Astrophysicists at Michigan State University have made progress in unraveling a century-old mystery about the origins of galactic cosmic rays. Their recent studies identified a pulsar wind nebula as a potential source behind a signal from China's LHAASO observatory. The findings, presented at the American Astronomical Society meeting, offer new insights into these high-energy particles.
Cosmic rays, high-energy particles traveling near the speed of light, were first discovered in 1912, but their precise origins within the Milky Way have puzzled scientists ever since. These particles likely emerge from extreme cosmic environments, such as black holes, star-forming regions, or remnants of exploded stars, which can also generate neutrinos.
Shuo Zhang, an assistant professor of physics and astronomy at Michigan State University, led two studies that provide fresh clues. In the first, postdoctoral researcher Stephen DiKerby analyzed a mysterious high-energy source detected by the Large High Altitude Air Shower Observatory (LHAASO) using data from the XMM-Newton space telescope. He identified it as a pulsar wind nebula, an expanding region energized by a pulsar and filled with particles like electrons and protons. This classifies it as one of the few known PeVatrons—natural accelerators capable of boosting particles to peta-electronvolt energies, far surpassing human-made facilities.
"Cosmic rays are a lot more relevant to life on Earth than you might think," Zhang said. "About 100 trillion cosmic neutrinos from far, far away sources like black holes pass through your body every second. Don't you want to know where they came from?"
The second study involved undergraduates Ella Were, Amiri Walker, and Shaan Karim, who used NASA's Swift X-ray telescope to investigate other LHAASO sources. They established upper limits on X-ray emissions, which could direct future research.
These results were shared at the 246th meeting of the American Astronomical Society in Anchorage, Alaska. Looking ahead, Zhang's team plans to integrate data from the IceCube Neutrino Observatory with X-ray and gamma-ray observations to explore why some sources produce neutrinos and others do not.
"Through identifying and classifying cosmic ray sources, our effort can hopefully provide a comprehensive catalogue of cosmic ray sources with classification," Zhang said. "That could serve as a legacy for future neutrino observatory and traditional telescopes to perform more in-depth study in particle acceleration mechanisms."
"This work will call for collaboration between particle physicists and astronomers," she added. "It's an ideal project for the MSU high-energy physics group."
The research, detailed in The Astrophysical Journal (DOI: 10.3847/1538-4357/adb7e0), is funded by NASA observation grants and a National Science Foundation IceCube analysis grant. Understanding PeVatrons could shed light on galaxy formation and dark matter.