TITLE: GCN CIRCULAR NUMBER: 20389 SUBJECT: GRB 170105A: AstroSat CZTI detection DATE: 17/01/07 13:28:52 GMT FROM: Vidushi Sharma at IUCAA V. Sharma, V. Bhalerao and D. Bhattacharya (IUCAA), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data showed clear detection of GRB170105A (POLAR Observation: R. Marcinkowski et al., GCN Circ. 20387) in the 40-200 keV energy range. The light curve shows a single peak structure with main peak at 06:14:06 UT in the quadrant A and B only, ~1 sec before the POLAR trigger at 06:14:07 UT. The measured peak count rate is 305.46 counts/sec above the background in combined data of four quadrants, with a total of 762 counts. The local mean background count rate was 287.54 counts/sec. Using cumulative rates, we measure a T90 of 2.86 secs. 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.