In 1912, astronomer Victor Hess discovered strange, high-energy particles known as “cosmic rays.” Since then, researchers have hunted for their birthplaces. Today, we know about some of the cosmic ray “launch pads”, ranging from the Sun and supernova explosions to black holes and distant active galactic nuclei. What astronomers are now searching for are sources of cosmic rays within the Milky Way Galaxy.
In a pair of presentations at the recent American Astronomical Society meeting, a team led by Michigan State University’s Zhuo Zhang, proposed an interesting place where cosmic rays originate: a pulsar wind nebula in our own Milky Way Galaxy. A pulsar is a rapidly rotating neutron star, formed as a result of a supernova explosion. High-energy particles and the neutron star’s strong magnetic field combine to interact with the nearby interstellar medium. The result is a pulsar wind nebula that can be detected across nearly the whole electromagnetic spectrum, particularly in X-rays. It makes sense that this object would be a source of cosmic rays. Pulsars are found throughout the Galaxy, which makes them a useful category in the search for cosmic ray engines in the Milky Way.
The Vela Pulsar is a good example of a pulsar wind nebula. The pulsar is at the center, and the surrounding cloudiness is the nebula. Courtesy NASA.
These high-energy particles tell us about more than extreme processes and objects, such as supernova explosions and pulsars. They also pass through matter – like our planet – as if it’s not there. They play a role in lightning, and are known to cause damage to electronics and even the DNA in our bodies. “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?”
All Hail The PeVatron!
Sources of cosmic rays and neutrinos have to be incredibly powerful and energetic to energize the protons and electrons that make them up to such high velocities. Zhang’s group searched out cosmic particle accelerators, known as PeVatrons, to identify the various sources. One way is to look for sources of X-rays and gamma rays, since those regions could also emit cosmic rays. Once they these sources, they can start to answer questions about how PeVatrons can accelerate particles to such high velocities.
To probe the nature of PeVatrons, one of Zhang’s postdoctoral students, Stephen DiKerby, examined X-ray data of a pulsar wind nebula associated with a cosmic ray source in the Milky Way called LHAASO PeVatron J0343+5254u. It was previously studied by the Chinese Large High Altitude Air Shower Observatory (LHAASO), a purpose-built detector to study gamma rays and cosmic rays by observing the air showers they trigger in our atmosphere. That’s when they found that this object is a pulsar wind bubble. It’s rich in high-speed electrons and positrons energized by energy from the pulsar and is a source of cosmic rays.
A bird’s-eye view of the Large High Altitude Air Shower Observatory in China. Courtesy LHAASO.
Several of Zhang’s undergraduate students also collaborated on observations using NASA’s Swift X-ray telescope to zero in on emissions from five possible PeVatron sources in the LHAASO data. The idea is to identify and do follow-up studies on all sources of galactic cosmic rays and other energetic particles. Four turned out to have no significant X-ray point sources matched to optical sources. Even though these turned out not to be sources of cosmic rays, the team’s study allowed members to characterize the limits of X-ray emissions in these cases.
Keep Looking for Galactic Sources
The search for cosmic ray sources in the Milky Way Galaxy is far from complete. Since neutrinos are also generated by high-energy galactic sources, Zhang’s team will compare data from the IceCube Neutrino detector in Antarctica to X-ray and gamma-ray data from other observatories. That should help them identify more sources of galactic cosmic rays. In addition to characterizing those sources, the team wants to understand why some emit neutrinos and others do not. Understanding more about them can help unlock fundamental questions about objects and processes in physics and astronomy, such as galaxy evolution and the nature of dark matter.
“Through identifying and classifying cosmic ray sources, our effort can hopefully provide a comprehensive catalog of cosmic ray sources with classification,” Zhang said. “That could serve as a legacy for future neutrino observatories and traditional telescopes to perform more in-depth studies of particle acceleration mechanisms.”
For More Information
Where Did Cosmic Rays Come From? Astrophysicists are Closer to Finding Out
Swift-XRT Observations and Upper Limits at Five LHAASO Galactic Sources