Century-old cosmic ray mystery is close to being solved

Cosmic rays -- high-energy particles moving close to the speed of light -- are known to arrive from within the Milky Way and from more distant regions of the universe. However, their specific points of origin have remained unclear since their discovery in 1912. Shuo Zhang, an assistant professor of physics and astronomy at MSU, and her research team conducted two studies that offer new clues about where these particles may have formed. The results were recently shared at the 246th meeting of the American Astronomical Society in Anchorage, Alaska.

These fast-moving particles may have been created in extreme environments such as black holes, star forming regions or the remnants of exploded stars. Such events are capable of generating neutrinos -- tiny, almost massless particles that constantly pass through space and Earth.

"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?"

Exploring Nature's Most Extreme Particle Accelerators

Sources that produce cosmic rays are powerful enough to propel protons or electrons to energies far beyond what the most advanced human-made particle accelerators can reach. Zhang's group focuses on understanding these natural accelerators, called PeVatrons, to determine what they are, where they are located and how they boost particles to such extraordinary energies. Gaining insight into these processes may also help scientists address broader questions about galaxy formation and the nature of dark matter.

New Insights From X-Ray Studies of PeVatron Candidates

In their latest publications, Zhang and her students investigated PeVatron candidates whose origins had not yet been identified. In the first study, postdoctoral researcher Stephen DiKerby examined a puzzling high-energy source found by the Large High Altitude Air Shower Observatory (LHAASO). Although LHAASO detected the source, its true nature was still unknown. Using X-ray observations from the XMM-Newton space telescope, DiKerby identified a pulsar wind nebula -- an expanding region filled with energetic electrons and particles receiving energy from a pulsar. This discovery confirmed the candidate as a pulsar wind nebula-type source of cosmic rays. Only a small number of PeVatrons have been classified in this way.

Student-Led Observations of Additional LHAASO Sources

The second study was conducted by MSU undergraduates Ella Were, Amiri Walker and Shaan Karim. They used NASA's Swift X-ray telescope to examine X-ray signals from several lesser-studied LHAASO cosmic ray sources. By calculating upper limits for the X-ray emission, their results may help guide future investigations of similar objects.

"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."

Combining Neutrino, X-Ray and Gamma-Ray Observations

Zhang's team will next examine cosmic ray sources by merging IceCube Neutrino Observatory data with results from X-ray and gamma-ray telescopes. Their aim is to understand why some cosmic ray sources emit neutrinos while others do not, as well as pinpoint where and how those neutrinos are produced.

"This work will call for collaboration between particle physicists and astronomers," Zhang said. "It's an ideal project for the MSU high-energy physics group."

This research is supported by several NASA observation grants and the National Science Foundation IceCube analysis grant.