelated CdZnTe detectors developed, built, and operated by the U.S. Naval Research Laboratory (NRL) in conjunction with H3D, Inc. The pixelated CdZnTe detectors have an energy range of 40 keV to 3 MeV per pixel. It was launched on 2025 April 21 aboard the Department of Defense Space Test Program’s STP-H10 to the ISS. On the ISS, it is located on the SOX external payload facility on the Columbus module. Analysis of TERI data at NRL is supported under contract with NASA as correlative science for the Fermi Gamma-ray Space Telescope.
[1] Shy, Daniel, et al. "Development of the cadmium zinc TElluride Radiation Imager." Journal of Astronomical Telescopes, Instruments, and Systems 10.4 (2024): 044009-044009 (arXiv:2408.04559).
Distribution Statement A: Approved for public release. Distribution is unlimited.

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