Clues to the photosphere emission origin of gamma-ray burst polarization

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 investigated the polarization samples of POLAR and AstroSAT, combining the light curve, spectral, and polarization parameters. Results. The power-law shape of the X-ray afterglows, the T90 ∝ (Liso)−0.5 correlation, and the hard low-energy spectral index α are revealed, thus supporting the photosphere origin. Furthermore, we discovered the positive correlation of α 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 θv = 0.015.