The cosmic gamma ray, recorded through the Milky Way, broke the record for the most vigorous since we found it, reaching 957 trillion electrovolts (teraelectronvolts or TeV).
This not only doubles the previous record, but also brings us closer to the petaelectronvolt range (it is a quadrillion electrovolts) – finally confirming the existence of cosmic superspells that can push photons to these energies in the Milky Way.
Such a super accelerator is called a PeVatron, and its presence can help us understand what creates high-energy gamma rays that permeate the galaxy.
“This groundbreaking work opens a new window for the study of the extreme universe,” said physicist Jing Huang of the Chinese Academy of Sciences in China. “Observations indicate an important stage in revealing the origin of cosmic rays, which have amazed humanity for centuries.”
The detection was most vigorous in attracting 23 ultra-high-energy gamma rays detected by a group with a range of 398 TeV, at the ASgamma facility, which has been jointly operated by China and Japan in Tibet since 1990.
Interestingly, unlike the previous record holder, which was traced to Crab Nebula, these 23 gamma rays did not seem to point back to the source, but were diffusely propagated across the galactic disk.
Above: gamma ray distribution. The galactic plane is the glow in the middle; the gray areas are out of sight of ASgamma.
However, they can still tell us where we can try to look for PeVatrons in the Milky Way – which in turn can lead us to the ultimate discovery where the most powerful cosmic rays of the universe are born.
First, we need to distinguish between cosmic rays and gamma rays. Cosmic rays are particles such as protons and atomic nuclei that constantly flow through space at almost the speed of light.
Cosmic rays of ultrahigh energies are thought to come from sources such as supernovae and supernova remnants, star-forming regions, and supermassive black holes where powerful magnetic fields can accelerate particles. But it was difficult to consolidate these ideas by observation, since cosmic rays carry an electric charge; this means that their direction changes when they travel in a magnetic field – which the galaxy is absolutely loaded with.
But! These powerful little particles don’t just increase space without consequences. They can interact with the interstellar medium – gas and dust hanging in the space between stars – which in turn produces high-energy gamma-ray photons with about 10 percent of the energy of the parents of cosmic rays.
This happens close to the PeVatron – and the gamma rays have no electric charge, so they just scale through space from A to B without completely interfering with the magnetic field.
If we are lucky, B is the Earth; the gamma ray collides with our atmosphere, creating a cascading shower of harmless particles. It is this shower that takes the surface of the ASgamma air shower.
Underground water cuttings were added in 2014 to detect muons produced by cosmic rays, allowing scientists who are here on Earth to extract cosmic ray data from the background for clearer detection and reconstruction of gamma-ray showers.
Here’s how the collaboration revealed their record-breaking gamma ray Crab Nebula; and now how they have found their 23 ultra-high energy gamma rays, including an even more record-breaking PeV gamma ray.
Their existence and diffuse propagation suggest the existence of protons accelerated, possibly even to the 10 PeV range – suggesting that ubiquitous PeVatrons are scattered along the Milky Way, the researchers note.
The next step will be to try to find them. It is possible that at least some of them are extinct and no longer operate, leaving as evidence only cosmic rays and gamma rays.
“Of the dead PeVatrons that became extinct like dinosaurs, we can only see a trace – the cosmic rays they produce over millions of years propagate across the galactic disk,” said astrophysicist Masato Takita of the University of Tokyo in Japan.
“If we can find real, active PeVatrons, we can explore many more questions. What type of star emits our sub-PeV gamma rays and related cosmic rays? How can a star accelerate cosmic rays to PeV energies? How do rays propagate within our galactic disk? “
Perhaps even – as with many things – there is more than one answer to all these questions.
Future work from both ASgamma and future detectors, such as the Great Air Shower Observatory at high altitude, the Cherenkov Telescope Array and the Southern Wide Field Observatory, may finally help us find them.
The study is published in Physical inspection letters.