TITLE: GCN CIRCULAR NUMBER: 40199 SUBJECT: GRB 250421B: AstroSat CZTI detection of a bright long burst DATE: 25/04/22 05:05:30 GMT FROM: Gaurav Waratkar at IIT Bombay gauravwaratkar@iitb.ac.in

M. Tembhurnikar (IUCAA), G. Waratkar (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 250421B.

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 2025-04-21 21:59:57.50 UTC. The measured peak count rate associated with the burst is 586 (+45, -49) counts/s above the background in the combined data of three quadrants (out of four), with a total of 3514 (+233, -248) counts. The local mean background count rate was 209 (+3, -4) counts/s. Using cumulative rates, we measure a T90 of 17 (+1, -1) 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 2025-04-21 21:59:57.06 UTC. The measured peak count rate associated with the burst is 1713 (+87, -96) counts/s above the background in the combined data of all quadrants, with a total of 9802 (+468, -504) counts. The local mean background count rate was 1168 (+7, -8) counts/s. We measure a T90 of 17 (+1, -1) 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

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