Is Gamma-ray Burst Polarization from Photosphere Emission?

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. Recent and upcoming high-sensitivity polarization observations can provide additional instructive information for model evaluation. In this work, we thoughtfully investigate the polarization samples of POLAR and AstroSAT, combining the light curve, the spectral, and the polarization parameters. 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 discover the positive correlation of the 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 θ ᵥ=0.015.