An international team of astronomers using the Einstein Probe has detected a highly unusual fast-evolving X-ray transient designated EP241021a. Detailed in a recent study published on the arXiv preprint server, this transient exhibits an unprecedented long-lasting X-ray emission phase that defies typical classifications. Launched in early 2024, the Einstein Probe (EP) is designed to monitor wide-field X-ray emissions and capture transient phenomena across the sky, providing critical insights into the temporal and spectral evolution of such events. EP241021a stands out due to its extraordinary duration and multiwavelength emission signatures, pushing the boundaries of current understanding of X-ray transient phenomena.
A Peculiar Transient With Unprecedented X-Ray Emission Duration
“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. The transient was initially detected by EP’s wide-field X-ray telescope (WXT) as an intense flare lasting approximately 92 seconds with a luminosity reaching about one quindecillion erg per second, accompanied by a relatively hard X-ray spectrum with a photon index of 1.8. Uniquely, its X-ray light curve revealed a nearly flat plateau phase during the first 7 days, followed by a sharp decline over the next 30 days before dropping below detection limits. This extended plateau phase is unprecedented among extragalactic fast-evolving X-ray transients (FEXTs), which typically exhibit much shorter emission durations. The prolonged high-energy emission suggests complex underlying physical processes sustaining the source far longer than expected for typical stellar flares, supernova shock breakouts, or gamma-ray bursts.
Multiwavelength Follow-Up Reveals Complex Emission Behavior
Beyond the initial X-ray flare, the team conducted extensive follow-up observations that uncovered additional emissions across the electromagnetic spectrum. Optical emissions linked to EP241021a appeared within 1.8 days after the initial X-ray detection, likely representing the afterglow of the prompt emission. Radio counterparts were also detected approximately 8.4 days after the event onset, indicating the presence of relativistic outflows or interactions with the surrounding medium. Such multiwavelength signatures are essential to piece together the physical mechanisms behind the transient, as the combination of X-ray, optical, and radio emissions can reveal jet formation, shock interactions, or engine activity. The coexistence of these emissions over weeks points to a complex source structure with multiple emission components contributing to the observed signals, setting EP241021a apart from more conventional fast transients.
Possible Origins: Magnetar Engine or Jetted Tidal Disruption Event
“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. A magnetar—a highly magnetized neutron star—could inject sustained energy to power the extended X-ray plateau, while a jetted tidal disruption event (where a star is torn apart by a supermassive black hole, producing jets) might explain the multiwavelength emissions and prolonged activity. However, neither model fully reconciles all the observed features, such as the precise light curve shape, spectral hardness, and timing of optical and radio counterparts. This ambiguity highlights the complexity of EP241021a and suggests that novel or hybrid astrophysical processes might be at play, inviting further theoretical and observational investigation into the nature of fast X-ray transients.
The Role of the Einstein Probe in Advancing Transient Astronomy
Since its launch, the Einstein Probe has revolutionized the detection and study of fast-evolving X-ray transients. With its sensitive wide-field X-ray telescope, EP can capture the dynamic evolution of such events with unprecedented temporal and spectral resolution. EP241021a represents one of the most extraordinary discoveries to date, showcasing EP’s ability to reveal phenomena that challenge existing models. This transient’s long duration and multiwavelength complexity exemplify the diverse population of fast transients that remain poorly understood. As EP continues to scan the sky, it is expected to uncover more unusual objects that will deepen astrophysicists’ understanding of high-energy processes and extreme environments in the universe.
