Astronomers have reported the discovery of an unusual X-ray transient detected by the Einstein Probe that does not fit any known class of cosmic explosions. The paper presenting its multiwavelength analysis was published in the journal Monthly Notices of the Royal Astronomical Society on June 13.
All eyes on it
On March 5, 2024, a space telescope called the Einstein Probe—designed to scan the sky for sudden X-ray flashes—caught a brief, never-before-seen source called EP240305a. It produced two brief X-ray flares, one right after the other, separated by about 200 seconds of quiet.
Researchers quickly pointed several telescopes at this source to gather more data in X-rays, infrared, optical and radio wavelengths; the analysis of these multiwavelength data is presented in the new study.
They noticed that the X-rays faded rapidly over the following days, while radio observations faded much more slowly over weeks, revealing evidence of an evolving jet. A faint, fading near-infrared source was spotted at the location, and there was no detection at optical wavelengths.
What it’s not
To understand its true nature, they compared its properties with other known events that send out bright flashes or short-lived explosions, such as: a jetted tidal disruption event where a star is shredded by a black hole and launches a jet, a black hole or neutron star “X-ray binary” outburst where a compact object feeding on its companion star suddenly flares up, a thermonuclear burst on a neutron star’s surface, a magnetar giant flare, a stellar flare, or an energetic gamma-ray burst (GRB).
They ruled out tidal disruption events and typical X-ray binary outbursts because they take months to fade, whereas EP240305a faded in days. Even a short-lived subclass of X-ray binary outbursts, which can fade within weeks, doesn’t show strong radio emission as this source did. A thermonuclear burst didn’t fit either, since its measured temperature was too low and its radio signal lasted far too long for that scenario.
A magnetar giant flare was also ruled out, as those events rise and fade in under a second—much faster than EP240305a’s two flares, which each lasted minutes. Similarly, the radio signal lasted far too long compared with typical stellar flares.
Match or no match?
Interestingly, its properties line up with several features of GRBs. The double-flash pattern resembles a known feature seen in some GRBs called “double bursts.” The X-ray brightness pattern matches the typical pattern seen in GRBs. The radio emission’s fading behavior over time matches what is expected from a GRB afterglow.
However, there was one problem: No actual gamma rays were ever detected. This makes it what astronomers call a “gamma-ray-dark” GRB-like event—meaning it has GRB-like behavior but didn’t actually show the namesake gamma-ray flash.
“The lack or weakness of gamma-ray emission in such cases may be attributed to several factors,” researchers explain in the paper. This could happen if the jet was pointed slightly off-axis from Earth, if the jet failed to fully break out of its surrounding material (a “choked jet”), or if the jet was a “dirty fireball,” loaded with extra material that dampens its gamma-ray output.
Hide and seek
Given the current data, the researchers can’t say for certain what EP240305a is. “In the case of EP240305a, the current data do not allow us to firmly establish a GRB origin, and we therefore conservatively classify it as a gamma-ray-dark GRB-like transient or more broadly an extragalactic fast X-ray transient,” they conclude.
Faint events like this one tend to get overshadowed by brighter, more obvious explosions, making them easy to miss with less sensitive instruments. The researchers note that highly sensitive telescopes like the Einstein Probe, combined with rapid follow-up observations across other wavelengths, are important for catching and studying this kind of faint, easily overlooked event.
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Publication details
Ruican Ma et al, Multiwavelength Analysis of the Einstein Probe X-ray Transient EP240305a, Monthly Notices of the Royal Astronomical Society (2026). DOI: 10.1093/mnras/stag1138. On arXiv: DOI: 10.48550/arxiv.2606.14341
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Einstein Probe detects mysterious X-ray transient that doesn’t fit any known class (2026, June 23)
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