GRB 240825A: Early Reverse Shock and Its Physical Implications

Wu, C ORCID: 0009-0001-7024-3863, Wang, Y ORCID: 0000-0002-8385-7848, Li, HL, Xin, LP ORCID: 0000-0002-9422-3437, Xu, D ORCID: 0000-0003-3257-9435, Schneider, B ORCID: 0000-0003-4876-7756, de Ugarte Postigo, A ORCID: 0000-0001-7717-5085, Lamb, G ORCID: 0000-0001-5169-4143, Reguitti, A ORCID: 0000-0003-4254-2724, Saccardi, A ORCID: 0000-0002-6950-4587, Gao, X, Li, XL, Wang, QL ORCID: 0009-0000-9352-6447, Zhang, B ORCID: 0000-0002-9725-2524, Wei, JY, Zhang, SN, Daigne, F, Atteia, JL ORCID: 0000-0001-7346-5114, Bernardini, MG ORCID: 0000-0001-6106-3046, Cai, HB et al (2025) GRB 240825A: Early Reverse Shock and Its Physical Implications. Research in Astronomy and Astrophysics, 25 (10). p. 105003. ISSN 1674-4527

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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 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.

Item Type:	Article
Uncontrolled Keywords:	5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	IOP Publishing
Date of acceptance:	27 June 2025
Date Deposited:	07 Nov 2025 10:27
Last Modified:	07 Nov 2025 10:30
DOI or ID number:	10.1088/1674-4527/adeaf0
URI:	https://researchonline.ljmu.ac.uk/id/eprint/27520

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