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

Xinwen Shu; Lei Yang; Haonan Yang; Fan Xu; Jin-Hong Chen; Rob A. J. Eyles-Ferris; Lixin Dai; Yunwei Yu; Rong-Feng Shen; Luming Sun; Hucheng Ding; WeiKang Zheng; Ning Jiang; Wenxiong Li; Ning-Chen Sun; Dong Xu; Zhumao Zhang; Chichuan Jin; Arne Rau; Tinggui Wang; Xue-feng Wu; Weimin Yuan; Bing Zhang; Kirpal Nandra; Alexei V. Filippenko; Frédérick Poidevin; Roberto Soria; Amit Kumar; David S. Aguado; Fangxia An; Tao An; Jie An; Moira Andrews; Rungrit Anutarawiramkul; Pietro Baldini; Thomas G. Brink; Pathompong Butpan; Zhiming Cai; Alberto J. Castro-Tirado; Huaqing Cheng; Weiwei Cui; Joseph Farah; Shaoyu Fu; Johan P.U. Fynbo; Xing Gao; Dawei Han; Xuhui Han; D. Andrew Howell; Jingwei Hu; Shuaiqing Jiang; Brajesh Kumar; Weihua Lei; Dongyue Li; Chengkui Li; Huaqiu Liu; Xing Liu; Yuan Liu; Xiaowei Liu; Alicia López-Oramas; David López Fernández-Nespral; Justyn R. Maund; Curtis McCully; Zexi Niu; Megan Newsome; Paul O’Brien; Haiwu Pan; Yu Pan; Estefania Padilla Gonzalez; Ismael Pérez-Fournon; Walter Silima; Hui Sun; Shengli Sun; Xiaojin Sun; Giacomo Terreran; Samaporn Tinyanont; Junxian Wang; Yanan Wang; Yun Wang; Klaas Wiersema; Yunfei Xu; Yongquan Xue; Yi Yang; Fabao Zhang; Juan Zhang; Pinpin Zhang; Wenda Zhang; Yonghe Zhang; Haisheng Zhao; Zipei Zhu; Liping Xin; Zhuheng Yao; Bertrand Cordier; Jianyan Wei; Yulei Qiu; Frédéric Daigne

doi:10.3847/2041-8213/adf4cd

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

Xinwen Shu, Lei Yang, Haonan Yang, Fan Xu, Jin-Hong Chen, Rob A. J. Eyles-Ferris, Lixin Dai, Yunwei Yu, Rong-Feng Shen, Luming Sun, Hucheng Ding, WeiKang Zheng, Ning Jiang, Wenxiong Li, Ning-Chen Sun, Dong Xu, Zhumao Zhang, Chichuan Jin, Arne Rau, Tinggui WangXue-feng Wu, Weimin Yuan, Bing Zhang, Kirpal Nandra, Alexei V. Filippenko, Frédérick Poidevin, Roberto Soria, Amit Kumar, David S. Aguado, Fangxia An, Tao An, Jie An, Moira Andrews, Rungrit Anutarawiramkul, Pietro Baldini, Thomas G. Brink, Pathompong Butpan, Zhiming Cai, Alberto J. Castro-Tirado, Huaqing Cheng, Weiwei Cui, Joseph Farah, Shaoyu Fu, Johan P.U. Fynbo, Xing Gao, Dawei Han, Xuhui Han, D. Andrew Howell, Jingwei Hu, Shuaiqing Jiang, Brajesh Kumar, Weihua Lei, Dongyue Li, Chengkui Li, Huaqiu Liu, Xing Liu, Yuan Liu, Xiaowei Liu, Alicia López-Oramas, David López Fernández-Nespral, Justyn R. Maund, Curtis McCully, Zexi Niu, Megan Newsome, Paul O’Brien, Haiwu Pan, Yu Pan, Estefania Padilla Gonzalez, Ismael Pérez-Fournon, Walter Silima, Hui Sun, Shengli Sun, Xiaojin Sun, Giacomo Terreran, Samaporn Tinyanont, Junxian Wang, Yanan Wang, Yun Wang, Klaas Wiersema, Yunfei Xu, Yongquan Xue, Yi Yang, Fabao Zhang, Juan Zhang, Pinpin Zhang, Wenda Zhang, Yonghe Zhang, Haisheng Zhao, Zipei Zhu, Liping Xin, Zhuheng Yao, Bertrand Cordier, Jianyan Wei, Yulei Qiu, Frédéric Daigne

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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 L0.5−4 keV ≈ 1048 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 1044 erg s−1 and 1041 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.

Original language	English
Number of pages	21
Journal	Astrophysical Journal Letters
Volume	990
Issue number	1
Early online date	29 Aug 2025
DOIs	https://doi.org/10.3847/2041-8213/adf4cd
Publication status	Published - 1 Sept 2025

Keywords

Relativistic jets
X-ray transient sources
Gamma-ray bursts
Tidal disruption
Black holes

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© 2025 The Author(s) . This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/
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Shu, X., Yang, L., Yang, H., Xu, F., Chen, J.-H., Eyles-Ferris, R. A. J., Dai, L., Yu, Y., Shen, R.-F., Sun, L., Ding, H., Zheng, W., Jiang, N., Li, W., Sun, N.-C., Xu, D., Zhang, Z., Jin, C., Rau, A., ... Daigne, F. (2025). EP241021a: A Months-duration X-Ray Transient with Luminous Optical and Radio Emission. Astrophysical Journal Letters, 990(1). https://doi.org/10.3847/2041-8213/adf4cd

@article{78a9cb400355439aae114ee04a8fd0ab,

title = "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 L0.5−4 keV ≈ 1048 erg s−1. Further observations with EP{\textquoteright}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 1044 erg s−1 and 1041 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.",

keywords = "Relativistic jets, X-ray transient sources, Gamma-ray bursts, Tidal disruption, Black holes",

author = "Xinwen Shu and Lei Yang and Haonan Yang and Fan Xu and Jin-Hong Chen and Eyles-Ferris, \{Rob A. J.\} and Lixin Dai and Yunwei Yu and Rong-Feng Shen and Luming Sun and Hucheng Ding and WeiKang Zheng and Ning Jiang and Wenxiong Li and Ning-Chen Sun and Dong Xu and Zhumao Zhang and Chichuan Jin and Arne Rau and Tinggui Wang and Xue-feng Wu and Weimin Yuan and Bing Zhang and Kirpal Nandra and Filippenko, \{Alexei V.\} and Fr{\'e}d{\'e}rick Poidevin and Roberto Soria and Amit Kumar and Aguado, \{David S.\} and Fangxia An and Tao An and Jie An and Moira Andrews and Rungrit Anutarawiramkul and Pietro Baldini and Brink, \{Thomas G.\} and Pathompong Butpan and Zhiming Cai and Castro-Tirado, \{Alberto J.\} and Huaqing Cheng and Weiwei Cui and Joseph Farah and Shaoyu Fu and Fynbo, \{Johan P.U.\} and Xing Gao and Dawei Han and Xuhui Han and Howell, \{D. Andrew\} and Jingwei Hu and Shuaiqing Jiang and Brajesh Kumar and Weihua Lei and Dongyue Li and Chengkui Li and Huaqiu Liu and Xing Liu and Yuan Liu and Xiaowei Liu and Alicia L{\'o}pez-Oramas and Fern{\'a}ndez-Nespral, \{David L{\'o}pez\} and Maund, \{Justyn R.\} and Curtis McCully and Zexi Niu and Megan Newsome and Paul O{\textquoteright}Brien and Haiwu Pan and Yu Pan and \{Padilla Gonzalez\}, Estefania and Ismael P{\'e}rez-Fournon and Walter Silima and Hui Sun and Shengli Sun and Xiaojin Sun and Giacomo Terreran and Samaporn Tinyanont and Junxian Wang and Yanan Wang and Yun Wang and Klaas Wiersema and Yunfei Xu and Yongquan Xue and Yi Yang and Fabao Zhang and Juan Zhang and Pinpin Zhang and Wenda Zhang and Yonghe Zhang and Haisheng Zhao and Zipei Zhu and Liping Xin and Zhuheng Yao and Bertrand Cordier and Jianyan Wei and Yulei Qiu and Fr{\'e}d{\'e}ric Daigne",

note = "{\textcopyright} 2025 The Author(s) . This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/",

year = "2025",

month = sep,

day = "1",

doi = "10.3847/2041-8213/adf4cd",

language = "English",

volume = "990",

journal = "Astrophysical Journal Letters",

issn = "2041-8205",

publisher = "American Astronomical Society",

number = "1",

}

Shu, X, Yang, L, Yang, H, Xu, F, Chen, J-H, Eyles-Ferris, RAJ, Dai, L, Yu, Y, Shen, R-F, Sun, L, Ding, H, Zheng, W, Jiang, N, Li, W, Sun, N-C, Xu, D, Zhang, Z, Jin, C, Rau, A, Wang, T, Wu, X, Yuan, W, Zhang, B, Nandra, K, Filippenko, AV, Poidevin, F, Soria, R, Kumar, A, Aguado, DS, An, F, An, T, An, J, Andrews, M, Anutarawiramkul, R, Baldini, P, Brink, TG, Butpan, P, Cai, Z, Castro-Tirado, AJ, Cheng, H, Cui, W, Farah, J, Fu, S, Fynbo, JPU, Gao, X, Han, D, Han, X, Howell, DA, Hu, J, Jiang, S, Kumar, B, Lei, W, Li, D, Li, C, Liu, H, Liu, X, Liu, Y, Liu, X, López-Oramas, A, Fernández-Nespral, DL, Maund, JR, McCully, C, Niu, Z, Newsome, M, O’Brien, P, Pan, H, Pan, Y, Padilla Gonzalez, E, Pérez-Fournon, I, Silima, W, Sun, H, Sun, S, Sun, X, Terreran, G, Tinyanont, S, Wang, J, Wang, Y, Wang, Y, Wiersema, K, Xu, Y, Xue, Y, Yang, Y, Zhang, F, Zhang, J, Zhang, P, Zhang, W, Zhang, Y, Zhao, H, Zhu, Z, Xin, L, Yao, Z, Cordier, B, Wei, J, Qiu, Y & Daigne, F 2025, 'EP241021a: A Months-duration X-Ray Transient with Luminous Optical and Radio Emission', Astrophysical Journal Letters, vol. 990, no. 1. https://doi.org/10.3847/2041-8213/adf4cd

TY - JOUR

T1 - EP241021a: A Months-duration X-Ray Transient with Luminous Optical and Radio Emission

AU - Shu, Xinwen

AU - Yang, Lei

AU - Yang, Haonan

AU - Xu, Fan

AU - Chen, Jin-Hong

AU - Eyles-Ferris, Rob A. J.

AU - Dai, Lixin

AU - Yu, Yunwei

AU - Shen, Rong-Feng

AU - Sun, Luming

AU - Ding, Hucheng

AU - Zheng, WeiKang

AU - Jiang, Ning

AU - Li, Wenxiong

AU - Sun, Ning-Chen

AU - Xu, Dong

AU - Zhang, Zhumao

AU - Jin, Chichuan

AU - Rau, Arne

AU - Wang, Tinggui

AU - Wu, Xue-feng

AU - Yuan, Weimin

AU - Zhang, Bing

AU - Nandra, Kirpal

AU - Filippenko, Alexei V.

AU - Poidevin, Frédérick

AU - Soria, Roberto

AU - Kumar, Amit

AU - Aguado, David S.

AU - An, Fangxia

AU - An, Tao

AU - An, Jie

AU - Andrews, Moira

AU - Anutarawiramkul, Rungrit

AU - Baldini, Pietro

AU - Brink, Thomas G.

AU - Butpan, Pathompong

AU - Cai, Zhiming

AU - Castro-Tirado, Alberto J.

AU - Cheng, Huaqing

AU - Cui, Weiwei

AU - Farah, Joseph

AU - Fu, Shaoyu

AU - Fynbo, Johan P.U.

AU - Gao, Xing

AU - Han, Dawei

AU - Han, Xuhui

AU - Howell, D. Andrew

AU - Hu, Jingwei

AU - Jiang, Shuaiqing

AU - Kumar, Brajesh

AU - Lei, Weihua

AU - Li, Dongyue

AU - Li, Chengkui

AU - Liu, Huaqiu

AU - Liu, Xing

AU - Liu, Yuan

AU - Liu, Xiaowei

AU - López-Oramas, Alicia

AU - Fernández-Nespral, David López

AU - Maund, Justyn R.

AU - McCully, Curtis

AU - Niu, Zexi

AU - Newsome, Megan

AU - O’Brien, Paul

AU - Pan, Haiwu

AU - Pan, Yu

AU - Padilla Gonzalez, Estefania

AU - Pérez-Fournon, Ismael

AU - Silima, Walter

AU - Sun, Hui

AU - Sun, Shengli

AU - Sun, Xiaojin

AU - Terreran, Giacomo

AU - Tinyanont, Samaporn

AU - Wang, Junxian

AU - Wang, Yanan

AU - Wang, Yun

AU - Wiersema, Klaas

AU - Xu, Yunfei

AU - Xue, Yongquan

AU - Yang, Yi

AU - Zhang, Fabao

AU - Zhang, Juan

AU - Zhang, Pinpin

AU - Zhang, Wenda

AU - Zhang, Yonghe

AU - Zhao, Haisheng

AU - Zhu, Zipei

AU - Xin, Liping

AU - Yao, Zhuheng

AU - Cordier, Bertrand

AU - Wei, Jianyan

AU - Qiu, Yulei

AU - Daigne, Frédéric

PY - 2025/9/1

Y1 - 2025/9/1

N2 - 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 L0.5−4 keV ≈ 1048 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 1044 erg s−1 and 1041 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.

AB - 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 L0.5−4 keV ≈ 1048 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 1044 erg s−1 and 1041 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.

KW - Relativistic jets

KW - X-ray transient sources

KW - Gamma-ray bursts

KW - Tidal disruption

KW - Black holes

U2 - 10.3847/2041-8213/adf4cd

DO - 10.3847/2041-8213/adf4cd

M3 - Article

SN - 2041-8205

VL - 990

JO - Astrophysical Journal Letters

JF - Astrophysical Journal Letters

IS - 1

ER -

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

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