Mexico’s High-Altitude Water Cherenkov (HAWC) observatory sits atop the Sierra Negra volcano, 13,000 feet above sea level. A joint venture between the American and Mexican governments, it is a continuously operating, wide-view telescope that searches for gamma and cosmic rays coming from space. It is primarily used to study extreme physical environments in the search for new physics.
Now, recent work by an international team co-led by Los Alamos National Laboratory has revealed the extremities of the environment at the center of the Milky Way, tracking the source of ultrahigh-energy gamma rays topping off at more than 100 teraelectron volts to the heart of our galaxy.
“These results are a glimpse at the center of the Milky Way to an order of magnitude higher energies than ever seen before,” explained Pat Harding, a Los Alamos physicist acting as the US Department of Energy’s lead on the experiments.
Seven Years of Research
For over seven years, HAWC has been monitoring gamma rays, recording close to 100 events involving powerful gamma rays exceeding 100 teraelectron volts. This is thanks to HAWC’s unique design, which is optimized to identify ultrahigh-energy gamma rays from far beyond the Earth.
HAWC is not an observatory in the sense of an optical device pointed at the sky, as usually comes to mind. Instead, it is a network of massive water tanks lined with optical sensors. When the ultrahigh-energy particles from the galactic center contact the water, they produce a blue glow, in light’s version of the sonic boom. The sensors record this glow, allowing “observation” of intense gamma rays.
By analyzing the data collected, researchers can understand the energies involved and even extrapolate the source of the ultrahigh-energy gamma rays. In recent research led by the Los Alamos team, the likely point of origin was at the center of our galaxy.
“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,” Harding said.
The Mysterious PeVatron
PeVatrons are named for the peta-electron-volt (PeV) measurement of the rays they produce. One PeV is equal to an astonishing one quadrillion electron volts and capable of reaching 99% of the speed of light. Although our knowledge of the PeVatron is increasing, much about them remains mysterious.
The leading candidates for their origins include some of the most dramatic events in the cosmos, such as one black hole consuming another, the birth of a star in nuclear fusion, or the fiery supernova deaths of stars.
“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 said. “For instance, a black hole eating another black hole would be an event only expected outside our galaxy.”
TO THE HEART OF THE Milky Way
While the research points toward PeVatron’s location in the broadest sense, much work still needs to be done. HAWC is already a next-generation facility, built on techniques used to create an observatory in the Jemez Mountains near Los Alamos, which ended service in 2008.
Currently, an even more advanced observatory, the Southern Wide-field Gamma-ray Observatory, is under construction in Chile’s Atacama Desert. Even closer to the equator, it will have a broader view than HAWC, hopefully enabling scientists to finally zero in on the specific location of the PeVatron and make a conclusive identification.
The paper, “Observation of the Galactic Center PeVatron beyond 100 TeV with HAWC” appeared in The Astrophysical Journal Letters on September 20, 2024.
Ryan Whalen covers science and technology for The Debrief. He holds a BA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.