For decades, modern astronomy has relied on brief, explosive flashes of gamma rays to identify some of the universe’s most violent events: the catastrophic collisions of neutron stars. Typically, these powerful bursts vanish into the cosmic background in less than two seconds, leaving scientists scrambling to capture their fading echoes. Now, groundbreaking new research indicates that these stellar smashups may also produce prolonged X-ray flashes that remain visible in the night sky for several minutes, completely reshaping how astrophysicists search for and interpret transient phenomena across the cosmos.
A comprehensive study published in the scientific journal Science Bulletin presents compelling evidence linking long-duration X-ray emissions directly to neutron star mergers. These findings raise the intriguing possibility that many mysterious cosmic flashes detected in recent years—phenomena whose underlying sources have stubbornly eluded explanation—may actually originate from the cataclysmic merging of dead stellar cores.
Since the successful launch of the Einstein Probe satellite in January 2024, astronomers have maintained a vigilant watch over the sky, detecting hundreds of remarkably bright X-ray flashes emanating from distant galaxies. These transient events, categorized by researchers as fast X-ray transients, possess a variety of potential cosmic origins. While some have been definitively linked to the dramatic deaths of massive stars collapsing into black holes, others have remained entirely unexplained, baffling researchers who lack a clear framework to determine their true nature.
Pinpointing the exact cause of these transient events is historically difficult. Astronomers frequently face severe observational hurdles, struggling to accurately determine how far away the events are located or precisely how much energy they release into the surrounding space. Without accurate distance measurements and multi-wavelength observations, distinguishing between different types of cosmic explosions remains an immense challenge.
A Possible Magnetar Birth
A dedicated team of researchers operating within Professor Eleonora Troja’s research group, supported by a prestigious European Research Council (ERC) Consolidator grant, recently managed to overcome these obstacles. They successfully obtained critical observations that helped identify the exact source of one particular elusive X-ray transient.
The breakthrough began when the Einstein Probe satellite issued an automated alert regarding a fresh cosmic explosion. Professor Troja’s team rapidly mobilized, organizing immediate follow-up observations utilizing a suite of powerful astronomical instruments. This coordinated response included the European Southern Observatory’s (ESO) Very Large Telescope (VLT) located in Chile and the Karl G. Jansky Very Large Array (VLA) in New Mexico. By carefully studying the fading remnants left behind in the wake of the explosion, the researchers concluded that they had likely witnessed the direct formation of a magnetar—a specialized, highly magnetized neutron star—produced by the collision and subsequent merger of two pre-existing neutron stars.
Neutron stars represent some of the most extreme objects in the universe. They are the extraordinarily dense stellar remnants left behind when massive stars exhaust their nuclear fuel and reach the dramatic ends of their lives, collapsing under their own gravity until protons and electrons are crushed together into neutrons. When two of these ultra-dense remnants spiral inward and finally merge, they generate ripples in the fabric of spacetime known as gravitational waves. Simultaneously, the intense light and radiation produced during these events provide astronomers with crucial diagnostic clues regarding what survives the violent impact.
Historically, short-duration gamma-ray bursts have served as the primary electromagnetic signature associated with neutron star mergers. However, theoretical models have long suggested that alternative signatures could exist depending on the remnant object’s ultimate fate.
"However, if the remnant of the collision is a magnetar, it could keep bursting for longer," explained Professor Troja, who serves as a member of the Einstein Probe European collaboration and is a co-corresponding author of the newly published study. "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. When I saw the X-ray data from this new event, I realized something was up."
A Record X-Ray Flash
The remarkable event, officially cataloged under the designations EP250704a and GRB 250704B, was initially detected on July 4, 2025, by a coordinated network of advanced orbital observatories, including the SVOM satellite, the Insight-HXMT mission, and the Einstein Probe itself.
Intriguingly, the gamma-ray burst component associated with the explosion lasted for only about half a second—a standard duration for such cosmic collisions. In stark contrast, the Einstein Probe recorded bright, continuous X-ray emission streaming from the source for nearly ten minutes.
"This is the longest lasting prompt X-ray flash ever observed from a neutron star merger," stated Niccolò Passaleva, a graduate student and researcher who spearheaded the intense follow-up observations using the VLT in Chile. "It is an opportunity to have a front-row seat to the most extreme forces of the Universe and discover more of its secrets."
For several years prior to this discovery, researchers had actively searched for a firm, undeniable connection between fast X-ray transients and neutron star mergers. Previous observational candidates had consistently faded far too quickly, vanishing before astronomers could gather sufficient data to confirm their origins.
This time, however, the observational pipeline moved with unprecedented speed. Passaleva responded to the automated alert within minutes, initiating a high-stakes scramble to secure telescope time while the transient event was still luminous enough to be studied in exhaustive detail.
"I was traveling home by train," Passaleva recalled, reflecting on the intense moment, "and all of a sudden I was rushing against time to commandeer one of the largest telescopes in the world from my laptop."
Measuring a Flash From More Than 6 Billion Years Away
Operating remotely while in transit, Passaleva and his colleagues utilized the VLT’s versatile X-Shooter instrument to split the event’s incoming light into its individual spectral components. This spectroscopic analysis allowed the team to identify distinct absorption patterns etched into the light as it passed through intervening gas clouds. These patterns enabled the researchers to calculate the event’s redshift—a standard astrophysical measurement that reveals how much the universe has expanded since the light was emitted, thereby indicating the true distance to the source.
The research team calculated a redshift value of $z=0.6610$. This precise measurement demonstrated that the cosmic explosion occurred long before our Sun and its surrounding planets even formed. In fact, the light captured by the VLT had been traveling across the vast expanses of space for more than six billion years before finally reaching Earth-based detectors.
With the distance firmly established, the astronomers immediately set out to search for another vital piece of the puzzle. Using deep, highly sensitive observations from the VLT’s FORS2 instrument, the team carefully scrutinized the region for any lingering signatures of a bright supernova. Under standard astrophysical expectations, a long-lasting X-ray flash of this nature would normally signal the catastrophic core-collapse of a massive star, an event that invariably leaves behind a distinct supernova remnant.
To the researchers’ anticipation, no supernova appeared.
When taken together, the precisely measured cosmological distance, the complete absence of an accompanying supernova signature, and the unique physical properties of the burst provided overwhelming evidence that the event originated from a binary neutron star merger rather than a collapsing massive star.
A New Way To Find Neutron Star Mergers
The identification of EP250704a/GRB 250704B opens up promising new avenues for observational astronomy. If astronomers can successfully locate and analyze additional events sharing these unique characteristics, they may finally be able to determine just how frequently neutron star mergers result in the formation of stable or transient magnetars.
"Finding more of these X-ray flashes could help reveal how often neutron star mergers create magnetars," concluded Passaleva, looking ahead to future technological milestones. "I am really excited for the next run of gravitational wave observations, when we could finally pair one of these X-ray flashes with a burst of gravitational waves from the same source."
The underlying research was conducted by a broad international collaboration of astronomers and astrophysicists from institutions around the world. The study’s lead authors include An Li of Beijing Normal University, Chen-Wei Wang of the Chinese Academy of Sciences, Niccolò Passaleva of the University of Rome Tor Vergata, and Jie An of the Chinese Academy of Sciences.
Corresponding authors for the published work include Binbin Zhang of Nanjing University, Eleonora Troja of the University of Rome Tor Vergata, Yi-Han Iris Yin of The University of Hong Kong, along with Jing-Wei Hu and Hua-Li Li, both representing the Chinese Academy of Sciences. The critical VLT observations utilized throughout the investigation were executed as part of the large scientific program 114.27LW, titled "QUEENB: a QUEst for Elusive Neutron star and Black hole mergers," led by Principal Investigator Eleonora Troja.