TITLE: GCN CIRCULAR NUMBER: 29011 SUBJECT: GRB 201209A: AstroSat CZTI detection DATE: 20/12/09 12:05:42 GMT FROM: Gaurav Waratkar at IIT Bombay D. Nadella (NITK), G. Waratkar (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 long GRB 201209A, which was also detected by Swift-BAT (GCN #29005). The source was clearly detected in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2020-12-09 05:44:56.500 UT. The measured peak count rate associated with the burst is 456 (+51, -54) cts/s above the background in the combined data of four quadrants, with a total of 3215 (+719, -885) cts. The local mean background count rate was 557 (+4, -3) cts/s. Using cumulative rates, we measure a T90 of 23 (+14, -12) s. It 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 2020-12-09 05:44:56.336 UT. The measured peak count rate is 697 (+81, -89) cts/s above the background in the combined Veto data of four quadrants, with a total of 4866 (+1506, -1667) cts. The local mean background count rate was 1856 (+6, -7) cts/s. We measure a T90 of 25 (+26, -15) 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.