Can We Find the Emission Mechanism Behind the Extremely Bright GRB 230812B?

Abstract

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 $T_0+2$ s to $T_0+32$ 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 $\hat{I}\gtrsim 50\%$ at the $1\ddot{I}ƒ$ 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 $\hat{I}{¸}_j=15_{-4}^{+6}$ degrees observed close to the jet axis with a viewing angle of $\hat{I}{¸}_v=0.9_{-0.6}^{+1.8}$ degrees. The inferred circumburst density is low, $n_0=1.2_{-0.1}^{+0.3}\times 10^{-4}{\text{cm}}^{-3}$, and the isotropic-equivalent kinetic energy of the jet is $E_{\mathrm{k},iso}=4.0_{-0.8}^{+1.5}\times 10^{53}$ 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.

16 pages, 7 figures, 3 Tables. Submitted to ApJ