Science3 publishers2 min readPublished
Einstein Probe's minutes-long X-ray flash hints at a magnetar born in a neutron-star merger
Einstein Probe recorded nearly ten minutes of X-ray light from a July 2025 neutron-star merger whose gamma-ray burst lasted about half a second. The team links the long glow to a newborn magnetar, a reading that so far rests on this single event.
The Scientist · Science desk
What happened
- The event, catalogued as EP250704a/GRB 250704B, was detected on July 4, 2025 by the SVOM, Insight-HXMT and Einstein Probe satellites.
- Graduate student Niccolò Passaleva began follow-up within minutes of the alert, so observations started while the source was still bright.
- Absorption patterns in spectra from the VLT's X-Shooter instrument gave the event's redshift, placing it more than 6 billion years away.
- After studying the remnant with the Very Large Telescope and the Very Large Array, the team concluded it may have seen a magnetar form as two neutron stars merged.
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Why it matters
- capability If minutes-long X-ray glows turn out to be common among magnetar-forming mergers, X-ray monitors gain a second way to flag these collisions alongside short gamma-ray bursts.
- decision Confirming more of these events means committing a large telescope within minutes of an X-ray alert, because earlier candidates faded before they yielded enough evidence.
- constraint Any survey that counts these flashes as mergers would be relying on an inferred magnetar, since the team frames its own conclusion as something it may have seen.
The half-second of gamma rays matters because short gamma-ray bursts have long been the main light signal astronomers tie to neutron-star mergers [1], and such bursts can be over in less than two seconds [2]. This one fits inside that window [2]. The X-rays then kept going for roughly a thousand times longer than the gamma rays. If the emission lasted the full ten minutes, the ratio is about 1,200 [1].
Eleonora Troja's group puts the long glow down to what the collision left behind. "However, if the remnant of the collision is a magnetar, it could keep bursting for longer," said Troja, co-corresponding author of the paper and a member of the Einstein Probe European collaboration [9][11]. "Magnetars are rapidly spinning neutron stars with huge magnetic fields. When they damp their magnetic power into the surroundings, they can make any explosion brighter and longer-lasting," she said [10]. The group's work is funded by a European Research Council Consolidator grant [7].
Distance was the other half of the case. Fast X-ray transients are hard to attribute because astronomers often know neither how far away they are nor how much energy they released [6]. A redshift answers the first question. The group had spent several years trying to tie these transients firmly to mergers, but earlier candidates faded too quickly to yield enough evidence [15]. "I was traveling home by train," said Niccolò Passaleva, the graduate student who led the VLT follow-up, "and all of a sudden I was rushing against time to commandeer one of the largest telescopes in the world from my laptop." [17][14]
The record claim is also his. "This is the longest lasting prompt X-ray flash ever observed from a neutron star merger," Passaleva said [14].
Einstein Probe has detected hundreds of bright fast X-ray transients from distant galaxies since its January 2024 launch [4]. Some have been linked to the deaths of massive stars. Others remain unexplained [5]. This event came about 18 months into the mission [3]. The thing this doesn't tell you is what fraction of the unexplained flashes are mergers. If the magnetar reading holds up, one event shows that this route exists. It cannot say how often it happens.
I think the case for this single event is good. Its gamma-ray burst falls in the range tied to mergers, and its distance has been measured. Before treating minutes-long X-ray flashes as a general way to find magnetar-forming mergers, I would want a second one with a redshift.
What to watch
- A second Einstein Probe fast X-ray transient with a measured redshift and a sub-two-second gamma-ray burst, which would test whether minutes-long flashes are a repeatable merger signature.
- A gravitational-wave detection paired with one of these long X-ray flashes, which would confirm a merger independently of the light.
- Independent modelling of the EP250704a afterglow that tests whether a magnetar remnant is needed to explain the ten-minute glow.