TITLE: GCN CIRCULAR NUMBER: 41537 SUBJECT: GRB 250821B: AstroSat CZTI detection DATE: 25/08/25 09:32:36 GMT FROM: Gaurav Waratkar at IIT Bombay <gauravwaratkar@iitb.ac.in> M. Tembhurnikar (IUCAA), A. Arya (IITB), 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 GRB 250821B which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 41473 and 41477), SVOM/GRM (SVOM team, GCN Circ. 41486), Konus-Wind (Frederiks et al., GCN Circ. 41508), and Insight-HXMT (Wang et al., GCN Circ. 41533). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-08-21 16:07:47.95 UTC. The measured peak count rate associated with the burst is 343 (+153, -46) counts/s above the background in the combined data of all quadrants, with a total of 218 (+60, -62) counts. The local mean background count rate was 252 (+10, -12) counts/s. Using cumulative rates, we measure a T90 of 1.2 (+0.2, -0.5) s. The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-08-21 16:07:47.16 UTC. The measured peak count rate associated with the burst is 1724 (+84, -94) counts/s above the background in the combined data of all quadrants, with a total of 2373 (+175, -194) counts. The local mean background count rate was 1175 (+9, -11) counts/s. We measure a T90 of 1.9 (+1.0, -0.2) 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 View this GCN Circular online at https://gcn.nasa.gov/circulars/41537. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...