The image of EP241021a detected in one of the WXT CMOS detectors. Credit: arXiv (2025). DOI: 10.48550/arxiv.2505.07665
An international team of astronomers using the Einstein Probe reports the discovery of a new peculiar fast-evolving X-transient. The newfound transient exhibits an unprecedented long-lasting X-ray emission. The finding was detailed in a paper published May 12 on the arXiv preprint server.
Intense bursts of soft X-ray emission lasting tens to thousands of seconds with a wide range of luminosities are known as fast-evolving X-ray transients (FEXTs). The nature of FEXTs is still puzzling. However, astronomers trying to explain their origin take into account several scenarios; for instance, stellar flares, supernova shock breakouts, or long gamma-ray bursts (GRBs).
One of the crucial space telescopes for the search and investigation of FEXTs is the Einstein Probe (EP), also known as the Tianguan telescope. Launched into space in early 2024, EP is capable of determining the temporal evolution properties of the afterglow X-ray emission in FEXTs, including the duration, the shape of light curve, and spectral evolution, which is essential to understanding the origin of these transients.
To date, EP has detected dozens of FEXTs, and now, a group of astronomers, led by Xinwen Shu of the Anhui Normal University in China, reports the detection of a new transient of this type with the help of EP’s wide-field X-ray telescope (WXT).
“We report the discovery of a peculiar X-ray transient, EP241021a, by EP/WXT on Oct 21, 2024, and the results from follow-up multiwavelength observations including X-ray, optical and radio,” the researchers wrote in the paper.
EP241021a was identified by WXT as an intense flare, which lasted for approximately 92 seconds and reached a luminosity of about one quindecillion erg/s. The X-ray spectrum of the flare was found to be relatively hard with a photon index of 1.8.
Follow-up observations of the flare up to 79 days after its detection revealed that its X-ray light curve shows a nearly plateau phase for the first 7 days, which is followed by a steep decline during the period of about 30 days. Afterward, the X-ray emission rapidly drops under the detection level.
Furthermore, the observations detected optical emission associated with EP241021a within 1.8 days after the X-ray emission was identified, possibly related to the afterglow of the prompt X-ray emission. The appearance of the radio counterpart was also detected about 8.4 days after the X-ray emission started.
The astronomers underlined that such a long duration of X-ray emission, as in the case of EP241021a, is unprecedented among the known extragalactic FEXTs. The team concluded that EP241021a is an extremely unusual transient, and multiple emission components might be behind its multi-wavelength properties.
Trying to explain the origin of EP241021a, the authors of the paper favor two scenarios.
“We have considered a variety of scenarios as origins of EP241021a, and favor a magnetar engine or jetted TDE [tidal disruption event], although neither can perfectly interpret the multiwavelength properties,” the scientists concluded.
More information:
Shu Xinwen et al, EP241021a: a months-duration X-ray transient with luminous optical and radio emission, arXiv (2025). DOI: 10.48550/arxiv.2505.07665
Journal information:
arXiv
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Einstein Probe detects a peculiar X-ray transient (2025, May 21)
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