This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 (27 quadrillion miles) from Earth—one that had never before been seen in full.
The tail extends about 42 light-years (250 trillion miles) 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 Sept. 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. 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 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. PSR J1740+1000 is a middle-aged pulsar, and XMM-Newton had previously discovered an X-ray tail several light-years long around it.
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