Hidden X-ray phase revealed in likely neutron star merger

Artist's impression of satellites observing high-energy prompt emission from a binary neutron star merger. SVOM and HXMT detected the short gamma-ray burst, while Einstein Probe captured the accompanying long-lasting X-ray emission. Credit: Yi-Han Iris Yin; reference images: EPSC, SVOM collaboration, NSSDC.

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An international team of astronomers, including researchers from the Department of Physics and the Hong Kong Institute for Astronomy and Astrophysics (HKIAA) at The University of Hong Kong (HKU), has discovered a previously hidden phase of high-energy activity following the merger of compact stars.

This fleeting cosmic explosion, known as EP250704a in X-rays and GRB 250704B in gamma rays, was jointly captured by the China-led Einstein Probe (EP), the Space Variable Objects Monitor (SVOM) and the Insight-Hard X-ray Modulation Telescope (HXMT)..

EP's wide-field X-ray telescope detected the event from its onset and continued to observe several episodes of soft X-ray emission after the gamma-ray signal disappeared. While the gamma-ray burst lasted only about 0.4 seconds, the source continued to shine in soft X-rays for nearly 10 minutes, revealing a phase of activity that conventional gamma-ray observations would likely have missed. These findings have been published as a cover article in Science Bulletin..

How Einstein Probe revealed the hidden X-ray phase.

Short gamma-ray bursts are commonly associated with the merger of compact objects such as neutron stars. As the two objects spiral together and collide, they release an intense burst of high-energy radiation. Traditionally, astronomers have identified these events mainly through gamma rays. Most X-ray telescopes can only turn to a burst after a gamma-ray detector has first located it, meaning the earliest X-ray emission may already be over by the time observations begin..

Einstein Probe changes this picture by continuously monitoring a wide area of the sky at soft X-ray energies. In this case, it revealed that what appeared to be a brief gamma-ray event was accompanied by much longer-lasting X-ray activity.

Yi-Han Iris Yin, a Ph.D. student at HKU's Department of Physics and the Hong Kong Institute for Astronomy and Astrophysics (HKIAA), is a co-corresponding author of the study. She led the analysis of the high-energy prompt emission and contributed to the physical interpretation of the event.

Yin's analysis of the X-ray signal revealed key insights into the merger's aftermath:

• The source remained active long after the gamma-ray burst. The soft X-ray emission continued for nearly 10 minutes after the brief gamma-ray flash.


• The X-rays point to continued central engine activity. Their rapid variability and changing spectrum indicate that the central engine created by the merger continued to release energy.


• The remnant may be a magnetar. One possible explanation is a rapidly rotating, highly magnetized neutron star that continued to power the X-ray emission.


• Similar X-ray activity may have been missed before. This suggests prolonged soft X-ray emission could be more common in compact-object mergers than previously recognized and may provide another way to study gravitational-wave sources.

Working with an international team led by An Li of Beijing Normal University (BNU) and Bin-Bin Zhang of Nanjing University (NJU), the researchers found that similar soft X-ray emission may have been present in previous short gamma-ray bursts but remained undetected because it was too soft and faint for conventional gamma-ray instruments.

"Einstein Probe is allowing us to uncover a part of compact star mergers that was hidden from previous gamma-ray observations," said Yin. "The short gamma-ray flash may represent only the beginning of the high-energy activity. By observing the universe at soft X-ray energies, we can now follow these systems for much longer and obtain a more complete view of what happens during and after the merger."

Fostering young scientific leadership

Bing Zhang, founding director of HKIAA and chair of astrophysics at HKU's Department of Physics, co-authored the study and contributed to the physical interpretation of the event. He has long proposed that binary neutron star mergers could produce fast X-ray transients detectable by missions such as Einstein Probe. Beyond the scientific findings, Zhang highlighted the significance of the study in nurturing young researchers and strengthening international collaboration.

"One of our goals at HKIAA is to create an environment where talented young researchers can take on scientific leadership and work at the forefront of international astronomy," Zhang said. "It is particularly encouraging to see our Ph.D. student Yin taking a leading role as one of the corresponding authors in this international collaboration."

"Her contribution reflects the quality and potential of the young researchers we are training at HKIAA, and this collaboration also demonstrates how Hong Kong can connect with major scientific facilities and research teams in mainland China and around the world."

International collaboration

The study brought together researchers from HKU, Nanjing University, the University of Rome "Tor Vergata," the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), the Institute of High Energy Physics of the Chinese Academy of Sciences and many other institutions worldwide.

The co-first authors include BNU's Li and colleagues from the Institute of High Energy Physics, the University of Rome "Sapienza" and the University of Chinese Academy of Sciences. The corresponding authors include NJU's Bin-Bin Zhang, Eleonora Troja from the University of Rome "Tor Vergata," HKU's Ph.D. student Yin and colleagues from NAOC.

Source: Phys.org/news

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More information

An Li et al, Minutes-long soft X-ray prompt emission from a compact object merger, Science Bulletin (2026).
DOI: 10.1016/j.scib.2026.08.021

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Measuring the Age of the Universe

An artist's visualization of the merger of a binary neutron star. Gravitational waves from the mergers of binary neutron stars and binary black holes have recently been detected by the LIGO and Virgo facilities. These measurements can be used to calculate the age of the universe in a way that is independent of the two conventional methods previously used. Astronomers have calculated that in the next five years it is probable that fifty such events will be detected; their statistics will enable able an age determination with a precision of 2%, enough to also resolve the current incompatibility between the other two estimates. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet. Low Resolution (jpg)

Cambridge, MA - The single most important puzzle in today's cosmology (the study of the universe as a whole) can be summarized in one question: How old is it? For nearly a century -- since the discoveries by Einstein, Hubble, LeMaitre and others led to the big bang model of creation -- we have known the answer. It is about 13.8 billion years old (using current data). But in just the past decade the two alternative measurement methods have narrowed the uncertainties in their results to a few percent to reach a stunning conclusion: The two do not agree with each other. Since both methods are based on exactly the same model and equations, our understanding of the universe is somehow wrong -- perhaps fundamentally so.

Enter the most exciting technical achievement in astronomy for decades, the detection of gravitational waves (GW) caused by the mergers of black holes or neutron stars with each other by LIGO-Virgo, soon to be joined by other similar GW detection facilities in other countries. The solution to the cosmological dilemma is likely to be settled soon by these instruments according to a new Nature paper by Hsin-Yu Chen of Harvard's Black Hole Initiative, Maya Fishbach and Daniel E. Holz of the University of Chicago. The authors describe how upcoming detections of GW will have enough statistics to settle the question of age, forcing either one or the other (or perhaps even both) methods to re-think their basic understanding, or possibly even forcing a new variation of the When and How of the creation.

The two currently conflicting methods rely on observations of vastly different parts of the cosmic order. The first method measures and models the cosmic microwave background radiation (the CMBR method) produced by the universe when, after about 380,000 years, it cooled down and allowed neutral hydrogen atoms to form and light to propagate without scattering. The second method, the one used by Hubble and interpreted by LeMaitre, measures galaxies. This method takes advantage of the expansion of the universe to correlate a galaxy’s distance with its recession velocity, the so-called Hubble-LeMaitre Law, and to derive the Hubble-LeMaitre parameter which describes how long these galaxies have been in motion, related to the age of the universe. All astronomers today rely on this expression to obtain the distances to galaxies too far away to measure directly but whose velocities are easily seen in the Doppler shifts (the redshift) of their spectral lines. While the most familiar use of the parameter is to obtain the age of the universe, its value influences all the other parameters in the cosmological model (about nine of them) which together also explain the shape and expansion character of the universe.

Hubble calibrated his set of distances with nearby galaxies, but today we are capable of seeing galaxies so remote their light has been traveling to us for over ten billion years. Supernovae (SN), or at least those whose brightness is thought to be well understood, can be seen at great distances and so have been used to bootstrap the distance scale calibration outward from Hubble’s original neighborhood. There are subtle complexities in SN that are not well understood, however, resulting in an uncertainty that has been getting smaller as our understanding of them has improved. Today those uncertainties are small enough to exclude the comparable result from CMBR measurements.

The GW method of distance measurement is completely independent of both galaxy and CMBR methods. General relativity alone provides the intrinsic strength of the GW signal from its peculiar ringing signal, and its observed strength provides a direct measure of its distance. (The velocity information is obtained from the redshift of atomic lines in the host galaxy). Dr. Chen and her colleagues simulated 90,000 merger events in binary black hole or binary neutron star systems, including the host galaxy properties, and included likely selection effects and other complexities. The GW strength, for example, depends on our viewing angle of inclination of the merger, while the number of events to expect is only roughly constrained by the detections so far. Including these and similar uncertainties, the astronomers conclude that within the next five years it is likely that the GW method will fix the Hubble-LeMaitre parameter (that is, the age of the universe) to a precision of 2%, and to 1% in a decade, good enough to exclude one or even both of the other methods. The new paper's conclusions are bolstered by the fact that one paper using the GW method to estimate an age has already appeared. It had an uncertainty of between 11.9 billion years to 15.7 billion years, spanning both the current CMBR and galaxy values. But the new paper shows that in five years another roughly fifty GW events will be detected and these should be enough to settle the matter … and usher in a new era in precision cosmology.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Tyler Jump
Public Affairs
Harvard-Smithsonian Center for Astrophysics
+1 617-495-7462
tyler.jump@cfa.harvard.edu

Source: Harvard-Smithsonian Center for Astrophysics (CfA)/News

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