Could a space telescope record an X-ray flash without raising the alert that usually sends astronomers running to follow it? That is what happened with the Einstein Probe X-ray flash EP250827b, linked to supernova SN 2025wkm. Einstein Probe recorded the signal, but it fell below the mission’s onboard trigger threshold and was not sent as a public alert. Researchers confirmed it as a real transient only after the Zwicky Transient Facility (ZTF) spotted a supernova in the same patch of sky. They then linked the optical discovery to X-ray data the spacecraft had already collected.
The event is an unusual case of discovery by cross-matching surveys, not a spacecraft catching a flash in real time. The team compared ZTF alerts with Einstein Probe’s X-ray detections and their error circles. The optical position was 0.27 arcminutes from the X-ray location, and the ZTF detection came 0.23 days after the X-ray timestamp. Their peer-reviewed study in The Astrophysical Journal Letters identifies EP250827b/SN 2025wkm as the fourth X-ray-flash supernova discovered with Einstein Probe, and the third of those studied in detail. A faint signal that never became a public alert still became useful once another survey revealed where to look.
What did Einstein Probe record?
EP250827b was a soft X-ray flash that lasted more than 1,000 seconds. Spectral fits put its peak energy below 1.5 kiloelectronvolts at 90 percent confidence, much softer than the emission from a typical gamma-ray burst. The associated supernova lies at redshift 0.1194 and was classified as a broad-lined Type Ic event, or SN Ic-BL.
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That classification tells us something about the star before it exploded. Its outer hydrogen and helium layers were missing, while broad absorption features in its spectrum point to fast-moving debris. But the spectrum and light curve do not show a magnetar directly. Astronomers have to infer the hidden engine by asking which physical models can account for the X-rays and the changing light across several wavelengths.
Why might a shock produce the X-rays?
The researchers propose that the collapse created a rapidly spinning magnetar, possibly surrounded by an accretion disk. Winds from the newborn neutron star and outflows from the disk could drive a fast shock into gas the star had shed before it died. In the model, the surrounding material extends to about 1013 centimetres from the star. As the shock breaks through that gas, the interaction can produce the observed X-ray flash.
This is a model for the signal, not a picture of an expanding shell. The proposed shock-breakout mechanism helps explain why X-rays appeared; it is separate from the question of what kept the supernova bright over the following weeks. Keeping those two questions apart is important: the flash alone does not prove that a magnetar powered every part of the explosion.
What does the magnetar explain?
SN 2025wkm’s optical light curve has two peaks in several filters. Its bolometric brightness—the estimated total energy output across wavelengths—settled into a plateau for about 20 days after the first peak. The researchers tested models in which a magnetar’s spin-down supplies energy to the late supernova, alongside radioactive nickel produced in the explosion.
One such fit gives the newborn magnetar a magnetic field of about 5 × 1014 gauss and an initial spin period near 1.9 milliseconds. These are inferred model parameters, not direct measurements of the neutron star. They show what the model requires to reproduce the later light curve; they do not tell us that astronomers have measured the star’s surface field or watched it spin.
The first optical peak is still an open part of the puzzle. The paper explores cooling from outflows as a possible explanation, but the authors did not fit the early and late emission with one joint model and say that further work is needed. The 20-day plateau and the first brightening therefore do not have equally settled explanations.
Did a jet point toward Earth?
Broad-lined Type Ic supernovae are related to some of the most energetic stellar explosions, including events accompanied by gamma-ray bursts. Yet a jet need not point toward Earth, and an X-ray flash is not the same thing as a conventional gamma-ray burst. The team detected no transient radio emission from SN 2025wkm. Given their assumed environment and standard radio-emission parameters, they rule out an energetic jet aimed directly at us. That narrows the possibilities, but it is not a claim that every kind of jet or viewing angle has been excluded.
For a wider look at what a magnetar’s extreme magnetic field can do, see our report on vacuum birefringence around magnetars. SN 2025wkm raises a different question: how can astronomers identify the engine of a stellar explosion when its strongest clue was too faint to trigger an alert?
Why this discovery route matters
Einstein Probe’s wide-field X-ray data and ZTF’s repeated optical scans supplied two complementary clues. One survey recorded a sub-threshold flash; the other found the supernova that gave the flash a likely home. The association turned an otherwise unreported signal into a studied event and expanded a still-small group of X-ray-flash supernovae.
That gives astronomers a practical next question: how many overlooked flashes can be recovered by systematically comparing X-ray archives with optical alerts? More examples, followed quickly in X-ray, optical, infrared and radio light, would help test whether shock breakout through surrounding gas and a newborn magnetar recur across this class—or whether SN 2025wkm followed an unusual path.
