Astronomers have linked an X-ray flash discovered by the Einstein Probe to a known type of supernova with unusual features that point to the birth of an ultra-dense, rapidly spinning magnetar. The paper outlining this finding was published in The Astrophysical Journal Letters on July 22.
Winking in X-rays
There is a category of events called extragalactic fast X-ray transients (EFXTs): brief flashes of X-rays lasting minutes to hours whose origins are still poorly understood. A rarer subset, called X-ray flashes, is especially puzzling. Some researchers think they originate from an entirely different kind of dying star. A few have been directly linked to a visible counterpart in the form of a supernova.
Their detection has been difficult because most space telescopes are built to monitor bursts of gamma rays, not lower-energy (softer) X-rays.
The Einstein Probe satellite, designed to hunt faint X-ray signals, has been contributing to the growing sample of extragalactic fast X-ray transients and X-ray flashes. It has found three X-ray flashes corresponding to supernovae so far.
In this new study, astronomers led by Gokul P. Srinivasaragavan of the University of Maryland analyzed an X-ray flash, EP250827b, and its supernova counterpart to determine the physical mechanism behind the explosion. This event, however, was found differently than the three before it. Rather than triggering as an obvious, bright signal, EP250827b appeared as a faint, borderline detection in the Einstein Probe’s data. It was too weak on its own to be confirmed as real.
A joint analysis
The confirmation came from a separate project pairing the Einstein Probe with the Zwicky Transient Facility (ZTF). It is a ground-based telescope that scans the same patches of sky the Einstein Probe is watching, looking for matching flashes of visible light. ZTF spotted a new optical transient (supernova) in the same location just 5.5 hours after the X-ray flash, confirming the X-ray signal as genuine and giving the event a name: SN 2025wkm. “Its confirmation as a real EFXT was only done after ZTF discovered its optical counterpart,” the team writes in the paper.
With data from both the X-ray flash and the supernova, the team tracked the object for weeks across X-ray, ultraviolet, optical, infrared and radio wavelengths and put together a detailed picture of the explosion.
The supernova data showed no trace of hydrogen or helium. That, combined with signatures of extremely fast-moving debris, was consistent with a “Type Ic-BL” supernova—the same violent, envelope-stripped category linked to previous X-ray flashes. Early spectra also showed ejecta racing outward at roughly 40,000 kilometers per second (25,000 miles per second).
A missing piece
The light curve (LC)—a graph of how the explosion’s light changes over time—told a different story. After the explosion’s initial brightness peak, most supernovae fade in a fairly predictable way, powered by the radioactive decay of nickel-56 formed in the blast. SN 2025wkm, however, did not follow that pattern.
Instead of declining smoothly, its total energy output flattened for about 20 days—the first time this kind of extended plateau has been seen in one of these X-ray flash supernovae. “This makes EP250827b/SN 2025wkm the first EP XRF-SN with a plateau in its bolometric luminosity LC, providing evidence for an extra central-engine-powering source,” the researchers note. They found that radioactive decay alone couldn’t explain this behavior.
The explanation that fit was a newborn magnetar, an ultra-dense neutron star with an extraordinarily powerful magnetic field, spinning several hundred times per second and slowly transferring energy into the expanding debris. Modeling the data, the team estimated a magnetic field strength around 5×10¹⁴ gauss—roughly a quadrillion times stronger than Earth’s average magnetic field of 0.5 gauss—and a spin period of just 1.9 milliseconds.
This rare X-ray flash was produced when a massive star’s core collapsed under gravity, leaving behind a powerful magnetar. It’s now the fourth such X-ray flash supernova found by the Einstein Probe. “Every new event has provided the community with unprecedented opportunities to understand the amount of diversity regarding SNe explosion mechanisms, relativistic jet formation, central-engine activity, and CSM properties surrounding massive stars,” the team concludes.
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Publication details
Gokul P. Srinivasaragavan et al, EP250827b/SN 2025wkm: An X-Ray Flash-supernova Powered by a Central Engine and Circumstellar Interaction, The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/ae7a68
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Einstein Probe catches rare X-ray flash from an exploding star (2026, August 16)
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