TITLE: GCN CIRCULAR NUMBER: 19011 SUBJECT: GRB160131A: detection of polarisation by Astrosat CZTI DATE: 16/02/12 13:23:06 GMT FROM: Dipankar Bhattacharya at IUCAA S. V. Vadawale (PRL), T. Chattopadhyay (PRL), N. P. S. Mithun (PRL), A. R. Rao (TIFR), D. Bhattacharya (IUCAA), Varun Bhalerao (IUCAA) report on behalf of the Astrosat CZTI collaboration: Astrosat CZT Imager detected GRB 160131A (Page and Barthelmy, GCN 18951) at an incident angle of 63 degrees from the telescope pointing axis. The T90 was measured to be 106 s and 97 s in 30 - 100 keV and 100 - 300 keV respectively. CZTI being sensitive to polarimetry [Chattopadhyay et al, 2014, EXPA, Vadawale et al 2015, A&A], we have analysed the double events due to Compton scattering and detect a clear azimuthal modulation of 0.30 +/- 0.12 %, with a detection significance of 2.5 sigma. Based on a preliminary mass modelling, we estimate that the source polarisation fraction lies between ~ 50 - 60 % in 100-300 keV band. A detailed mass modeling to refine this measurement and its estimated uncertainty is in progress. In an earlier event, GRB 151006A (Bhalerao et al. GCN 18422) CZT Imager data showed a marginal evidence for polarisation signal. From the higher detection confidence for this brighter GRB160131A, we estimate that CZT Imager can provide time resolved spectro-polarimetry for GRBs with fluence larger than 10^-4 erg per cm^2. This message may be cited.