Discovery offers clues to cosmic particles' travel

A joint observation by two of China's flagship observatories has traced a 42-light-year-long stream of high-energy particles traveling in a clear direction through interstellar space, providing strong evidence that such particles can retain a preferred direction long after leaving the places where they were accelerated.

The discovery, made through a joint campaign by the Einstein Probe and the Large High Altitude Air Shower Observatory or LHAASO, challenges the long-standing assumption that high-energy particles quickly spread in all directions after being accelerated.

Launched in January 2024, the Einstein Probe is a space-based X-ray observatory designed to detect and study powerful, short-lived cosmic events, such as bursts of X-rays from black holes, neutron stars and other extreme objects.

Put into operation in 2023, the LHAASO is a major ground-based cosmic ray observatory in Sichuan province that detects extremely high-energy gamma rays and cosmic rays to study their origins and how they travel through space.

The study, conducted by researchers from the Institute of High Energy Physics of the Chinese Academy of Sciences and Nanjing University, was published in the journal Science China: Physics, Mechanics & Astronomy on Monday.

High-energy particles in space include ultrafast electrons accelerated by powerful objects such as pulsars and supernova remnants. As the electrons travel through space, they emit X-rays. When they interact with light particles around them, they can also produce gamma rays. By observing both types of radiation, scientists can trace how these particles travel after leaving their sources, offering clues to the century-old mystery of cosmic rays.

"The most important finding is that these high-energy particles do not immediately diffuse isotropically after escaping their source, providing a new perspective on how high-energy particles escape and travel through the Milky Way," said Feng Hua, a corresponding author of the study and a researcher at the Institute of High Energy Physics.

The finding may also help explain so-called "orphan" gamma ray sources — ultrahigh-energy gamma ray sources that have no obvious astronomical object at their location. Feng said these sources may be produced when high-energy particles travel a long distance from their birthplace before producing detectable radiation.

Chi Yiheng, another corresponding author of the study and a PhD student at Nanjing University, said the Einstein Probe's Follow-up X-ray Telescope, or FXT, played a crucial role in the discovery.

The region had previously been observed by the European Space Agency's X-ray Multi-Mirror Mission Newton telescope, but only a short section of the structure was detected. Using about 70,000 seconds of observations, FXT mapped a 42-light-year-long tail extending southwest from the vicinity of the pulsar.

Its large field of view and exceptionally low background interference in orbit enabled FXT to reveal that the structure is much longer than previously observed. The pulsar is about 4,600 light-years from Earth, and the structure is now the longest known X-ray tail associated with a pulsar wind nebula, Chi said.

A pulsar wind nebula is a cloud of high-energy particles produced by a rapidly rotating neutron star, or pulsar. The particles streaming away from the pulsar can emit X-rays, allowing scientists to trace their movement through space.

Meanwhile, LHAASO detected an ultrahigh-energy gamma ray source extending in the same direction as the X-ray tail.

The combined observations indicate that the X-rays and gamma rays are "footprints" of the same high-energy particles, seen at different wavelengths. The close alignment of the two emissions, together with their similar energy characteristics, provides strong evidence that the 42-light-year trail was formed by particles traveling in a preferred direction.

Xi Shaoqiang, another corresponding author of the study and a distinguished young researcher at the Institute of High Energy Physics, said scientists do not yet know exactly why the particles can maintain such a clear direction over such a long distance.

However, he said, the finding provides a new way of understanding ultrahigh-energy gamma ray sources in the Milky Way.

"What we see as the radiation location is likely just the 'footprint' of the particles' long journey, not necessarily their birthplace," Xi said.

He said future studies should broaden the search for ultrahigh-energy gamma ray sources. Rather than focusing only on the immediate vicinity of possible source objects, scientists should also examine regions farther away, where particles may eventually produce detectable radiation.

An accompanying commentary in the same issue, written by Gabriele Ponti of the Italian National Institute for Astrophysics, also highlighted the significance of the discovery.

"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," Ponti wrote.

Cao Zhen, chief scientist of LHAASO and a member of the Chinese Academy of Sciences, described the project as "a model for national large facility cooperation", saying the collaboration demonstrated how combining observations from major scientific facilities can produce results greater than the sum of their individual contributions.

limenghan@chinadaily.com.cn