China's Tiangguan (Einstein Probe) satellite and the LHAASO ground-based observatory have jointly observed a 42-light-year-long particle trail from pulsar PSR J1740+1000, resolving a century-old mystery about how cosmic rays travel through space.
Establishing the Scale
The full Moon spans about 31 arcminutes in the night sky—a familiar sight to everyone. The elongated structure observed by Tiangguan measures roughly 32 arcminutes—comparable to the full Moon—yet lies 4,600 light-years away. At that distance, this angular extent unfolds into a 42-light-year strip; light itself takes 42 years to travel from one end to the other.
The pulsar PSR J1740+1000 spins approximately 6.5 times per second, releasing energy each second about 100 times the Sun's total luminous output.
A Century-Old Puzzle, Missing One Piece
Cosmic rays were discovered in 1912 by physicists flying hot-air balloons to high altitudes. Where they come from, how they are accelerated, and how they reach Earth remained unanswered for over a century. The difficulty lies with the charged particles themselves: cosmic rays are deflected by interstellar magnetic fields along the way, so by the time they travel hundreds or thousands of light-years to Earth, all original directional information is lost.
Fortunately, particles leave footprints along the way. High-energy electrons moving in magnetic fields emit synchrotron radiation, part of which appears as X-rays seen by Tiangguan. The same electrons collide with low-energy cosmic photons, boosting them into ultra-high-energy gamma rays that fall into LHAASO's view. Photons travel in straight lines, so footprints in both bands can be traced back to their origin.
Chi Yiheng, a PhD student at Nanjing University, offered an analogy: "It's like shining a flashlight in fog—you can see a beam of light. The X-rays and gamma rays are the beam we see, and the pulsar is the flashlight."
One in the Sky, One on the Ground
The two facilities are naturally complementary. LHAASO, built at 4,410 meters in Daocheng, Sichuan, is one of the world's most sensitive ultra-high-energy gamma-ray detectors, having already discovered over 70 such gamma-ray sources—the problem being that no visible object can be found near some of them as the source. Tiangguan, launched in early 2024 and formally known as the Einstein Probe, contributed its Follow-up X-ray Telescope, nicknamed Fengxingtian.
Fengxingtian's two strengths are a combination unique worldwide: a single field of view large enough to fit the entire lunar surface, and a low in-orbit particle background sharp enough to resolve very faint, very diffuse structures. Researcher Xiong Shaolin stated that combining these two capabilities makes it one of a kind globally.
| Observer | Investment | Trail observed | |---|---|---| | XMM-Newton (previously) | Effective area over 4× larger, observing time over 6× longer, combined effective exposure 20–30× higher | A few light-years, only the brightest segment | | Tiangguan Fengxingtian (this time) | ~70,000 seconds | ~32 arcminutes extending southwest from the pulsar, about 42 light-years |
A telescope with 20–30 times less effective exposure saw a far more complete structure, relying precisely on large field of view plus low background. XMM-Newton isn't inadequate—its strengths simply don't apply to this faint, diffuse target.
LHAASO likewise spent a long time grinding. According to Xi Shaoqiang, a distinguished young researcher at the Institute of High Energy Physics, the team stacked more than 1,900 days of data before the gamma rays emerged from a dot into a ~0.6-degree streak. His analogy: it's like exposure time—too short and nothing appears; long enough, and a long strip emerges.
The Moment the Puzzle Closed
Previously, XMM-Newton had seen a small tail of a few light-years around the pulsar, and LHAASO had detected ultra-high-energy gamma radiation along its extension direction. The problem: the gamma rays appeared beyond the small tail, and researchers could not confirm both were footprints of the same particles.
This time, Fengxingtian extended the tail. The 42-light-year X-ray trail aligns closely in space with LHAASO's gamma-ray streak. Same particles, two kinds of radiation, one path. The paper concludes that after high-energy particles leave the pulsar wind nebula, they can propagate in an orderly direction over considerable distances rather than dispersing rapidly.
This overturns a default assumption: lacking observational evidence, the field had generally assumed cosmic rays scatter randomly in all directions immediately upon leaving their source. Researcher Feng Hua summarized: "This discovery shows that, at least over a considerable distance, propagation is directional." LHAASO Chief Scientist Cao Zhen praised it further: "This result provides a comprehensive explanation for the entire radiation mechanism puzzle—something neither instrument could achieve alone, perfectly demonstrating the 1+1>2 effect of collaboration."
Beyond the Puzzle
Sourceless gamma-ray sources are an old LHAASO problem: particles glow only after traveling far, so the source naturally can't be found. Once directional propagation holds, the remaining sourceless gamma-ray sources may each drag an unseen long tail, waiting for the next large-field-of-view, low-background X-ray telescope to connect.
Why can the trail maintain such long-distance directional propagation, and how is the magnetic field structured? Beyond the paper's mechanism explanations, many questions remain. The Tiangguan–LHAASO combination spans over ten orders of magnitude in energy—whether their next targets include a second 42-light-year-scale tail is the most worth-following serial from this instrument pair.
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Sources: Paper in *SCIENCE CHINA Physics, Mechanics & Astronomy* (English edition, 2026-09-21); Xinhua report (2026-09-28, reposted by CAS Network Information Office); China Youth Daily (2026-09-28); Guangming Online/Science and Technology Daily (2026-10-01).
Limitations: The original paper's data tables were not verified; figures including 42 light-years, 32 arcminutes, 70,000 seconds, 1,900+ days, and 0.6 degrees come from institutional press releases and media coverage; details of the magnetic-structure mechanism were not elaborated in reports; the English edition's impact factor and volume/page numbers were not verified.