Time-resolved spectro-polarimetric analysis of extremely bright GRB 230307A: Possible evidence of evolution from photospheric to synchrotron dominated emission

Context. The radiation mechanisms powering gamma-ray bursts (GRBs) and their physical processes remain one of the unresolved questions in high-energy astrophysics. Spectro-polarimetric observations of exceptionally bright GRBs provide a powerful diagnostic tool to investigate these challenges. Aims. GRB 230307A, the second-brightest long-duration GRB ever detected, exhibits a rare association with a kilonova, offering a unique and rare probe into the emission processes of GRBs originating from compact object mergers. Methods. We present a comprehensive time-averaged and time-resolved spectro-polarimetric analysis of GRB 230307A using joint observations from the AstroSat Cadmium Zinc Telluride Imager (CZTI), the Fermi Gamma-ray Burst Monitor (GBM), and Konus-Wind. Results. Spectral analysis reveals a temporal evolution in the low-energy photon index, α, transitioning from a hard to a softer state over the burst duration. Time-averaged polarimetric measurements yield a low polarization fraction (< 12.75%), whereas time-resolved polarization analysis unveils a marked increase in polarization fractions (> 49.0%) in the later stages of the emission episode. Conclusions. This spectro-polarimetric evolution suggests a transition in the dominant radiative mechanism: The initial phase, possibly characterized by thermal-dominated photospheric emission (unpolarized or weakly polarized), gives way to a regime dominated by non-thermal synchrotron emission (highly polarized). This transition provides possible evidence of the evolving influence of magnetic fields in shaping the GRB emission process and jet dynamics.