Evolution of the Three Spectral Components in the Prompt Emission of GRB 240825A

Abstract

The prompt emission of gamma-ray bursts (GRBs) could be composed of different spectral components, such as a dominant nonthermal Band component in the keV–MeV range, a subdominant quasi-thermal component, and an additional hard nonthermal component extending into the GeV range. The existence and evolutionary behaviors of these components could provide essential constraints on physical models, such as ejecta composition and dissipation processes. Although numerous GRBs have been found to exhibit one or two spectral components, reports of GRBs containing all three components remain rare. In this Letter, based on the joint observations of GRB 240825A from multiple gamma-ray telescopes, we conduct a comprehensive temporal and spectral analysis to identify the presence and evolution of all three components. The bulk Lorentz factor of this bright and relatively short-duration burst is independently obtained from the thermal and hard nonthermal components, supporting a jet penetration scenario. The multisegment broken power-law feature observed in the flux lightcurves suggests the presence of an early afterglow in the keV–MeV band and hints at a possible two-jet structure. Furthermore, the observed transition from positive to negative on the spectral lag can be interpreted as misalignment in the cross-correlation function analysis of pulses, which is caused by evolution of the soft and hard components.