Astronomers have traced an enigmatic source of very-high-energy gamma rays back to a pulsar 4,600 light-years from Earth, following a trail of X-ray emission that stretches roughly 42 light-years through interstellar space. The finding, led by researchers at the Chinese Academy of Sciences and Nanjing University, offers direct evidence that cosmic rays can retain a preferred direction long after leaving the site where they were accelerated.
The result challenges a long-standing assumption in astrophysics: that high-energy particles diffuse rapidly in all directions once they escape their source. Instead, the new observations suggest that a population of cosmic rays can stream outward along a well-defined path, leaving detectable signatures across multiple wavelengths.
At the center of the study is PSR J1740+1000, a pulsar estimated to be about 114,000 years old with a rotation period of 154 milliseconds. Pulsars are highly magnetized, rapidly spinning neutron stars that drive winds of charged particles into their surroundings, creating what astronomers call a pulsar-wind nebula. Previous observations with the XMM-Newton space telescope had revealed an X-ray tail extending a few light-years from the pulsar. The new campaign, using the Follow-up X-ray Telescope aboard China's Einstein Probe satellite, pushed that detection much farther.
In roughly 70,000 seconds of observation, Einstein Probe tracked the structure to about 32 arcminutes from the pulsar, corresponding to approximately 42 light-years. According to the research team, this is the longest X-ray tail ever observed in association with a pulsar-wind nebula.
But the most significant aspect of the work emerged from a coordinated observing campaign. The Large High Altitude Air Shower Observatory, one of the world's most sensitive facilities for cosmic-ray and gamma-ray studies, observed the same region of sky and detected a very-high-energy gamma-ray source extending in the same direction as the X-ray tail. The spatial coincidence is unlikely to be accidental.
Analysis of the X-ray and gamma-ray spectra indicates that both emissions can be explained by the same population of cosmic rays. The researchers propose that these particles gained very high energies near the pulsar's wind nebula. As they propagated along the tail, they interacted with surrounding magnetic fields and emitted synchrotron radiation, part of which Einstein Probe detected as X-rays. The same electrons may also have interacted with low-energy photons in space, transferring energy to them and boosting them to extremely high energies, producing the gamma rays seen by LHAASO.
In this scenario, the X-ray and gamma-ray signals are two different fingerprints left by the same particle population during its journey. Their spatial distribution and spectral properties are compatible with that picture, providing strong evidence that the comet-like tail is the result of energetic particles streaming away from the pulsar.
The study, published in Science China: Physics, Mechanics & Astronomy, also offers a new way to interpret so-called orphan gamma-ray sources. These are very-high-energy gamma-ray emitters for which no obvious astronomical object nearby can explain the emission. The new work suggests that in some cases the particles responsible have simply traveled so far from their acceleration site that the gamma-ray emission appears far from its source.
The research adds to a growing body of evidence that cosmic-ray transport is more structured than once thought. It also raises new questions about how particles escape their acceleration sites and how they propagate through the interstellar medium. For now, the 42-light-year tail of PSR J1740+1000 stands as the clearest example yet of cosmic rays maintaining a preferred direction across vast distances.
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