China Einstein Probe and LHAASO trace 42 light year X-ray tail from pulsar PSR J1740+1000

Cosmic rays have puzzled astronomers for more than a century. Where they are born, how they are accelerated to extreme energies and how they cross interstellar space remain central questions in high-energy astrophysics. Xinhua reported on September 21, 2026 that Chinese scientists have traced a single, remarkably long path taken by such particles, by combining data from a space telescope and a ground-based observatory in a way that neither instrument could manage alone.

The trail runs for about 42 light-years away from a middle-aged pulsar named PSR J1740+1000, which lies roughly 4,600 light-years from Earth. The discovery, made with China's Einstein Probe satellite and the Large High Altitude Air Shower Observatory, known as LHAASO, revealed the longest X-ray tail ever recorded from a pulsar wind nebula. The trail runs in precisely the same direction as the ultra-high-energy gamma-ray radiation detected by LHAASO. Feng Hua, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, called the origin, acceleration and propagation of high-energy cosmic rays a century-old puzzle in astrophysics.

Pulsars are rapidly spinning, intensely magnetized neutron stars left behind by supernova explosions. Their spin drives winds of relativistic particles into surrounding space, producing glowing structures called pulsar wind nebulae, and such nebulae are among the leading candidate sources of cosmic rays in the Milky Way. PSR J1740+1000 has a characteristic age of about 114,000 years, a rotation period of 154 milliseconds and a spin-down power of about 2.3 x 10^35 ergs per second. It sits at a high Galactic latitude of about 20 degrees and powers a well-studied bow-shock nebula.

Einstein Probe was launched in January 2024 as a Chinese Academy of Sciences led space science mission, with contributions from the European Space Agency, the Max Planck Institute for Extraterrestrial Physics and France's CNES. LHAASO sits at about 4,410 meters above sea level in Sichuan Province and covers 1.36 square kilometers. The new study was published on September 21, 2026 in Science China: Physics, Mechanics & Astronomy.

Key Facts

Nanowerk reported on September 21, 2026 that Einstein Probe's Follow-up X-ray Telescope, using about 70,000 seconds of observations, found the tail extending southwest from near PSR J1740+1000 for up to about 32 arcminutes on the sky. The pulsar lies at a distance of about 4,600 light-years, or roughly 1.4 kiloparsecs, and at that distance the angular span corresponds to about 42 light-years, the longest X-ray pulsar wind nebula tail known to date.

The gamma-ray evidence comes from LHAASO. The Chinese Academy of Sciences said on September 21, 2026 that the observatory detected an ultra-high-energy gamma-ray source, LHAASO J1740+0948, above 25 TeV with a significance of 17.1 sigma. Updated LHAASO observations show the gamma-ray emission is elongated, with its major axis aligned with the extended X-ray tail seen by Einstein Probe. The paper describes this as the first detection of an X-ray pulsar tail associated with a spatially coincident extended ultra-high-energy gamma-ray emission.

The two signals appear to come from the same population of particles. The INSPIRE-HEP record dated August 8, 2026 described the paper's finding that the X-ray and gamma-ray spectra can be explained by a single population of relativistic electrons, through synchrotron and inverse Compton radiation respectively, removing any need for re-acceleration during propagation. Electrons and positrons above 100 TeV are escaping the pulsar wind nebula and are transported anisotropically over at least about 10 parsecs, or roughly 33 light-years.

Earlier work provides a sharp contrast. ESA's XMM-Newton satellite had previously found a much shorter X-ray tail around the same pulsar, extending about 6 arcminutes to the southwest, and its searches for faint diffuse emission missed the full structure. Nanowerk reported on September 21, 2026 that XMM-Newton's exposure was more than six times that of Einstein Probe, yet it detected only a small fraction of the tail.

Corresponding authors on the paper come from the Institute of High Energy Physics and Nanjing University. The paper was published under the title 'Anisotropic particle transport from a pulsar wind nebula revealed by Einstein Probe and LHAASO'. Gabriele Ponti and colleagues at the Italian National Institute for Astrophysics published a commentary in the same issue of the journal.

Analysis

What this really means is that the standard picture of how cosmic rays leave their sources needs revision. Normally, after entering interstellar space, high-energy particles are continually scattered by fluctuations in the magnetic field and gradually diffuse in all directions, like smoke spreading through a room. That assumption underpins much of how astronomers link a gamma-ray source to a candidate object nearby. Here, across a span of 42 light-years, the particles still carry a clear direction. If that behavior is common, then models that assume isotropic diffusion will systematically misplace the origin of the highest-energy particles.

The team proposes two explanations. One is a highly ordered interstellar magnetic field that forms a narrow magnetic track guiding particles along a preferred path. The other is a fast collimated outflow carrying particles far from their birthplace, much like water from a fire hose. Both ideas can be tested by mapping how the tail fades and widens at greater distances from the pulsar. Either way, the X-rays and gamma rays are, in Feng's phrasing, footprints left by the same group of high-energy particles, recorded in different wavebands.

The bigger picture here is that the place where we detect ultra-high-energy gamma rays may be only the footprint of a particle's long journey, not its birthplace. The INAF commentary makes the point directly: a spatial offset between an ultra-high-energy gamma-ray source and its candidate astronomical counterpart does not necessarily mean a false association, because it may instead trace the propagation path of particles after they leave their acceleration site.

Why It Matters

Astronomers have catalogued a growing number of ultra-high-energy gamma-ray sources in the Milky Way. Feng noted that LHAASO detects extremely high-energy gamma rays, some from particles with energies exceeding peta-electronvolts, and that some of these show no obvious celestial object that could serve as their source. If particles routinely stream away from their accelerators for tens of light-years, then some apparent mismatches between gamma-ray sources and candidate counterparts are not failures of observation but evidence of transport.

The measurement also shows how much instrument design matters. Einstein Probe's Follow-up X-ray Telescope combines a wide field of view with a low background, a combination that made the faint, diffuse structure detectable. XMM-Newton observed the region for more than six times as long yet captured only a small part of the tail, which suggests that sensitivity to extended, low surface brightness emission, rather than raw exposure time, decided the outcome.

The result also strengthens the case that pulsar wind nebulae are major accelerators of cosmic rays. If electrons and positrons above 100 TeV can escape a nebula and travel at least about 33 light-years while keeping a clear direction, then pulsars can inject particles deep into the interstellar medium and leave a measurable imprint far from home, which matters for understanding the total energy budget that pulsars contribute to the Galaxy's cosmic ray population.

Next Up

The immediate next step is to work out which transport mechanism is at work. A magnetic track and a fire-hose outflow make different predictions about how the emission should fade and widen with distance, and longer Einstein Probe observations combined with further LHAASO data can separate them.

Wider searches will matter too. LHAASO's growing catalogue of ultra-high-energy sources gives the search a natural target list, and the team's result suggests that other elongated gamma-ray sources with offset counterparts deserve a second look, because their shapes may record a particle's journey rather than its birthplace.