AstroSat-CZTI Detection of Variable Prompt Emission Polarization in GRB 171010A

Abstract

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.