Spectro-polarimetry of GRB 180427A: evidence for distinct emission sites with varying polarisation

Abstract

The dynamics of the origin of gamma-ray emissions in gamma-ray bursts (GRBs) remains an enigma. Through a joint analysis of GRB 180427A, observed by the Fermi Gamma-ray Space Telescope and AstroSat's Cadmium Zinc Telluride Imager, we identify emissions from two distinct regions with varying polarisation properties. Time-resolved polarisation analysis reveals a synchronous evolution of the polarisation angle (PA) and fraction (PF) with two emission pulses, peaking with a delay of $ 5.09 \pm 0.29\, \mathrm{s}$. Spectral analysis indicates that the first pulse is characterised by a stronger blackbody component, while the second pulse exhibits a more prominent non-thermal spectrum (power law with an exponential cutoff). Using a bottom-to-top approach through simulations, we decouple the polarisation properties of the individual spectral components, revealing polarisation fractions of 25\% - 40\% for the blackbody spectrum and 30\% - 60\% for the non-thermal spectrum. At a redshift of $z \sim 0.22$, the blackbody emission originates from the jet photosphere at $\sim$ a few $10^{11}\, \mathrm{cm}$, whereas the non-thermal emission arises from an optically thin region at a few $10^{13}\, \mathrm{cm}$. The changing dominance of these emissions explains the observed PA shift of $60^\circ \pm 22^\circ$. The spectral cutoff at 1 MeV suggests pair opacity due to the jet's relatively lower bulk Lorentz factor ($\Gamma \sim$ a few tens). The high polarisation fraction and hard low energy spectral slopes ($\alpha > -0.5$) imply a top-hat jet structure observed off-axis, near the jet's edge. This off-axis viewing introduces anisotropy in the observed radiation within the viewing cone ($1/\Gamma$), accounting for the observed polarisation.