Time resolved spectro-polarimetry of GRB prompt emission

Spectroscopic study of prompt gamma ray emission of GRBs, while providing valuable information, has not proven adequate to fully discriminate between various emission models. Measurement of polarisation simultaneously with spectroscopy, however, adds a very important dimension that allows one to put strong constraints on the emission mechanism, composition and geometry of the GRB outflow. AstroSat Cadmium Zinc Telluride Imager (CZTI) is actively measuring linear polarisation of GRBs in 100-400 keV range. This paper presents the results of spectro-polarimetric analysis of GRB 160325A and GRB 160821A. (a) In GRB 160325A, a transition is seen in the spectrum from thermal+non-thermal to pure non-thermal emission, with an accompanying change in polarisation. A composite modelling of polarisation, spectrum and the afterglow light curve allows us to infer that the outflow is baryon dominated with mild magnetisation. (b) GRB 160821A, the third most energetic burst detected by the Fermi, exhibited (i) high linear polarisation, and (ii) near-orthogonal transition of polarisation angle twice during a single emission episode. The emission mechanism that best explains these observations is synchrotron radiation produced in a magnetic field which is highly ordered on angular scales of 1/$\Gamma$.