Is gamma-ray burst polarization from photosphere emission?

Abstract

Context: Despite more than half a century of research, the dominant radiation mechanism of gamma-ray burst (GRB) prompt emission remains unsolved. Some progress has been made through the analyses of the observational spectra of Swift/BAT, Konus/Wind, and Fermi/GBM, as well as the spectra of the photosphere or synchrotron models, but it is still insufficient to pin down the answer. Aims: Combining the spectral and polarization observations, we seek new criteria for model evaluation. Methods: We thoughtfully investigate the polarization samples of POLAR and AstroSAT, combining the light curve, the spectral and the polarization parameters. Results: The power-law shape of the X-ray afterglows, the $T_{90} \propto (L_{\text{iso}})^{-0.5}$ correlation, and the hard low-energy spectral index $\alpha$ are revealed, thus supporting the photosphere origin. Furthermore, we discover the positive correlation of the $\alpha$ and the polarization degree (PD), which can be consistently explained by the photosphere polarization scenario involving the jet asymmetry from a moderate viewing angle of $\theta_{v}$=0.015.