EP240801a/XRF 240801B: An X-ray Flash Detected by the Einstein Probe and Implications of its Multiband Afterglow

Jiang, S-Q, Xu, D, Hoof, APCV, Lei, W-H, Liu, Y, Zhou, H, Chen, Y, Fu, S-Y, Yang, J, Liu, X, Zhu, Z-P, Filippenko, AV, Jonker, PG, Pozanenko, AS, Gao, H, Wu, X-F, Zhang, B, Lamb, GP ORCID: 0000-0001-5169-4143, Pasquale, MD, Kobayashi, S et al (2025) EP240801a/XRF 240801B: An X-ray Flash Detected by the Einstein Probe and Implications of its Multiband Afterglow. The Astrophysical Journal Letters, 988 (1). ISSN 2041-8205

Preview

Text EP240801aXRF 240801B An X-Ray Flash Detected by the Einstein Probe and the Implications of its Multiband Afterglow.pdf - Published Version Available under License Creative Commons Attribution. Download (4MB) | Preview

Abstract

We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on August 1, 2024 by the Einstein Probe (EP) satellite, with a weak contemporaneous signal also detected by Fermi/GBM. Optical spectroscopy of the afterglow, obtained by GTC and Keck, identified the redshift of $z = 1.6734$. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma-rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP/WXT and Fermi/GBM data yields an isotropic energy $E_{\gamma,\rm{iso}} = (5.57^{+0.54}_{-0.50})\times 10^{51}\,\rm{erg}$, a peak energy $E_{\rm{peak}} =14.90^{+7.08}_{-4.71}\,\rm{keV}$, a fluence ratio $\rm S(25-50\,\rm{keV})/S(50-100\,\rm{keV}) = 1.67^{+0.74}_{-0.46}$, classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early $R$-band behavior and EP/FXT observations suggest multiple components in the afterglow. Three models are considered: two-component jet model, forward-reverse shock model and forward-shock model with energy injection. Both three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs.

Item Type:	Article
Uncontrolled Keywords:	astro-ph.HE; astro-ph.HE
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society
Date of acceptance:	30 May 2025
Date of first compliant Open Access:	22 July 2025
Date Deposited:	22 Jul 2025 09:40
Last Modified:	22 Jul 2025 09:45
DOI or ID number:	10.3847/2041-8213/addebf
URI:	https://researchonline.ljmu.ac.uk/id/eprint/26808

View Item