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

Abstract

The physical origins of gamma-ray burst (GRB) prompt emission (photosphere or synchrotron) are still subject to debate, after more than five decades. Here, we find that many of the observed characteristics of 15 long GRBs, which have the highest prompt emission efficiency ϵ γ ( ϵ γ ≳ 80%), strongly support photosphere (thermal) emission origin, in the following ways: (1) the relation between E p and E iso is almost E p ∝ ( E iso ) 1 / 4 and the dispersion is quite small; (2) the simple power-law shape of the X-ray afterglow light curves and the presence of significant reverse shock signals in the optical afterglow light curves; (3) the best fits using the cutoff power-law model for time-integrated spectra; and (4) the consistent efficiency from observations (with E iso / E k ) and the predictions from the photosphere emission model (with η /Γ). We then further investigate the characteristics of the long GRBs for two distinguished samples ( ϵ γ ≳ 50% and ϵ γ ≲ 50%). It is found that the different distributions for E p and E iso , and the similar observed efficiency (from the X-ray afterglow) and theoretically predicted efficiency (from the prompt emission or the optical afterglow), follow the predictions of the photosphere emission model well. Also, based on the same efficiency, we derive an excellent correlation of Γ ∝ E iso 1 / 8 E p 1 / 2 / T 90 1 / 4 to estimate Γ. Finally, we show that different distributions for E p and E iso , and the consistent efficiency, exist for short GRBs. We also give a natural explanation of the extended emission ( ϵ γ ≲ 50%) and the main pulse ( ϵ γ ≳ 50%).