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

## TL;DR

This paper analyzes the multi-episode emission of the extremely bright GRB 160625B, revealing different physical origins for each episode through spectral fitting and supporting a complex ejecta evolution model.

## Contribution

It provides the first detailed spectral analysis of all three emission episodes of GRB 160625B, proposing distinct physical mechanisms for each and evidence for long-term ejecta evolution.

## Key findings

- Episode I fits a multi-color black body model, suggesting cocoon emission.
- Episode II's spectrum includes a multi-color black body plus cutoff power-law, indicating photosphere and internal shocks.
- Episode III's spectrum fits a cutoff power-law plus power-law, consistent with internal and external shocks.

## Abstract

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 find that a multi-color black body (mBB) model can be used to fit the spectra of initial short episode (Episode I) very well within the hypothesis of photosphere emission of a fireball model. 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) of the initial episode, can be fitted with a model composing of an mBB component plus a cutoff power-law (CPL) component. 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 may be 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.