2025ApJ...990..215S

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 polarization properties. Time-resolved polarization analysis reveals a synchronous evolution of the polarization angle (PA) and fraction with two emission pulses, peaking with a delay of 5.09 ± 0.29 s. Spectral analysis indicates that the first pulse is characterized by a stronger blackbody component, while the second pulse exhibits a more prominent nonthermal spectrum (power law with an exponential cutoff). Using a bottom-to-top approach through simulations, we decouple the polarization properties of the individual spectral components, revealing polarization fractions (PFs) of 25%-40% for the blackbody spectrum and 30%-60% for the nonthermal spectrum. At a redshift of z ∼ 0.22, the blackbody emission originates from the jet photosphere at ∼a few 10¹¹ cm, whereas the nonthermal emission arises from an optically thin region at a few 10¹³ cm. The changing dominance of these emissions explains the observed PA shift of 60° ± 22°. The spectral cutoff at 1 MeV suggests pair opacity due to the jet's relatively lower bulk Lorentz factor (Γ∼ a few tens). The high PF and hard low-energy spectral slopes (α > -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/Γ), accounting for the observed polarization.