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EP241021a: a months-duration X-ray transient with luminous optical and radio emission

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arxiv 2505.07665 v2 pith:QY5U2PTP submitted 2025-05-12 astro-ph.HE

EP241021a: a months-duration X-ray transient with luminous optical and radio emission

Shu Xinwen , Yang Lei , Yang Haonan , Xu Fan , Chen Jinhong , Eyles-Ferris Rob A. J. , Dai Lixin , Yu Yunwei
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Shen Rongfeng Sun Luming Ding Hucheng Jiang Ning Li Wenxiong Sun Ningchen Xu Dong Zheng Weikang Zhang Zhumao Jin Chichuan Rau Arne Wang Tinggui Wu Xuefeng Yuan Weimin Zhang Bing Nandra Kirpal Aguado David S. An Fangxia An Tao An Jie Andrews Moira Anutarawiramkul Rungrit Baldini Pietro Brink Thomas G. Butpa Pathompong Cai Zhiming Castro-Tirado Alberto J. Cheng Huaqing Cui Weiwei Farah Joseph Filippenko Alexei V. Fu Shaoyu Johan P.U. Fynbo Gao Xing Han Dawei Han Xuhui Howell D. Andrew Hu Jingwei Jiang Shuaiqing Kumar Amit Lei Weihua Li Dongyue Li Chengkui Liu Huaqiu Liu Xing Liu Yuan L\'opez-Oramas Alicia Fern\'andez-Nespral David L\'opez Maund Justyn R. McCully Curtis Niu Zexi Newsome Megan Paul O'Brien Pan Haiwu Padilla Gonzalez Estefania P\'erez-Fournon Ismael Poidevin Fr\'ed\'erick Silim Walter Soria Roberto Sun Hui Sun Shengli Sun Xiaojin Terreran Giacomo Tinyanont Samaporn Wang Junxian Wang Yanan Wang Yun Wiersema Klass Xu Yunfei Xue Yongquan Zhang Fabao Zhang Juan Zhang Pinpin Zhang Wenda Zhang Yonghe Zhao Haisheng Zhu Zipei
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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 at z=0.748. 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 declines as 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. A bright counterpart at optical and radio wavelengths was also detected, with high peak luminosities in excess of 10^44 erg/s and 10^41 erg/s, respectively. In addition, EP241021a exhibits a non-thermal 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 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 transients with months-duration evolution timescales, and 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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Cited by 6 Pith papers

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  1. AT2019ijn: a fast-rising, slow-decaying blue optical transient with exceptionally bright radio emission

    astro-ph.HE 2026-07 conditional novelty 7.0

    AT2019ijn combines LFBOT-like fast optical rise and blue color with slow decay and radio luminosity peaking late at 2e31 erg/s/Hz, best fit as an off-axis jetted IMBH TDE.

  2. Detectability of Gravitational-wave counterparts of EP-FXTs observed during the O4b LIGO-Virgo-KAGRA Observing Run

    astro-ph.HE 2026-07 conditional novelty 6.0

    No gravitational-wave counterpart is statistically associated with any of 47 Einstein Probe fast X-ray transients in the O4b run; 90% exclusion distances of ~178 Mpc (BNS) and ~349 Mpc (NSBH) disfavor nearby compact-b...

  3. Multi-wavelength Constraints on the Transient EP250905a

    astro-ph.HE 2026-07 conditional novelty 5.0

    EP250905a is best explained as a mildly off-axis structured-jet afterglow at z=2.714, possibly weakly magnified by a foreground galaxy at z=0.374.

  4. A Unified Model for the Emission of Supernova-Associated Fast X-ray Transients: Case Studies of EP240414a, EP250108a, and GRB~171205A

    astro-ph.HE 2026-06 unverdicted novelty 5.0

    A magnetar model unifies emission phases of supernova-associated fast X-ray transients EP240414a, EP250108a, and GRB 171205A via jet, cocoon, wind, and PWN components.

  5. In-flight calibration of the Wide-field X-ray Telescope on board the Einstein Probe

    astro-ph.IM 2026-06 accept novelty 4.0

    In-flight calibration of the Einstein Probe WXT confirms 3-6 arcmin spatial resolution across most of the field of view, 1.3 arcmin positioning accuracy, and effective area within 10% of models in the 0.5-4 keV band, ...

  6. Discovery of a Supernova Following the Einstein Probe Transient EP250302a at z = 1.131

    astro-ph.HE 2026-06 unverdicted novelty 4.0

    The paper identifies supernova emission matching a scaled SN 1998bw template in the late-time light curve of EP250302a at z=1.131, with early data constraining the jet Lorentz factor above 25.