2019ApJ...874...70C

We present a spectropolarimetric analysis of GRB 171010A using data from AstroSat, Fermi, and Swift to provide insights into the physical mechanisms of the prompt radiation and jet geometry. The prompt emission from GRB 171010A was very bright (fluence >10^–4 erg cm^–2) and had a complex structure that was composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low-energy peak ∼15 keV, which we interpret as a power law with two breaks, with a synchrotron origin. Alternatively, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. A time-resolved analysis confirms the presence of the low-energy component, while the peak energy is found to be confined in the range of 100-200 keV. The afterglow emission detected by Fermi-LAT is typical of an external shock model, and we constrain the initial Lorentz factor using the peak time of the emission. Swift-XRT measurements of the afterglow show an indication for a jet break, allowing us to constrain the jet opening angle to <6°. The detection of a large number of Compton-scattered events by AstroSat-CZTI provides an opportunity to study the hard X-ray polarization of the prompt emission. We find that the burst has a high time-variable polarization, and the emission has a higher polarization at energies above the peak energy. We discuss all observations in the context of GRB models and polarization arising as a result of physical or geometric effects: synchrotron emission from multiple shocks with ordered or random magnetic fields, a jet dominated by Poynting flux that undergoes abrupt magnetic dissipation, subphotospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model.