GRB 240825A: Early Reverse Shock and Its Physical Implications

Early multiwavelength observations offer crucial insights into the nature of the relativistic jets responsible for gamma-ray bursts and their interaction with the surrounding medium. We present data of GRB 240825A from 17 space- and ground-based telescopes/instruments, covering wavelengths from NIR/optical to X-ray and GeV, and spanning from the prompt emission to the afterglow phase triggered by Swift and Fermi. The early afterglow observations were carried out by SVOM/C-GFT, and spectroscopic observations of the afterglow by GTC, VLT, and TNG determined the redshift of the burst (z = 0.659) later. A comprehensive analysis of the prompt emission spectrum observed by Swift-BAT and Fermi-GBM/LAT reveals a rare and significant high-energy cutoff at 76 MeV. Assuming this cutoff is due to γγ absorption allows us to place an upper limit on the initial Lorentz factor, Γ0 < 245. The optical/NIR and GeV afterglow light curves can be described by the standard external shock model, with early-time emission dominated by a reverse shock (RS) and a subsequent transition to forward shock (FS) emission. Our afterglow modeling yields a consistent estimate of the initial Lorentz factor (Γ0 ∼ 234). Furthermore, the RS-to-FS magnetic field ratio ( R B ∼ 302 ) indicates that the RS region is significantly more magnetized than the FS region. An isotropic-equivalent kinetic energy of Ek,iso = 5.25 × 1054 erg is derived, and the corresponding γ-ray radiation efficiency is estimated to be ηγ = 3.1%. On the other hand, the standard afterglow model cannot reproduce the X-ray light curve of GRB 240825A, calling for improved models to characterize all multiwavelength data.