TITLE: GCN CIRCULAR NUMBER: 29410 SUBJECT: GRB 210204A: AstroSat CZTI Detection DATE: 21/02/06 10:46:42 GMT FROM: Gaurav Waratkar at IIT,Bombay G. Waratkar (IITB), V. Shenoy (IITB), A. Vibhute (IUCAA), S. Gupta (IUCAA), P. Sawant (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (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, 2020, arxiv:2011.07067) showed detection of a bright long GRB 210204A, which was also detected by Fermi-GBM (GCN #29390, #29393) and GECAM-B (GCN #29392). The source was clearly detected in the 20-200 keV energy range. The light curve showed two isolated emission peaks with the stronger peak at 2021-02-04 06:32:45 UT in all four quadrants. The measured peak count rate associated with the burst is 789 (+58, -61) cts/s above the background in the combined data of four quadrants, with a total of 18141 (+1631, -1364) cts. The initial fainter peak contributed to about 6% of the reported total counts. The local mean background count rate was 480 (+2, -3) cts/s. Using cumulative rates, we measure a T90 of 200 (+7, -10) s. It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed two isolated peaks of emission with the stronger peak at 2021-02-04 06:32:45 UT. The initial fainter peak contributed less than 5% of total counts and hence was excluded from the following analysis. The measured peak count rate is 754 (+76, -80) cts/s above the background in the combined Veto data of four quadrants, with a total of 15971 (+827, -996) cts. The local mean background count rate was 1575 (+2, -3) cts/s. We measure a T90 of 57 (+7, -5) s from the cumulative Veto light curve. CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.