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

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arxiv 2503.04306 v1 pith:GFAIUR6H submitted 2025-03-06 astro-ph.HE

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

Shuai-Qing Jiang , Dong Xu , Agnes P. C. van Hoof , Wei-Hua Lei , Yuan Liu , Hao Zhou , Yong Chen , Shao-Yu Fu
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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. C. 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 Bj{\o}rn Malesani Maria E. Ravasio Jonathan Quirola-V\'asquez Joyce N. D. van Dalen Javi S\'anchez-Sierras Daniel Mata S\'anchez Stuart P. Littlefair Jennifer A. Chac\'on 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 P\'erez-Garc\'ia Emilio J. Fern\'andez-Garc\'ia Moira Andrews Joseph Farah Zhou Fan Estefania Padilla Gonzalez D. Andrew Howell Dieter Hartmann Jing-Wei Hu P\'all 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
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keywords modelep240801aenergyafterglowtwo-componentx-raysanalysisburst
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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.

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