"These results are a glimpse at the center of the Milky Way to an order of magnitude higher energies than ever seen before," said Pat Harding, a Los Alamos physicist and principal investigator with the Department of Energy for the project. "The research for the first time confirms a PeVatron source of ultrahigh-energy gamma rays at a location in the Milky Way known as the Galactic Center Ridge, meaning the galactic center is home to some of the most extreme physical processes in the universe."
Over the past seven years, the HAWC observatory has detected nearly 100 high-energy gamma-ray events above 100 teraelectron volts. This data, as detailed in a study led by Sohyoun Yun-Carcamo and published in the *Astrophysical Journal Letters*, enables researchers to directly investigate cosmic ray interactions with the PeVatron. This finding provides new insights into both the mechanisms behind gamma-ray emission and their pinpoint location at the Milky Way's core.
The PeVatron phenomenon remains enigmatic, representing one of the universe's most extreme processes. The galactic center, home to a supermassive black hole, neutron stars, and white dwarfs, is an area dense with gas clouds and intensely heated to millions of degrees, making it difficult to observe through traditional optical methods.
Gamma-ray detection is therefore invaluable for probing this energetic environment. Ultrahigh-energy gamma rays signify the presence of a PeVatron source capable of accelerating particles to a million billion electron volts (PeV), which is a quadrillion times more intense than typical visible light. The PeVatron propels cosmic-ray protons to over 99% of light speed, where they interact with the surrounding gas to produce these high-energy gamma rays.
However, the precise nature of PeVatrons remains unclear. Harding noted that their immense energy levels suggest a connection to some of the universe's most violent occurrences, like supernova explosions, star formations in high-energy regions, or even black holes consuming other black holes. "A lot of those processes are so rare you wouldn't expect them to be happening in our galaxy, or they occur on scales that don't correlate with the size of our galaxy," Harding explained. "For instance, a black hole eating another black hole would be an event only expected outside our galaxy."
HAWC's unique capabilities allow it to capture ultrahigh-energy gamma rays traveling vast interstellar distances. Its detection method involves 300 water-filled tanks equipped with photomultiplier detectors. When these high-energy particles reach Earth's atmosphere, they trigger cascades of secondary particles. As these secondary particles pass through the water tanks, they emit Cherenkov radiation, a blue glow generated as particles move faster than the phase velocity of light in water. Researchers then analyze particle distribution across the tanks, using this data to infer the original energy levels and pinpoint the source of the gamma rays.
Building on previous data from the Milagro experiment, which operated near Los Alamos, the HAWC observatory has relocated to its current site to improve access to the galactic center. Future research will extend these findings with the Southern Wide-field Gamma-ray Observatory in Chile's Atacama Desert, where researchers aim to narrow down the PeVatron's exact location and gain deeper insights into the cosmic phenomena at the heart of our galaxy.
Research Report:Observation of the Galactic Center PeVatron beyond 100 TeV with HAWC
