EP240801a/XRF 240801B: An X-Ray Flash Detected by the Einstein Probe and the Implications of Its Multiband Afterglow
Jiang S.-Q. Xu D. van Hoof A.P.C. Lei W.-H. Liu Y. Zhou H. Chen Y. Fu S.-Y. Yang J. Liu X. Zhu Z.-P. Filippenko A.V. Jonker P.G. Pozanenko A.S. Gao H. Wu X.-F. Zhang B. Lamb G.P. De Pasquale M. Kobayashi S. Bauer F.E. Sun H. Pugliese G. An J. D’Elia V. Fynbo J.P.U. Zheng W. Castro-Tirado A.J. Yin Y.I. Zou Y.-C. Deller A.T. Pankov N.S. Volnova A.A. Moskvitin A.S. Spiridonova O.I. Oparin D.V. Rumyantsev V. Burkhonov O.A. Egamberdiyev Sh.A. Kim V. Krugov M. Tatarnikov A.M. Inasaridze R. Levan A.J. Malesani D.B. Ravasio M.E. Quirola-Vásquez J. van Dalen J.N.D. Sánchez-Sierras J. Mata Sánchez D. Littlefair S.P. Chacón J.A. Torres M.A.P. Chrimes A.A. Sarin N. Martin-Carrillo A. Dhillon V. Yang Y. Brink T.G. Davies R.L. Yang S. Aryan A. Chen T.-W. Kong A.K.H. Li W.-X. Li R.-Z. Mao J. Pérez-García I. Fernández-García E.J. Andrews M. Farah J. Fan Z. Padilla Gonzalez E. Howell D.A. Hartmann D. Hu J.-W. Jakobsson P. Li C.-K. Ling Z.-X. McCully C. Newsome M. Schneider B. Tinyanont K.S. Sun N.-C. Terreran G. Tang Q.-W. Wang W.-X. Xu J.-J. Yuan W.-M. Zhang B.-B. Zhao H.-S. Zhang J.
20 July 2025American Astronomical Society
Astrophysical Journal Letters
2025#988Issue 1
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 γ , iso = ( 5.5 7 − 0.50 + 0.54 ) × 1 0 51 erg , a peak energy E peak = 14.9 0 − 4.71 + 7.08 keV , and a fluence ratio S(25-50 keV)/S ( 50 - 100 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.
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National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100101, China
School of Astronomy and Space Science, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing, 100049, China
Altay Astronomical Observatory, Xinjiang, 836500, China
Radboud University Nijmegen, Department of Astrophysics, IMAPP, P.O. Box 9010, Nijmegen, 6500 GL, Netherlands
Department of Astronomy, School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, China
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210023, China
Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China
School of Astronomy and Space Science, Nanjing University, Nanjing, 210093, China
Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University, Ministry of Education, China
Department of Astronomy, University of California, Berkeley, 94720-3411, CA, United States
Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997, Russian Federation
National Research University “Higher School of Economics, ” Myasnitskaya ul. 20, Moscow, 101000, Russian Federation
Moscow Institute of Physics and Technology (MIPT), Institutskiy Pereulok, 9, Dolgoprudny, 141701, Russian Federation
School of Physics and Astronomy, Beijing Normal University, Beijing, 100875, China
Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University, Beijing, 102206, China
Nevada Center for Astrophysics, University of Nevada Las Vegas, 89154, NV, United States
Department of Physics and Astronomy, University of Nevada Las Vegas, 89154, NV, United States
Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, United Kingdom
University of Messina, MIFT Department, Via F.S. D’Alcontres 31, Messina, 98166, Italy
Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
Anton Pannekoek Institute of Astronomy, University of Amsterdam, Science Park 904, Amsterdam, 1098 XH, Netherlands
Space Science Data Center (SSDC), Agenzia Spaziale Italiana (ASI), Roma, I-00133, Italy
Osservatorio Astronomico di Roma, via Frascati 33, Monte Porzio Catone, I-00040, Italy
The Cosmic Dawn Centre (DAWN), Denmark
Niels Bohr Institute, University of Copenhagen, Jagtvej 155, Copenhagen N, DK-2200, Denmark
Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, Granada, 18008, Spain
Ingeniería de Sistemas y Automática, Universidad de Málaga, Unidad Asociada al CSIC por el IAA, Escuela de Ingenierías Industriales, Arquitecto Francisco Peñalosa, 6, Campanillas, Málaga, 29071, Spain
Center for Astrophysics and Supercomputing, Swinburne University of Technology, P.O. Box 218, Hawthorn, 3122, VIC, Australia
ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), Hawthorn, 3122, VIC, Australia
Special Astrophysical Observatory, the Russian Academy of Sciences, Nizhny Arkhyz, 369167, Russian Federation
Crimean Astrophysical Observatory
Ulugh Beg Astronomical Institute, Uzbek Academy of Sciences, Tashkent, Uzbekistan
National University of Uzbekistan, Tashkent, Uzbekistan
Fesenkov Astrophysical Institute, Almaty, 050020, Kazakhstan
Sternberg Astronomical Institute, Lomonosov Moscow State University, Moscow, 119191, Russian Federation
Evgeni Kharadze Georgian National Astrophysical Observatory, Kanobili, Abastumani, Mt., Adigeni, 0301, Georgia
Samtskhe-Javakheti State University, Rustaveli Str. 113, Akhaltsikhe, 0080, Georgia
University of Warwick, Department of Physics, Coventry, CV4 7AL, United Kingdom
Osservatorio Astronomico di Brera, Istituto Nazionale di Astrofisica, Merate, 23807, Italy
Instituto de Astrofísica de Canarias, La Laguna Tenerife, E-38205, Spain
Universidad de La Laguna, Departamento de Astroísica, La Laguna Tenerife, E-38206, Spain
University of Sheffield, Department of Physics and Astronomy, Sheffield, S3 7RH, United Kingdom
Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Macul Av. Vicuña Mackenna 4860, Santiago, 7820436, Chile
Millennium Institute of Astrophysics, Providencia Nuncio Monseñor Sótero Sanz 100, Of 104, Santiago, Chile
European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, Noordwijk, 2201 AZ, Netherlands
The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, Stockholm, Stockholm, SE-106 91, Sweden
Stockholm University, KTH Royal Institute of Technology, Hannes Alfvéns väg 12, Stockholm, Stockholm, SE-106 91, Sweden
School of Physics, Centre for Space Research, University College Dublin, Dublin, D04 V1W8, Ireland
Physics Department, Tsinghua University, Beijing, 100084, China
Institute for Gravitational Wave Astronomy, Henan Academy of Sciences, Henan, Zhengzhou, 450046, China
Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, Jhongli, 32001, Taiwan
Institute of Astronomy, National Tsing Hua University, No. 101 Sect. 2 Kuang-Fu Road, Hsinchu, 30013, Taiwan
Yunnan Observatories, Chinese Academy of Sciences, Kunming, 650216, China
Center for Astronomical Mega-Science, Chinese Academy of Sciences, Beijing, 100012, China
Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming, 650216, China
Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, 93117-5575, CA, United States
Department of Physics, University of California, Santa Barbara, 93106-9530, CA, United States
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, 21218, MD, United States
Department of Physics and Astronomy, Clemson University, Clemson, 29631, SC, United States
Center for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, Reykjavík, 107, Iceland
Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, Cambridge, MA, United States
Aix Marseille Université, CNRS, CNES, LAM, Marseille, France
National Astronomical Research Institute of Thailand, 260 Moo 4, T. Donkaew, A. Maerim, Chiangmai, 50180, Thailand
Adler Planetarium, 1300 South DuSable Lake Shore Drive, Chicago, 60605, IL, United States
Department of Physics, School of Physics and Materials Science, Nanchang University, Nanchang, 330031, China
National Astronomical Observatories
School of Astronomy and Space Science
Altay Astronomical Observatory
Radboud University Nijmegen
Department of Astronomy
Purple Mountain Observatory
Key Laboratory of Particle Astrophysics
School of Astronomy and Space Science
Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University
Department of Astronomy
Space Research Institute
National Research University “Higher School of Economics
Moscow Institute of Physics and Technology (MIPT)
School of Physics and Astronomy
Institute for Frontier in Astronomy and Astrophysics
Nevada Center for Astrophysics
Department of Physics and Astronomy
Astrophysics Research Institute
University of Messina
Instituto de Alta Investigación
Anton Pannekoek Institute of Astronomy
Space Science Data Center (SSDC)
Osservatorio Astronomico di Roma
The Cosmic Dawn Centre (DAWN)
Niels Bohr Institute
Instituto de Astrofísica de Andalucía (IAA-CSIC)
Ingeniería de Sistemas y Automática
Center for Astrophysics and Supercomputing
ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)
Special Astrophysical Observatory
Crimean Astrophysical Observatory
Ulugh Beg Astronomical Institute
National University of Uzbekistan
Fesenkov Astrophysical Institute
Sternberg Astronomical Institute
Evgeni Kharadze Georgian National Astrophysical Observatory
Samtskhe-Javakheti State University
University of Warwick
Osservatorio Astronomico di Brera
Instituto de Astrofísica de Canarias
Universidad de La Laguna
University of Sheffield
Instituto de Astrofísica
Millennium Institute of Astrophysics
European Space Agency (ESA)
The Oskar Klein Centre
Stockholm University
School of Physics
Physics Department
Institute for Gravitational Wave Astronomy
Graduate Institute of Astronomy
Institute of Astronomy
Yunnan Observatories
Center for Astronomical Mega-Science
Key Laboratory for the Structure and Evolution of Celestial Objects
Las Cumbres Observatory
Department of Physics
Space Telescope Science Institute
Department of Physics and Astronomy
Center for Astrophysics and Cosmology
Massachusetts Institute of Technology
Aix Marseille Université
National Astronomical Research Institute of Thailand
Adler Planetarium
Department of Physics
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