GCN Circular 41843

Subject
GRB 250916A: AstroSat CZTI detection of a bright long burst
Event
GRB 250916A
Date
2025-09-16T18:23:31Z (5 days ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
A. Arya (IITB), A. Goyal (IITB), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), S. Salunke (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 a bright long-duration GRB 250916A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 41839Loading...
 
 ), and CALET (Trig ID. 1442064382). Note that we detected the bright burst roughly 250 s after the initial fainter spike as seen in the GBM lightcurve. 

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 2025-09-16 13:33:28.50 UTC. The measured peak count rate associated with the burst is 79 (+29, -4) counts/s above the background in the combined data of two quadrants (out of four), with a total of 1695 (+232, -241) counts. We caution that there is a 0.3 s readout dead time in CZT data during the burst which affects the calculated total counts. The local mean background count rate was 135 (+1, -2) counts/s. Using cumulative rates, we measure a T90 of 48 (+2, -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 2025-09-16 13:33:26.00 UTC. The measured peak count rate associated with the burst is 556 (+80, -24) counts/s above the background in the combined data of all quadrants, with a total of 13048 (+742, -896) counts. The local mean background count rate was 1180 (+5, -4) counts/s. We measure a T90 of 50 (+2, -5) 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