EP241021a: A Months-duration X-Ray Transient with Luminous Optical and Radio Emission

Abstract We present the discovery of a peculiar X-ray transient, EP241021a, by the Einstein Probe (EP) mission and the results from multiwavelength follow-up observations. The transient was first detected with the Wide-field X-ray Telescope as an intense flare lasting for ∼100 s, reaching a luminosity of L 0.5−4 keV ≈ 10 48 erg s −1 . Further observations with EP’s Follow-up X-ray Telescope reveal a huge drop in the X-ray flux by a factor of >1000 within 1.5 days. After maintaining a nearly plateau phase for ∼7 days, the X-ray flux decreases ∝ t −1.2 over a period of ∼30 days, followed by a sudden decrease to an undetectable level by EP and XMM-Newton, making it the longest afterglow emission detected among known fast X-ray transients. Bright counterparts at optical and radio wavelengths were also detected, with high peak luminosities in excess of 10 44 erg s −1 and 10 41 erg s −1 , respectively. In addition, EP241021a exhibits a nonthermal X-ray spectrum, red optical color, X-ray and optical rebrightenings in the light curves, and fast radio spectral evolution, suggesting that relativistic jets may have been launched. We discuss the possible origins of EP241021a, including a choked jet with supernova shock breakout, a merger-triggered magnetar, a highly structured jet, and a repeating partial tidal disruption event involving an intermediate-mass black hole, but none can perfectly explain the multiwavelength properties. EP241021a may represent a new type of X-ray transient with month-duration evolution timescales; future EP detections and follow-up observations of similar systems will provide statistical samples to understand the underlying mechanisms at work.

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Journal
The Astrophysical Journal Letters
Published
2025-08-29
DOI
https://doi.org/10.3847/2041-8213/adf4cd
Citations
10
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
11.55

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EP241021a: A Months-duration X-Ray Transient with Luminous Optical and Radio Emission

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EP241021a: A Months-duration X-Ray Transient with Luminous Optical and Radio Emission

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article en
10 citations

Abstract

Abstract We present the discovery of a peculiar X-ray transient, EP241021a, by the Einstein Probe (EP) mission and the results from multiwavelength follow-up observations. The transient was first detected with the Wide-field X-ray Telescope as an intense flare lasting for ∼100 s, reaching a luminosity of L 0.5−4 keV ≈ 10 48 erg s −1 . Further observations with EP’s Follow-up X-ray Telescope reveal a huge drop in the X-ray flux by a factor of >1000 within 1.5 days. After maintaining a nearly plateau phase for ∼7 days, the X-ray flux decreases ∝ t −1.2 over a period of ∼30 days, followed by a sudden decrease to an undetectable level by EP and XMM-Newton, making it the longest afterglow emission detected among known fast X-ray transients. Bright counterparts at optical and radio wavelengths were also detected, with high peak luminosities in excess of 10 44 erg s −1 and 10 41 erg s −1 , respectively. In addition, EP241021a exhibits a nonthermal X-ray spectrum, red optical color, X-ray and optical rebrightenings in the light curves, and fast radio spectral evolution, suggesting that relativistic jets may have been launched. We discuss the possible origins of EP241021a, including a choked jet with supernova shock breakout, a merger-triggered magnetar, a highly structured jet, and a repeating partial tidal disruption event involving an intermediate-mass black hole, but none can perfectly explain the multiwavelength properties. EP241021a may represent a new type of X-ray transient with month-duration evolution timescales; future EP detections and follow-up observations of similar systems will provide statistical samples to understand the underlying mechanisms at work.

The Astrophysical Journal LettersVol. 990(1)
University of Copenhagen (DK), University of Science and Technology of China (CN), Las Cumbres Observatory Global Telescope Network (US), Centre National de la Recherche Scientifique (FR), The University of Sydney (AU), Space Telescope Science Institute (US), Adler Planetarium (US), University of Nevada, Las Vegas (US), University of Hertfordshire (GB), University of Leicester (GB), University of California, Santa Barbara (US), University of Cape Town (ZA), Sun Yat-sen University (CN), Universidad de La Laguna (ES), Royal Holloway University of London (GB), Yunnan University (CN), Chinese Academy of Sciences (CN), Beijing Normal University (CN), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Shanghai Astronomical Observatory (CN), Xinjiang Astronomical Observatory (CN), Yunnan Observatories (CN), Institut d'Astrophysique de Paris (FR), Purple Mountain Observatory (CN), Sorbonne Université (FR), Central China Normal University (CN), Astrophysique, Instrumentation et Modélisation (FR), Shanghai Micro Satellite Engineering Center (CN), Max Planck Institute for Extraterrestrial Physics (DE), Osservatorio Astrofisico di Torino (IT), DAWN Center for Independent Living (US), Altaiskiy State Nature Biosphere Reserve (RU), Université Bourgogne Franche-Comté (FR), CEA Paris-Saclay (FR), Instituto de Astrofísica de Canarias (ES), Shanghai Institute of Technical Physics (CN), Institute of High Energy Physics (CN), Instituto de Astrofísica de Andalucía (ES), National Astronomical Observatories (CN), Beijing Chaoyang Emergency Medical Center (CN), University of Chinese Academy of Sciences (CN), Institut de Recherche sur les Lois Fondamentales de l'Univers (FR), Innovation Academy for Microsatellites of Chinese Academy of Sciences, Anhui Normal University (CN), Huazhong University of Science and Technology (CN), Universidad de Málaga (ES), National Astronomical Research Institute of Thailand (TH), University of Hong Kong (HK), University of California, Berkeley (US), Tsinghua University (CN)
National Science Foundation, National Aeronautics and Space Administration, W. M. Keck Foundation, California Institute of Technology, UK Research and Innovation, Ministerio de Ciencia, Innovación y Universidades, Australian Government, Commonwealth Scientific and Industrial Research Organisation, National Research Foundation, University of Pretoria, National Natural Science Foundation of China, Chinese Academy of Sciences, Centre National d’Etudes Spatiales, Government of Jiangsu Province, University of the Western Cape, Max-Planck-Gesellschaft, Natural Science Foundation of Jiangsu Province, Ministerio de Ciencia e Innovación, University of Cape Town, Yunnan University, Science and Technology Facilities Council, National Key Research and Development Program of China
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