Evidence of Photosphere Emission Origin for Gamma-Ray Burst Prompt Emission

Abstract

The physical origin of gamma-ray burst (GRB) prompt emission (photosphere or synchrotron) is still subject to debate after five decades. Here, firstly we find that many observed characteristics of 15 long GRBs, which have the highest prompt emission efficiency $\epsilon _{\gamma}$ ($\epsilon_{\gamma }\gtrsim 80\%$), strongly support the photosphere (thermal) emission origin: (1) the relation between $E_{\text{p}}$ and $E_{\text{iso}}$ is almost $E_{\text{p}}\propto (E_{\text{iso}})^{1/4}$ , and the dispersion is quite small; (2) the simple power-law shape of the X-ray afterglow light curves and the significant reverse shock signals in the optical afterglow light curves; (3) best-fitted by the cutoff power-law model for the time-integrated spectrum; (4) the consistent efficiency from observation (with $E_{\text{iso}}/E_{k}$) and the prediction of photosphere emission model (with $\eta /\Gamma $). Then, we further investigate the characteristics of the long GRBs for two distinguished samples ($\epsilon _{\gamma }\gtrsim 50\%$ and $\epsilon _{\gamma }\lesssim 50\%$). It is found that the different distributions for $E_{\text{p}}$ and $E_{\text{iso}}$, and the similar observed efficiency (from the X-ray afterglow) and theoretically predicted efficiency (from the prompt emission or the optical afterglow) well follow the prediction of photosphere emission model. Also, based on the same efficiency, we derive an excellent correlation of $\Gamma \propto E_{\text{iso}}^{1/8}E_{\text{p}}^{1/2}/(T_{90})^{1/4}$ to estimate $\Gamma $. Finally, the different distributions for $E_{\text{p}}$ and $E_{\text{iso}}$, and the consistent efficiency exist for the short GRBs. Besides, we give a natural explanation of the extended emission ($\epsilon _{\gamma }\lesssim 50\%$) and the main pulse ($\epsilon _{\gamma }\gtrsim 50\%$).