On the 21st of October 2024, China’s newly launched Einstein Probe satellite registered something no one expected. From a quiet patch of sky, a sudden explosion of X-rays burst into view. It lasted just over ninety seconds. But what followed over the next forty days left astronomers struggling to explain what they had witnessed. The signal, now known as EP241021a, has become one of the most perplexing events seen in modern high-energy astrophysics.
The initial blast reached luminosities exceeding one hundred thousand trillion times the output of the Sun. It unfolded in the 0.5–4 keV X-ray band and was so powerful that it briefly rivalled gamma-ray bursts, despite the fact that no gamma-ray emission was detected at all. This absence of high-energy gamma rays would prove to be only the first in a series of anomalies.
Roughly a day and a half later, follow-up X-ray observations found that the source had faded by a factor of more than one thousand. Then, unexpectedly, it held steady for nearly a week in a plateau phase before fading again over the next month. All observations were consistent with the same hard X-ray spectrum. Then it vanished completely. By day 89, it was gone from the view of even the most sensitive X-ray telescopes. In terms of duration, EP241021a is now considered the longest-lasting fast X-ray transient ever observed.
The mystery deepened when ground-based telescopes began searching for counterparts in visible light. Within two days of the initial trigger, an optical source had been confirmed. Then, six days in, something strange happened. The optical light began to brighten. Within a span of two days, it reached an absolute magnitude of minus 21.5. In terms of energy, this was equivalent to hundreds of millions of Suns shining all at once. Then it faded. Weeks later, another weaker brightening was detected in the same spot. That one too faded and disappeared.
Unlike most optical events associated with explosive transients, this source showed a persistent red colour. Across forty days of observation, the red tone remained. This behaviour is not typical of supernovae or optical afterglows from gamma-ray bursts. In those cases, the colour profile usually evolves over time. EP241021a didn’t follow that path.
Just over a week after the initial X-ray flash, radio telescopes also picked up signals from the location. The data revealed a delayed but luminous radio afterglow, peaking around ten gigahertz. Over the next two months, the signal remained strong as the peak frequency shifted downward toward two gigahertz. By the end of the monitoring campaign, the radio signal was declining but still visible. This multi-wavelength persistence suggests that whatever caused the explosion had launched an outflow of material, potentially at relativistic speeds.
Using spectral modeling and synchrotron emission analysis, astronomers estimated the energy involved in the radio afterglow alone to be in the range of ten to the power of fifty ergs. For context, this is comparable to the entire energy output of a typical supernova. The expansion speed of the source, based on the equipartition modeling, points to a bulk Lorentz factor of around three. This is fast enough to require relativistic treatment but not as extreme as those seen in traditional gamma-ray burst jets.
Crucially, every major gamma-ray observatory had a clear line of sight to the event when it began. Fermi-GBM, Konus-Wind, and others saw nothing. Not even the weakest signal could be linked to the location or time of EP241021a’s burst. This stands in stark contrast to other transients with similar X-ray characteristics, which typically have high-energy gamma-ray precursors or are directly associated with supernovae or magnetar events.
Attempts to identify the host galaxy led to further complications. Spectroscopic analysis with the Keck telescope found narrow emission lines indicating a redshift of 0.748. That places the galaxy over seven billion light-years away. The host appears to be faint, with moderate star formation activity. No obvious signs of an active galactic nucleus were present, though the emission line ratios could allow for some level of hidden activity. This distance, however, makes any associated supernova features far more difficult to detect, and indeed none were clearly identified in follow-up optical spectra.
Several hypotheses have been proposed. One model suggests a failed gamma-ray burst, in which a jet is formed during the collapse of a massive star but fails to escape the surrounding stellar envelope. In such cases, the energy may leak out more slowly, producing X-ray and optical signals without the accompanying gamma-ray flash. This has been used to explain a handful of previous Einstein Probe events, but those events also showed signs of supernovae. EP241021a does not.
Another explanation points to a newborn magnetar — a highly magnetised neutron star formed in the aftermath of a binary neutron star merger or a white dwarf collapse. The idea here is that such an object could continuously pump energy into the surrounding environment, causing extended emission across multiple wavelengths. This could potentially account for the prolonged X-ray plateau and the late-time optical rebrightening. However, the velocity of the ejecta inferred from the optical data appears too high for typical magnetar-driven outflows unless the system was unusually stripped of mass and compact.
Others have looked to tidal disruption events, where a star is torn apart by a black hole. EP241021a does not behave like typical disruptions of this type, which tend to occur near supermassive black holes and produce softer X-ray spectra over longer timescales. But the idea of a tidal disruption by an intermediate-mass black hole — still a largely theoretical category — has gained some traction. The data, however, don’t line up perfectly with this scenario either.
Perhaps the most intriguing idea under discussion is that of a highly structured jet observed at an off-axis angle. In this model, the core of the jet is not directly visible, but its edges — slower, fainter, and broader — produce the observed emission. As the jet evolves, different regions enter the observer’s line of sight, causing fluctuations in the light curves across different bands. This could explain the rebrightenings, the changing radio spectrum, and the mismatch between expected and observed gamma-ray signatures. However, such a configuration would require very specific viewing angles and jet structures that are not yet well understood.
What makes EP241021a so remarkable is that none of the proposed models fully fit. Each accounts for some features, but leaves others unexplained. The event defies the usual classification systems. It does not behave like a gamma-ray burst, a magnetar wind, a choked jet, a tidal disruption, or a kilonova. Instead, it exhibits a mixture of all of them, along with properties that are simply not predicted by current models.
This confusion is precisely why it has drawn so much attention. The Einstein Probe was launched with the goal of finding transients like this — fast, powerful, and previously hidden. Its wide-field scanning and rapid follow-up capabilities have opened a new observational window. EP241021a may be the first of many similar events to be uncovered in the coming years.
It raises the possibility that we have only been seeing part of the picture. For decades, our understanding of high-energy astrophysics has been shaped by instruments that could only capture the brightest or most conventional examples. As new missions like the Einstein Probe, SVOM, and ULTRASAT come online, more events like EP241021a could be discovered. Some may offer clearer clues. Others may push the mystery even further.
Whatever its true origin, EP241021a has revealed the existence of a new kind of celestial explosion. It has shown that the universe still hides events of staggering power that do not conform to known categories. And it has done so using no gamma rays, no supernova features, and no long-term remnants. It appeared, blazed, shifted, and disappeared.
Future detections will be critical. If even a handful of similar cases can be found, a pattern may emerge. Until then, EP241021a stands alone — not as a one-off curiosity, but as a challenge to current models of how the universe explodes, radiates, and resets. Observations continue, and more updates will follow if related events are confirmed.
Source:
Shu, X., Yang, L., Yang, H., et al. (2025). EP241021a: a months-duration X-ray transient with luminous optical and radio emission. Draft version, May 13, 2025. arXiv:2505.07665
