# Lorentz Factor Evolution of an Expanding Jet Shell Observed in Gamma-ray Burst: Case study of GRB 160625B

## TL;DR

This study analyzes the evolution of the Lorentz factor in a gamma-ray burst jet by identifying a high-energy spectral cutoff in GRB 160625B, revealing that the jet's radiation location increases over time while the Lorentz factor remains nearly constant.

## Contribution

It provides the first estimation of the Lorentz factor and radiation location evolution during a GRB jet expansion based on spectral cutoff analysis.

## Key findings

- The high-energy spectral cutoff appears in the first pulse of the second emission episode.
- The radiation location increases with time during the jet expansion.
- The Lorentz factor remains nearly constant, indicating low to intermediate magnetization.

## Abstract

The Lorentz factor of a relativistic jet and its evolution during the jet expansion are difficult to estimate, especially for the jets in gamma-ray bursts (GRBs). However, it is related to the understanding of jet physics. Owing to the absorption of two-photon pair production ($\gamma\gamma{\leftrightarrow}e^+e^-$), a high-energy spectral cutoff may appear in the radiation spectrum of GRBs. We search such kind of high-energy cutoff in GRB 160625B, which is one of the brightest bursts in recent years. It is found that the high-energy spectral cutoff is obvious for the first pulse in the second emission episode of GRB 160625B (i.e., $\sim$186-192 s after the burst first trigger), which is smooth and well-shaped. Then, we estimate the Lorentz factor and radiation location of the jet shell associated with the first pulse in the second emission episode of GRB 160625B. It is found that the radiation location increases with time. In addition, the Lorentz factor remains almost constant during the expansion of the jet shell. This reveals that the magnetization of the jet is low or intermediate in the emission region, event though the jet could be still Poynting flux dominated at smaller radii to avoid a bright thermal component in the emission episode.