Extremely bright GRB 160625B with multi-episodes emission: Evidences for long-term ejecta evolution

GRB 160625B is an extremely bright GRB with three distinct emission episodes. By analyzing its data observed with the GBM and LAT on board the {\em Fermi} mission, we show that the spectrum of its initial short episode (Episode I) can be fitted with a multi-color black body (mBB) model, and the derived maximum temperature ($T_{\rm max}$) is $\sim 25$ keV. The time-resolved spectra of its main episode (Episode II), which was detected with both GBM and LAT after a long quiet stage ($\sim 180$ seconds) after the initial episode, can be fitted with a model composing of an mBB component plus a cutoff power-law (CPL) component. The derived $T_{\rm max}$ initially increases up to $\sim 1100$ keV, then decays down to $\sim 300$ keV. The photon index and the cutoff energy of the CPL component do not show clear evolution trend. This GRB was detected again in the GBM and LAT bands with a long extended emission (Episode III) after a quiet period of $\sim 300$ seconds. The spectrum of Episode III is adequately fitted with a CPL plus a single power-law models, and no mBB component is required. These features may imply that the emission of three episodes are dominated by distinct physics process, i.e., Episode I is possible from cocoon emission surrounding the relativistic jet, Episode II is from photosphere emission and internal shock of relativistic jet, and Episode III is contributed by internal and external shocks of relativistic jet. On the other hand, both X-ray and optical afterglows are consistent with standard external shocks model.