TITLE: GCN CIRCULAR NUMBER: 31367 SUBJECT: GRB 211231A: AstroSat CZTI detection DATE: 22/01/02 12:16:26 GMT FROM: Gaurav Waratkar at IIT,Bombay V. Prasad (IUCAA), A. Suresh (IITB), G. Waratkar (IITB), A. Vibhute (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., 2021, JApA, 42, 73) showed detection of a long GRB 211231A which was also detected by Fermi-GBM (GCN 31343) and GECAM (Guo et al. GCN 31345). The source was clearly detected in the 20-200 keV energy range. The light curve peaks at 2021-12-31 07:00:35.75 UT. The measured peak count rate associated with the burst is 193 (+60, -27) cts/s above the background in the combined data of three quadrants, with a total of 599 (+188, -196) cts. The local mean background count rate was 362 (+4, -4) cts/s. Using cumulative rates, we measure a T90 of 13 (+1, -7) s. It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2021-12-31 07:00:35.88 UT. The measured peak count rate is 562 (+73, -75) cts/s above the background in the combined Veto data of four quadrants, with a total of 1536 (+452, -569) cts. The local mean background count rate was 1590 (+4, -4) cts/s. We measure a T90 of 8 (+3, -6) 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.