TITLE: GCN CIRCULAR NUMBER: 31178 SUBJECT: GRB 211204C: AstroSat CZTI detection DATE: 21/12/07 06:17:15 GMT FROM: Gaurav Waratkar at IIT,Bombay V. Prasad (IUCAA), 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 the long GRB 211204C, which was also detected by Fermi (GCN 31171), GECAM (Zhang et al., GCN 31173) and Swift/BAT-GUANO (DeLaunay et al., GCN 31174) with IPN triangulation reported (GCN 31177). The source was clearly detected in the 20-200 keV energy range. The light curve peaks at 2021-12-04 21:37:06.50 UT. The measured peak count rate associated with the burst is 426 (+57, -23) cts/s above the background in the combined data of two quadrants, with a total of 3173 (+195, -220) cts. The local mean background count rate was 500 (+2, -2) cts/s. Using cumulative rates, we measure a T90 of 13 (+2, -1) s. It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2021-12-04 21:37:04.16 UT. The measured peak count rate is 592 (+80, -46) cts/s above the background in the combined Veto data of four quadrants, with a total of 4150 (+285, -354) cts. The local mean background count rate was 1677 (+4, -4) cts/s. We measure a T90 of 10 (+1, -1) 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.