A cosmic gamma ray detected by a zipper through the Milky Way broke the record for the most energetic we have ever discovered, reaching a whopping 957 trillion electron volts (teraelectronvolts or TeV).
This not only doubles the previous record, but brings us closer to the range of the fifth electron volt (this is a quadrillion electron volts) – finally confirming the existence of cosmic super-accelerators that can stimulate photons to these energies in the Milky Way.
Such a super-accelerator is called a PeVatron, and finding them can help us understand what produces the high-energy gamma rays scattered across the galaxy.
“This groundbreaking work opens a new window for the study of the extreme universe,”
The discovery was the most vigorous in a shot of 23 ultra-high-energy gamma rays detected by the team, over the 398 TeV range, at ASgamma, a facility jointly operated by China and Japan in Tibet since 1990.
Interestingly, unlike the previous record, which was traced to the Cancer Nebula, these 23 gamma rays did not seem to point back to the source, but were diffusely propagated throughout the galactic disk.
Above: Distribution of gamma rays. The galactic plane is the glow in the middle; the gray areas are out of sight of ASgamma.
They could still tell us where we could try to find PeVatrons in the Milky Way – which in turn could lead us to finally discovering where the most powerful cosmic rays in the universe are born.
First, we need to distinguish between cosmic rays and gamma rays. Cosmic rays are particles like protons and atomic nuclei that are constantly moving in space at almost the speed of light.
Superhigh-energy cosmic rays are thought to come from sources such as supernovae and remnants of supernovae, star-forming regions, and supermassive black holes where powerful magnetic fields can accelerate particles. But it was difficult to reinforce these ideas with observations, because cosmic rays carry an electric charge; this means that their direction changes when they travel through a magnetic field – with which the galaxy is absolutely loaded.
But! These powerful small particles don’t just scale without consequences. They can interact with the interstellar medium – gas and dust that hover in space between stars – which in turn produces high-energy gamma quanta with about 10 percent of their parents’ energy from cosmic rays.
This happens near PeVatron – and the gamma rays have no electric charge, so they just scale directly through space from A to B, completely disturbed by magnetic fields.
If we are lucky, B is the Earth; the gamma ray collides with our atmosphere, producing a cascading shower of harmless particles. It is this shower that lifts the surface of ASgamma’s Air Shower.
Cherenkov’s underground water detectors were added in 2014 to detect muons produced by cosmic rays, which allows scientists here on Earth to extract cosmic ray data from the background so they can more accurately detect and reconstruct gamma rays.
Here’s how the collaboration unveiled their record-breaking Crab Nebula gamma ray; and now, how they discovered their 23 ultra-high energy gamma rays, including the even more record-breaking gamma ray of PeV.
Their existence and diffuse propagation suggest the existence of protons accelerated to perhaps even the range of 10 PeV – suggesting the ubiquitous PeVatrons scattered throughout the Milky Way, the researchers said.
The next step will be to try to find them. It is possible that at least some of them have disappeared and are no longer active, leaving only cosmic rays and gamma rays as evidence.
“Of the dead PeVatrons that have disappeared like dinosaurs, we can only see the imprint – the cosmic rays they produce for millions of years are propagating on the galactic disk,” said astrophysicist Masato Takita of the University of Tokyo in Japan.
“If we can find real, active PeVatrons, we can study 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? inside our galactic disk? “
It is possible – as with so many things – to have more than one answer to all these questions.
Future work from both ASgamma and upcoming detectors, such as the Great High-Altitude Air Shower Observatory, the Cherenkov Telescope Array and the Southern Wide-Range Gamma-ray Observatory, can finally help us find them.
The study was published in Physical examination letters.