GRB 251023C
GCN Circular 42429
R. Woolf, C.C. Cheung, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 251023C, which was also detected by Swift/BAT-GUANO (GCN 42412), CALET (GCN 42414), and AstroSat CZTI (GCN 42419).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-10-23 16:24:17.240 with a duration of 30.7 s and a total significance of about 110 sigma. The light curve comprises two primary peaks at ~T0+10s and ~T0+14s, and a tertiary peak around ~T0+28s. The Glowbug onset is ~7.7 s earlier than the CALET-reported T0 because we detect low-level emission prior to the first peak at ~T0+10s.
The best-fit localization is RA, Decl. (J2000, deg) = 333.9, 58.4 with a radius of 2.1 deg (95% confidence), with a highly uncertain systematic uncertainty. We note that this localization is offset by ~40 deg from the Swift/BAT-GUANO-reported localization.
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS, and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12.
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular 42419
S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (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 a long,bright GRB 251023C which was also detected by SWIFT BAT (DeLaunay et. al., GCN Circ. 42412), and CALET (Asaoka et. al., GCN Circ. 42414).
The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-10-23 16:24:32.50 UTC. The measured peak count rate associated with the burst is 682 (+55, -51) counts/s above the background in the combined data of all quadrants, with a total of 4344 (+401, -229) counts. The local mean background count rate was 294 (+1, -7) counts/s. Using cumulative rates, we measure a T90 of 18.2 (+2.8, -1.6) s.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-10-23 16:24:28.05 UTC. The measured peak count rate associated with the burst is 942 (+83, -89) counts/s above the background in the combined data of all quadrants, with a total of 5554 (+521, -528) counts. The local mean background count rate was 1676 (+7, -8) counts/s. We measure a T90 of 15.8 (+1.9, -6.2) 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 42414
Y. Asaoka (ICRR), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), 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 251023C (Swift/BAT-GUANO localization skymap: DeLaunay et al.,
GCN Circ 42412) triggered the CALET Gamma-ray Burst Monitor (CGBM) at
16:24:24.93 UTC on 23 October 2025
(https://cgbm.calet.jp/cgbm_trigger/flight/1445271383/index.html).
The burst signal was seen by all CGBM detectors.
The burst light curve shows a multi-peaked structure that starts
at T+1.9 sec, peaks at T+3.7 sec, and ends at T+23.4 sec.
The T90 and T50 durations measured by the SGM data are 18.7 +/- 1.4 sec
and 5.7 +/- 0.8 sec (40-1000 keV), respectively.
The ground-processed light curve is available at
https://cgbm.calet.jp/cgbm_trigger/ground/1445271383/
The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.
GCN Circular 42412
James DeLaunay (PSU), Samuele Ronchini (GSSI), Aaron Tohuvavohu (Caltech), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC), Maia Williams (Northwestern) report:
Swift/BAT did not localize GRB 251023C onboard (T0: 2025-10-23T16:24:24.92 UTC, CALET Trig 1445271383)
The CALET notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 90 seconds of BAT event-mode data from [-45,+45] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 26.0 in a 8.192 s analysis time bin, starting at T0 + 2.048 s.
Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)
The 90% credible area is 1,100 deg2 and the 50% credible area is 229 deg2. The integrated probability inside the coded field of view is <1%.
A plot of the probability skymap can be viewed here:
The probability skymap file can be downloaded from the link here
Instructions on how to read and manipulate this map can be found here:
https://guano.swift.psu.edu/documentation
More details about this burst can be found on the trigger report page here:
https://guano.swift.psu.edu/trigger_report?id=782929501
GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/