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

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

We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on 2024 August 1 by the Einstein Probe (EP) satellite with a weak contemporaneous signal also detected by the Fermi Gamma-ray Burst Monitor (GBM). Optical spectroscopy of the af...

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Main Authors: Shuai-Qing Jiang, Dong Xu, Agnes P. C. van Hoof, Wei-Hua Lei, Yuan Liu, Hao Zhou, Yong Chen, Shao-Yu Fu, Jun Yang, Xing Liu, Zi-Pei Zhu, Alexei V. Filippenko, Peter G. Jonker, A. S. Pozanenko, He Gao, Xue-Feng Wu, Bing Zhang, Gavin P Lamb, Massimiliano De Pasquale, Shiho Kobayashi, Franz Erik Bauer, Hui Sun, Giovanna Pugliese, Jie An, Valerio D’Elia, Johan P. U. Fynbo, WeiKang Zheng, Alberto J. Castro-Tirado, Yi-Han Iris Yin, Yuan-Chuan Zou, Adam T. Deller, N. S. Pankov, A. A. Volnova, A. S. Moskvitin, O. I. Spiridonova, D. V. Oparin, V. Rumyantsev, O. A. Burkhonov, Sh. A. Egamberdiyev, V. Kim, M. Krugov, A. M. Tatarnikov, R. Inasaridze, Andrew J. Levan, Daniele Bjørn Malesani, Maria E. Ravasio, Jonathan Quirola-Vásquez, Joyce N. D. van Dalen, Javi Sánchez-Sierras, Daniel Mata Sánchez, Stuart P. Littlefair, Jennifer A. Chacón, Manuel A. P. Torres, Ashley A. Chrimes, Nikhil Sarin, Antonio Martin-Carrillo, Vik Dhillon, Yi Yang, Thomas G. Brink, Rebecca L. Davies, Sheng Yang, Amar Aryan, Ting-Wan Chen, Albert K. H. Kong, Wen-Xiong Li, Rui-Zhi Li, Jirong Mao, Ignacio Pérez-García, Emilio J. Fernández-García, Moira Andrews, Joseph Farah, Zhou Fan, Estefania Padilla Gonzalez, D. Andrew Howell, Dieter Hartmann, Jing-Wei Hu, Páll Jakobsson, Cheng-Kui Li, Zhi-Xing Ling, Curtis McCully, Megan Newsome, Benjamin Schneider, Kaew Samaporn Tinyanont, Ning-Chen Sun, Giacomo Terreran, Qing-Wen Tang, Wen-Xin Wang, Jing-Jing Xu, Wei-Min Yuan, Bin-Bin Zhang, Hai-Sheng Zhao, Juan Zhang
Format: Article
Language:English
Published: IOP Publishing 2025-01-01
Series:The Astrophysical Journal Letters
Subjects:
Gamma-ray bursts
X-ray transient sources
Online Access:https://doi.org/10.3847/2041-8213/addebf
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Summary:We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on 2024 August 1 by the Einstein Probe (EP) satellite with a weak contemporaneous signal also detected by the Fermi Gamma-ray Burst Monitor (GBM). Optical spectroscopy of the afterglow, obtained by Gran Telescopio Canarias 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 that the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP Wide-field X-ray Telescope and Fermi/GBM data yields an isotropic energy ${E}_{\gamma ,{\rm{iso}}}=(5.5{7}_{-0.50}^{+0.54})\times 1{0}^{51}\,{\rm{erg}}$ , a peak energy ${E}_{{\rm{peak}}}=14.9{0}_{-4.71}^{+7.08}\,{\rm{keV}}$ , and a fluence ratio S (25–50 keV)/ S $(50\,\unicode{x02013}\,100\,{\rm{keV}})=1.6{7}_{-0.46}^{+0.74}$ , 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 Follow-up X-ray Telescope observations suggest multiple components in the afterglow. Three models are considered: a two-component jet model, a forward-reverse shock model, and a forward shock model with energy injection. All 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.
ISSN:2041-8205

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