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

Abstract

We present spectro-polarimetric analysis of GRB 171010A using data from AstroSat, Fermi, and Swift, to provide insights into the physical mechanisms of the prompt radiation and the jet geometry.Prompt emission from GRB 171010A was very bright (fluence > 10 −4 ergs cm −2 ) and had a complex structure 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. Alternately, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. 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.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 • .Detection of a large number of Compton scattered events by AstroSat-CZTI provides an opportunity to study hard X-ray polarization of the prompt emission. We find that the burst has high, time-variable polarization, with the emission have higher polarization at energies above the peak energy.We discuss all observations in the context of GRB models and polarization arising due to due to physical or geometric effects: synchrotron emission from multiple shocks with ordered or random magnetic fields, Poynting flux dominated jet undergoing abrupt magnetic dissipation, sub-photospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model.

Proton–synchrotron as the radiation mechanism of the prompt emission of gamma-ray bursts?

“…More physically, Oganesyan et al (2019) also successfully reproduced GRB spectra with the synchrotron spectrum produced by a non-thermal electron energy distribution (see also Chand et al 2019, Burgess et al 2019, Ronchi et al 2019. The top panel of Fig.…”

Spectropolarimetric analysis of prompt emission of GRB 160325A: jet with evolving environment of internal shocks

“…Linear polarisation measurement of the prompt emission of GRBs is a powerful tool, in addition to spectral analysis, to strongly constrain the radiation process, geometry of the emitting region and the composition of the GRB outflow (Covino & Gotz 2016). Currently, polarisation measurement of prompt emission has been reported for only a handful of cases (Wigger et al 2004;Willis et al 2005;McGlynn et al 2007;Götz et al 2009;McGlynn et al 2009;Yonetoku et al 2011Yonetoku et al , 2012Götz et al 2013Götz et al , 2014Zhang et al 2019;Burgess et al 2019b;Chand et al 2018Chand et al , 2019Sharma et al 2019;Chattopadhyay et al 2019; for a recent review see McConnell 2017). Linear polarisation has also been measured in early optical afterglows of a few GRBs (Wijers et al 1999;Covino et al 2002;Greiner et al 2003;King et al 2014;Gorbovskoy et al 2016;Troja et al 2017;Laskar et al 2019).…”

The AstroSat mass model: Imaging and flux studies of off-axis sources with CZTI

“…An older version of AstroSat CZTI mass model with minor differences has been successfully used to study the polarisation of prompt emission in several GRBs (Rao et al 2016;Chattopadhyay et al 2019;Chand et al 2019). Singhal et al (2021) used the same older version to measure fluxes of bright sources by the earth occultation technique.…”

“…The off-axis sensitivity of CZTI has also been leveraged for Earth-occultation studies of sources (Singhal et al 2021). Compton scattering within the CZT detectors is sensitive to polarisation of incoming photons, and has been successfully used to measure polarisation of GRBs (Chand et al 2019;Chattopadhyay et al 2019) and the Crab pulsar (Vadawale et al 2018).…”