2019ApJ...883..187L

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, the Lorentz factor is related to the jet physics. Owing to the absorption of two-photon pair production (γγ↔e⁺e^–), a high-energy spectral cutoff may appear in the radiation spectrum of GRBs. We search for such high-energy cutoffs 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., ∼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, even though the jet could still be Poynting-flux-dominated at smaller radii to avoid a bright thermal component in the emission episode.