2026/07/20 by Utkarsh Pathak, Sameer K. Patil, Hitesh Tanenia +23
#astro-ph.HE
GRB 230812B is a bright long-duration GRB with a luminous, long-lived afterglow and an AstroSat/CZTI polarization measurement during the prompt phase, enabling a joint study of its prompt spectral evolution, polarization, and broadband afterglow. Time-resolved spectroscopy of the prompt emission shows that during the rising phase, the low-energy Band-function index exceeds the synchrotron line of death, favoring the presence of an additional thermal component. At later times, from T0+2 s to T0+32 s, the prompt spectra are consistent with predominantly non-thermal emission. Polarization analysis of the prompt emission in the 300-600 keV band yields a marginal lower limit on the polarization fraction of Π\gtrsim 50% at the 1σ level. The long X-ray monitoring of the afterglow shows no jet break over the observed baseline. Multiwavelength afterglow modeling favors a wide jet with an inferred half-opening angle of θj = 15+6-4 degrees observed close to the jet axis with a viewing angle of θv = 0.9+1.8-0.6 degrees. The inferred circumburst density is low, n0 = 1.2+0.3-0.1×10-4 \textrmcm-3, and the isotropic-equivalent kinetic energy of the jet is E_\rm k, iso = 4.0+1.5-0.8 × 1053 erg. Taken together, the prompt spectral evolution favors an early phase with a thermal contribution followed by a later phase dominated by non-thermal emission. The polarization constraint in the late prompt phase is consistent with synchrotron emission, although a higher-significance polarization measurement will be required to robustly constrain the magnetic-field geometry and the relative contribution of photospheric emission.