GRB 240825A: Early Reverse Shock and Its Physical Implications

Abstract

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 ${\hat{I}}^3{\hat{I}}^3$ absorption allows us to place an upper limit on the initial Lorentz factor, $\hat{I}{“}_0<245$. The optical/NIR and GeV afterglow light curves 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 modelling yields a consistent estimate of the initial Lorentz factor ($\hat{I}{“}_0\sim 234$). Furthermore, the RS-to-FS magnetic field ratio (${\mathcal{R}}_B\sim 302$) indicates that the reverse shock region is significantly more magnetized than the FS region. An isotropic-equivalent kinetic energy of $E_{\text{k,iso}}=5.25\times 10^{54}$ erg is derived, and the corresponding ${\hat{I}}^3$-ray radiation efficiency is estimated to be $\hat{I}{\cdot }_{\hat{I}}^3$ = 3.1%. On the other hand, the standard afterglow model can not reproduce the X-ray light curve of GRB 240825A, calling for improved models to characterize all multiwavelength data.

31 pages, 9 Figures, 10 Tables