China's space- and ground-based observatories trace long journey of cosmic rays

BEIJING, Sept. 21 (Xinhua) -- By combining data from a satellite and a ground-based observatory, Chinese scientists have discovered an X-ray trail stretching 42 light-years across the cosmos. This finding allows them to trace the long journey of high-energy particles through interstellar space.

In a recent study, researchers used joint observations from China's Einstein Probe (EP) satellite and the Large High Altitude Air Shower Observatory (LHAASO) to discover, around a pulsar roughly 4,600 light-years from Earth, an extraordinarily long X-ray trail never before seen in its entirety. 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 (CAS), noted that the origin, acceleration, and propagation of high-energy cosmic rays are known as a "century-old puzzle" in astrophysics.

The new discovery shows that high-energy particles produced in the pulsar wind nebula, after leaving their source, do not immediately diffuse uniformly in all directions. Instead, they can propagate along a certain direction for dozens of light-years. This provides new observational evidence for understanding how high-energy particles escape from acceleration sites and how they propagate through interstellar space, Feng said.

The findings were published in the Chinese scientific journal Science China: Physics, Mechanics & Astronomy on Monday.

The universe is teeming with extremely energetic particles. Near extreme celestial bodies such as supernova remnants, pulsars, and black holes, these particles can be accelerated to near the speed of light, and they continue to journey through interstellar space long after leaving their birthplaces.

LHAASO is capable of detecting extremely high-energy gamma rays in the universe, some of which originate from particles with energies exceeding peta-electronvolts. Curiously, some of these ultra-high-energy gamma rays show no obvious celestial object that could serve as their source.

One possibility is that these high-energy particles travel a great distance after leaving their accelerator and only produce detectable gamma rays when they are far from the source. The key to understanding this phenomenon lies in determining where these particles come from and how exactly they propagate, Feng said.

The pulsar, coded PSR J1740+1000, is a middle-aged pulsar. Previously, European Space Agency (ESA)'s XMM-Newton satellite had discovered an X-ray trail several light-years long around it, while the ultra-high-energy gamma-ray radiation found by LHAASO lay along the very direction in which that trail extended.

Through observations, China's EP satellite revealed that this trail is far longer than previously seen, about 42 light-years. It is the longest X-ray pulsar wind nebula trail known to date.

"This means what we are seeing is not merely a 'small tail' confined to the vicinity of the pulsar, but a colossal trace of high-energy particle propagation spanning tens of light-years," Feng said.

The researchers believe that the X-ray trail observed by EP and the ultra-high-energy gamma rays detected by LHAASO come from the same population of particles.

"X-rays and gamma rays are like footprints left by the same group of high-energy particles, recorded in different wavebands," Feng said.

The joint observation by EP and LHAASO not only demonstrates the unique advantages of coordinated work between space- and ground-based observation facilities, but also provides a new observational window for studying how high-energy particles produced by extreme celestial bodies escape their sources and how they propagate through the Milky Way, Feng added.

Launched in January 2024, EP is one of a series of space science missions under the leadership of CAS. It is also an international collaboration mission with contributions from the ESA, the Max Planck Institute for Extraterrestrial Physics in Germany, and the French space agency CNES.

Located on a mountain at an elevation of about 4,410 meters above sea level in southwest China's Sichuan Province, and covering an area of 1.36 square km, LHAASO is mainly used for cosmic ray and gamma-ray observation and research.