For decades, astronomers have traced neutron star mergers mainly through their brief gamma-ray bursts (short GRBs). The earliest soft X-ray emission right after the explosion has been almost impossible to catch, because most narrow-field X-ray telescopes need a gamma-ray alert before they can slew to the source. Einstein Probe (EP), with its wide-field soft X-ray monitoring, has now exposed this previously unseen phase.
Artist’s rendering of the Einstein Probe satellite in space — Image: China News Service / Wikimedia Commons, CC BY 4.0
A half-second flash with a 10‑minute tail
On 4 July 2025, EP detected the event EP250704a/GRB 250704B. At first it looked like a textbook short GRB: a sub‑half‑second bright flash seen simultaneously in gamma rays by SVOM‑GRM and Insight‑HXMT, and in X‑rays by EP‑WXT.
But when the team dug into the data, the surprise emerged: instead of fading, the source kept producing episodes of soft X‑rays for nearly 10 minutes. This extended activity carried a lot of energy, yet its spectrum was so soft that instruments like Swift’s Burst Alert Telescope would have missed it entirely at this distance. Earlier missions would have recorded only the brief gamma spike, not the rich, long‑lasting X‑ray phase that EP revealed.
A compact object merger and a magnetar engine
EP’s alert triggered an extensive multiwavelength follow‑up campaign, from X‑ray and optical to radio. Spectroscopy pinned down the host galaxy and redshift, and crucially ruled out a supernova, pointing instead to a compact object merger – the collision of neutron stars.
Detailed analysis showed that the prolonged X‑ray emission is powered by the central remnant, not just the outward‑moving blast wave. A compelling explanation is that the merger created a rapidly rotating, highly magnetized neutron star – a magnetar – which kept injecting energy long after the initial short GRB faded, driving the extended soft X‑ray emission and later afterglows.
A new probe for multi‑messenger astronomy
This discovery has major implications for gravitational‑wave astronomy. It shows that fast X‑ray transients can act as electromagnetic counterparts to gravitational‑wave sources and may commonly accompany short GRBs, simply hidden below gamma‑ray sensitivity.
By revealing this hidden soft X‑ray phase, Einstein Probe extends our view of neutron star mergers beyond the brief gamma flash and offers a new way to study merger remnants – potentially tightening constraints on the neutron star equation of state and enriching the toolkit of multi‑messenger astronomy.
Source: ScienceDaily
