The discovery emerged from a coordinated campaign between two of China’s flagship observatories: the Einstein Probe, a space-borne X-ray telescope, and the Large High Altitude Air Shower Observatory, known as LHAASO, which sits at high altitude in Sichuan province and detects the showers of secondary particles produced when ultrahigh-energy gamma rays strike Earth’s atmosphere. Together, the two instruments observed PSR J1740+1000, a middle-aged pulsar located roughly 4,600 light-years from Earth. What they found was an X-ray tail of unprecedented length, stretching southwest from the pulsar for about 32 arcminutes on the sky, which translates to approximately 42 light-years at the pulsar’s distance. This is the longest X-ray tail ever associated with a pulsar wind nebula.

The significance of the measurement lies in how it transforms our picture of particle transport. The prevailing assumption has long been that once high-energy particles escape their acceleration sites, magnetic turbulence in interstellar space scatters them rapidly, causing them to diffuse isotropically, meaning equally in all directions. Under that model, any directional structure should be erased within a relatively short distance of the source. The new observations defy that expectation. Over a scale of tens of light-years, the particles streaming away from PSR J1740+1000 maintain a coherent propagation direction, leaving behind an elongated trail of synchrotron X-ray emission that astronomers can read like a breadcrumb path.

The story began with earlier observations by the XMM-Newton satellite, which had detected an X-ray tail several light-years long in the vicinity of the pulsar. But XMM-Newton’s field of view and observing strategy limited how much of the structure it could capture. The Follow-up X-ray Telescope aboard the Einstein Probe, by contrast, was designed with a wide field of view and a low instrumental background, making it exceptionally well suited to searching large swaths of sky for faint, diffuse X-ray structures. In roughly 70,000 seconds of observations, the Einstein Probe revealed that the tail extends far beyond what XMM-Newton had seen. Remarkably, although XMM-Newton accumulated more than six times the observing time, it detected only a small fraction of the full structure, underscoring how much of the high-energy sky may still be hiding beyond the edges of conventional surveys.

The crucial connection came from LHAASO. In recent years, the observatory has catalogued a series of ultrahigh-energy gamma-ray sources, some of which reach energies exceeding one petaelectronvolt, a million billion electronvolts, produced by particles accelerated in the vicinity of supernova remnants, pulsars, and black holes. A persistent puzzle has been that several of these gamma-ray sources have no obvious astronomical counterpart nearby, no pulsar, remnant, or stellar cluster that could plausibly serve as their engine. These so-called orphan sources have been difficult to interpret. One possibility is that the parent particles simply traveled far from their acceleration site before producing detectable gamma rays, meaning the visible emission marks a waypoint on a long journey rather than the point of origin.

PSR J1740+1000 provides a striking confirmation of that idea. The ultrahigh-energy gamma-ray emission detected by LHAASO extends in the same direction as the newly mapped X-ray tail, and the two structures match closely in position on the sky. More importantly, the physical interpretation ties them together at the level of the particles themselves. The electrons responsible gain enormous energies within the pulsar wind nebula, the bubble of magnetized plasma that a rapidly spinning neutron star blows into its surroundings. As these electrons propagate along the tail, they spiral through magnetic fields and emit synchrotron radiation, a portion of which falls into the X-ray band and is recorded by the Einstein Probe. Simultaneously, the same electrons collide with low-energy photons that pervade interstellar space and boost them to extreme energies through inverse Compton scattering, producing the ultrahigh-energy gamma rays that LHAASO detects from the ground.

In effect, the X-rays and gamma rays are two footprints left by a single population of high-energy electrons, imprinted in different wavelength bands along the same path. The spatial alignment and the consistency of the energy spectra of the two emissions provide strong evidence that the extraordinarily long tail is genuinely the record of extended particle propagation, not a chance superposition of unrelated structures. For the first time, astronomers can follow a coherent trail of PeV-scale particles across a distance comparable to the spacing between neighboring stars in the galactic neighborhood, connecting the accelerator directly to the far-flung region where its particles finally reveal themselves.

Why the particles hold their direction over such vast distances remains an open question, and the researchers propose two candidate mechanisms. The first is that the interstellar magnetic field in this region is highly ordered, allowing the electrons to stream efficiently along the field lines while suppressing motion perpendicular to them, effectively creating a narrow magnetic track through the galaxy. The second is that a fast, collimated outflow, perhaps a jet-like wind, is continuously ejected from the pulsar wind nebula in that direction, carrying the particles outward the way water from a fire hose carries a jet far from its nozzle. The current data cannot yet distinguish between these scenarios, but both lead to the same consequential conclusion: high-energy particles do not always scatter into randomness within a short distance of their source, and anisotropic transport can persist across scales of tens of light-years.

The implications reach well beyond a single pulsar. As Gabriele Ponti and colleagues at the Italian National Institute for Astrophysics note in a commentary published in the same issue of the journal, the significance of the discovery extends past PSR J1740+1000 itself, because it demonstrates that a spatial offset between an ultrahigh-energy gamma-ray source and its candidate astronomical counterpart does not necessarily mean the association is false. Instead, such an offset may trace the propagation path of particles after they leave their acceleration site. In other words, the location where astronomers detect gamma rays may be merely the footprints of a long journey, not the birthplace of the particles. This reframing could resolve the identities of several of LHAASO’s orphan sources and reshape how scientists match gamma-ray detections to their astrophysical engines.

The study also stands as a demonstration of the power of coordinated, multiwavelength astronomy. By pairing a wide-field X-ray telescope in orbit with a ground-based gamma-ray observatory covering an entirely different energy regime, the research team stitched together a continuous physical narrative, following particles from the turbulent environment near a neutron star across 42 light-years of interstellar space. Each band captures a different stage of the electrons’ lives: synchrotron light marks their passage through magnetic fields close to the source, while inverse Compton gamma rays record their later interactions with ambient starlight far from it. As surveys by the Einstein Probe and LHAASO continue, astronomers expect this approach to uncover more such trails, opening a new window onto how the galaxy’s most extreme accelerators seed interstellar space with particles whose energies dwarf anything achievable in human laboratories, and gradually dismantling a century-old mystery one footprint at a time.

Subject of Research: Anisotropic propagation of high-energy cosmic-ray electrons from the pulsar wind nebula PSR J1740+1000, traced by joint X-ray and gamma-ray observations

Article Title: Einstein probe and LHAASO reveal a 42-light-year directed journey of cosmic rays

Article References: Einstein probe and LHAASO reveal a 42-light-year directed journey of cosmic rays. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cosmic rays, pulsar wind nebula, PSR J1740+1000, Einstein Probe, LHAASO, X-ray astronomy, ultrahigh-energy gamma rays, synchrotron radiation, inverse Compton scattering, interstellar magnetic fields, particle transport, high-energy astrophysics

The reason this phase had remained hidden for so long lies in the architecture of previous X-ray missions. Most narrow-field X-ray telescopes could not find transient events on their own; they relied on gamma-ray detections to provide an initial localization before repointing their instruments toward the source. By the time they arrived, the earliest and softest X-ray emission had already passed. Einstein Probe, with its wide-field soft X-ray monitoring capability, was watching the sky at the right moment and in the right energy band, providing a direct observation of the earliest stage of the short gamma-ray burst explosion. The finding, published in Science Bulletin, suggests that the standard picture of these mergers, in which the action is essentially over within a couple of seconds, is incomplete.

An Li, a PhD student at Beijing Normal University who serves as a Transient Advocate for Einstein Probe, was on shift when the event unfolded. According to Li, the event initially appeared to be an ordinary short gamma-ray burst, producing a bright flash lasting less than half a second that was detected simultaneously in gamma rays by SVOM-GRM and Insight-HXMT and in X-rays by EP-WXT. He swiftly responded to the onboard alerts and conducted a preliminary analysis. Rather than disappearing, the source kept producing episodes of soft X-ray emission for nearly ten minutes, an behavior that immediately set the event apart from the hundreds of short bursts recorded by earlier missions.

The energy budget of this extended emission was substantial, but its spectral character made it extraordinarily difficult to detect with conventional instrumentation. Professor Bin-Bin Zhang of Nanjing University, a co-corresponding author of the paper who initiated the in-depth study, explained that the spectrum was so soft that, for a burst at a typical cosmological distance, it would have fallen below the detection threshold of conventional gamma-ray instruments such as Swift’s Burst Alert Telescope. Previous missions would therefore have recorded only the brief gamma-ray flash and missed the prolonged activity that Einstein Probe revealed. As Zhang put it, the observations show that what appears to be a typical short gamma-ray burst can actually conceal a much longer and richer episode of activity at soft X-ray energies.

Determining the physical nature of the explosion required more than a single instrument. The team mounted an extensive international follow-up campaign spanning X-ray, optical, and radio wavelengths, coordinating facilities across the globe and in orbit. Professor Eleonora Troja of the University of Rome Tor Vergata, a co-corresponding author whose group obtained the key redshift information from spectroscopic analysis, emphasized that the coordinated multiwavelength observations were essential. They allowed the team to identify and study the burst’s host galaxy, measure its distance, and, critically, rule out an accompanying supernova. That exclusion provided strong evidence linking the extraordinary X-ray emission to a compact object merger rather than the death of a massive star, which produces long gamma-ray bursts and supernovae.

The deeper analysis delivered perhaps the most consequential result of the study: the long-lasting X-ray emission appeared to be powered directly by the merger remnant itself, not by the expanding blast wave slamming into surrounding material. Yi-Han Iris Yin, a PhD student in the Department of Physics and the Hong Kong Institute of Astronomy and Astrophysics at The University of Hong Kong, led the analysis of the high-energy emission. As a co-corresponding author, she found that the event’s rapid variability, its spectral evolution, and the subsequent behavior of both the X-ray and optical afterglows all pointed toward sustained activity from a central engine operating long after the initial short gamma-ray burst had faded. In standard afterglow models, emission is dominated by the external shock; here, the data told a different story.

One plausible explanation advanced by the team is that the merger produced a rapidly rotating, highly magnetized neutron star, known as a magnetar, which powered the extended X-ray emission through continued injection of energy. A stable magnetar remnant of this kind would rotate hundreds of times per second, winding up extreme magnetic fields that can tap the enormous rotational energy of the newborn object and channel it into electromagnetic radiation. Such a remnant has major implications for fundamental physics, because whether a merger produces a long-lived neutron star or collapses promptly into a black hole depends sensitively on the true maximum mass of neutron stars, which in turn depends on the poorly constrained equation of state of matter at nuclear densities.

The discovery also resonates strongly with the field of multi-messenger astronomy. Since the first joint detection of electromagnetic signals and gravitational waves from merging neutron stars in 2017, astronomers have been searching for electromagnetic counterparts to gravitational-wave sources that can reveal what happens during and after these violent cosmic collisions. Troja noted that the newly discovered soft X-ray component provides a new probe, indicating that fast X-ray transients are also electromagnetic counterparts to gravitational-wave sources and may originate from compact object mergers. If confirmed by future joint detections, fast X-ray transients could join kilonovae as standard signposts of mergers, helping telescopes and gravitational-wave detectors triangulate events across the sky.

Importantly, the researchers argue that this phenomenon may not be rare at all. Similar soft X-ray emission could accompany many more short gamma-ray bursts, but it may have escaped detection simply because previous missions lacked the capability to capture prompt emission below gamma-ray energies. In that sense, EP250704a may be the first clearly observed member of a population that has been hiding in plain sight, its soft X-ray glow too faint and too long-lived for gamma-ray monitors, and too prompt for repointing X-ray telescopes. Wide-field, sensitive soft X-ray monitoring changes the observational calculus entirely, potentially multiplying the number of merger events whose central engines can be studied directly.

The broader stakes of the finding extend to some of the deepest questions in astrophysics. Yin observed that the discovery extends our view of neutron star mergers beyond the brief gamma-ray flash, and that by revealing this previously hidden soft X-ray phase, Einstein Probe opens a new window for studying neutron star merger remnants and may ultimately help constrain the neutron star equation of state. Zhang added that the findings demonstrate Einstein Probe’s unique capability to uncover new classes of transient phenomena and strengthen its role in the era of multi-messenger astronomy, in which gravitational waves and electromagnetic radiation are studied together to understand some of the most extreme events in the Universe. As more wide-field X-ray monitors come online and gravitational-wave detectors grow more sensitive, events like EP250704a are likely to become a routine part of the astronomer’s toolkit, transforming short gamma-ray bursts from split-second flashes into extended laboratories for probing matter under the most extreme conditions nature allows.

Subject of Research: Soft X-ray emission from a neutron star merger associated with a short gamma-ray burst

Article Title: A hidden X-ray chapter of cosmic collisions comes to light

Article References: A hidden X-ray chapter of cosmic collisions comes to light. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Einstein Probe, short gamma-ray burst, neutron star merger, magnetar, soft X-ray transient, gravitational waves, multi-messenger astronomy, central engine, kilonova, SVOM, Insight-HXMT, equation of state