TITLE: GCN CIRCULAR NUMBER: 20412 SUBJECT: GRB170105A: AstroSat CZTI localisation DATE: 17/01/10 05:46:39 GMT FROM: Varun Bhalerao at IUCAA V. Bhalerao and D. Bhattacharya (IUCAA), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Detailed analysis of AstroSat CZTI data for GRB170105A (POLAR Observation: R. Marcinkowski et al., GCN Circ. 20387; CZTI detection: Sharma et al., GCN Circ. 20389) was carried out for localising the GRB. We modelled the counts in various quadrants using a raytracing code based on the instrument geometry. The GRB is localised to a 550 sq. deg. area, centred roughly on RA, Dec = 60, +80. While spectral fits are not possible at the current stage, the data indicate a rather soft spectrum, with strongest detection around 60-80 keV and almost no counts above 100 keV. Along with the T90 measurement in Sharma et al., GCN Circ. 20389, this suggests that GRB170501A was a long GRB. A pdf showing the localisation region is available at: http://www.iucaa.in/~astrosat/czti_grb/GRB170105A/heal/GRB170105A_localisation.pdf A healpix map with the localisation region is available at: http://www.iucaa.in/~astrosat/czti_grb/GRB170105A/heal/GRB170105A_localisation.fits 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.