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

¹ Bhabha Atomic Research Center, Mumbai, Maharashtra 400094, India
² Homi Bhabha National Institute, Mumbai, Maharashtra 400094, India
³ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA
⁴ Kavli Institute of Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA
⁵ Ioffe Institute, 26 Politekhnicheskaya, St. Petersburg 194021, Russia
⁶ Physical Research Laboratory Thaltej, Ahmedabad, Gujarat 380009, India
⁷ Department of Physics, Ashoka University, Sonipat, Haryana 131029, India
⁸ Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra 400076, India
⁹ Department of Physics, University of Cagliari, SP Monserrato-Sestu, km 0.7, 09042 Monserrato, Italy

^⋆ Corresponding authors: soumya.gupta1512@gmail.com, soumyag@barc.gov.in, rahulbhu.c157@gmail.com

Received: 6 April 2025
Accepted: 24 July 2025

Abstract

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.