A 42-light-year-long X-ray tail allows us to trace the long journey of high-energy cosmic particles through interstellar space.

Understanding the origin, acceleration, and propagation of high-energy cosmic rays has been a "century-old mystery" in astrophysics. Recently, joint observations from China's Einstein Probe (EP) satellite and the Large High Altitude Air Shower Observatory (LHAASO) showed an extraordinarily long X-ray tail near a pulsar about 4,600 light-years from Earth—one that had never before been seen in full. The tail extends about 42 light-years and stretches in the same direction as ultrahigh-energy gamma-ray emission detected by LHAASO, with the two showing a close spatial match.

This finding indicates that high-energy particles produced in the pulsar wind nebula do not immediately diffuse isotropically after leaving their source; instead, they can travel in a preferred direction for tens of light-years. It provides new observational evidence for understanding how high-energy particles escape their acceleration sites and propagate through interstellar space. It also offers a new way to explain some ultrahigh-energy gamma-ray sources that lack clear astronomical counterparts.

The work was published in Science China: Physics, Mechanics & Astronomy on September 21. The corresponding authors are from the Institute of High Energy Physics of the Chinese Academy of Sciences and Nanjing University. In the same issue, Gabriele Ponti and colleagues at the Italian National Institute for Astrophysics (INAF) published a commentary on this work.

"Orphan" Gamma Rays Seen by LHAASO

The universe contains a vast number of extremely energetic particles. They can be accelerated to near the speed of light in the vicinity of extreme objects such as supernova remnants, pulsars, and black holes, and continue to travel through interstellar space after leaving these objects. LHAASO can detect extremely high-energy gamma rays from space, some of which are produced by high-energy particles with energies exceeding one petaelectronvolt (PeV).

In recent years, LHAASO has discovered a series of ultrahigh-energy gamma-ray sources, but some of them have no obvious astronomical object nearby that could serve as their source. This does not mean these gamma rays have no origin. One possibility is that the high-energy particles have already traveled far from their acceleration site and only produce detectable gamma rays at a location far from their origin. To understand this phenomenon, the key is to determine where the high-energy particles come from and how they actually propagate.

Einstein Probe Discovers a Hidden 42-light-year Tail

PSR J1740+1000 is a middle-aged pulsar, and XMM-Newton had previously discovered an X-ray tail several light-years long around it. In addition, the ultrahigh-energy gamma-ray emission detected by LHAASO extends in the same direction as the tail.

The Follow-up X-ray Telescope onboard the Einstein Probe has a wide field of view and low background, making it well suited to searching large areas of sky for faint, diffuse X-ray structures. With about 70,000 seconds of observations, the Einstein Probe found that the tail is far longer than previously seen: It extends southwest from near the pulsar for up to about 32 arcminutes, which corresponds to about 42 light-years at a distance of 4,600 light-years.

This finding means that what we are seeing is not a "little tail" confined to the vicinity of the pulsar, but a giant trace of high-energy particle propagation spanning tens of light-years. The observations show that this is the longest X-ray tail of a pulsar wind nebula yet discovered.

X-Rays and Gamma Rays Come from the Same Population of Particles

The key to this joint observation is not only that the positions of the X-rays and gamma rays in the sky match; their radiation properties can also be explained by the same population of high-energy electrons.

These electrons gain very high energies in the pulsar wind nebula. As they propagate along the tail, they move through magnetic fields and emit synchrotron radiation, part of which is detected by the Einstein Probe as X-rays. At the same time, these high-energy electrons interact with low-energy photons already present in space, boosting them to extremely high energies and producing the ultrahigh-energy gamma rays detected by LHAASO.

Thus, the X-rays and gamma rays are like "footprints" left by the same population of high-energy particles, recorded in different bands. The spatial positions and energy spectra of the two types of radiation are consistent with each other, providing strong evidence that this extraordinarily long tail is indeed formed by the extended propagation of high-energy particles.

Why Can High-energy Particles Travel So Far?

Normally, after entering interstellar space, high-energy particles are continually scattered by magnetic-field fluctuations and gradually diffuse in all directions. But this observation shows that, over a scale of 42 light-years, the high-energy particles still maintain a clear propagation direction.

The researchers propose two possible explanations. One possibility is that the interstellar magnetic field is highly ordered, allowing high-energy particles to travel along the field direction while motion perpendicular to it is suppressed—essentially creating a narrow "magnetic track." Another possibility is that a fast, collimated outflow is continuously ejected in that direction and carries the high-energy particles far away, much like water from a fire hose.

It is not yet possible to determine which mechanism dominates, but both explanations point to the same important fact: After leaving the pulsar wind nebula, high-energy particles do not always scatter completely at random within a very short distance; they may retain their propagation direction over a considerable distance.

A New Perspective for Finding Ultrahigh-energy Gamma-Ray Sources

This finding also offers a new perspective for understanding ultrahigh-energy gamma-ray sources in the Milky Way: The location where we detect the radiation may be merely the "footprints" of a long journey by the particles, not their "birthplace." As the commentary puts it: "The significance of this discovery goes beyond PSR J1740+1000 itself—it shows that a spatial offset between an ultrahigh-energy gamma-ray source and its candidate astronomical counterpart does not necessarily mean a false association; it may instead trace the propagation path of particles after they leave their acceleration site."

This study, using the Einstein Probe and LHAASO, connects observations in two different bands—X-rays and ultrahigh-energy gamma rays—allowing scientists to follow a 42-light-year-long tail and trace high-energy particles from near the pulsar all the way to distant space. It shows the advantages of coordinated space-borne and ground-based high-energy observatories. It also opens a new window for studying how high-energy particles produced by extreme objects escape their sources and propagate through the Milky Way.

Multiwavelength view of J1740+1000. Purple indicates the ultrahigh-energy gamma-ray emission measured with LHAASO. Red shows the partial X-ray tail of the pulsar wind nebula measured with the XMM-Newton satellite (left), and blue shows the complete X-ray tail of the pulsar wind nebula observed with Einstein Probe (right). Although XMM-Newton's observing time was more than six times that of Einstein Probe, it detected only a small fraction of the tail. (Credit: Y.-H. Chi et al. )