Kavli Affiliate: Jing Wang
| First 5 Authors: Chao Wu, Chao Wu, , ,
| Summary:
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 $gammagamma$ absorption allows us to place an
upper limit on the initial Lorentz factor, $Gamma_0 < 245$. 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 ($Gamma_rm 0
sim 234$). Furthermore, the RS-to-FS magnetic field ratio ($mathcalR_B sim
302$) indicates that the reverse shock region is significantly more magnetized
than the FS region. An isotropic-equivalent kinetic energy of $E_textk,iso
= 5.25 times 10^54$ erg is derived, and the corresponding $gamma$-ray
radiation efficiency is estimated to be $eta_gamma$ = 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.
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