A cosmic gamma ray rushing through the Milky Way has broken the record for the most energetic we’ve found to date, with a whopping 957 trillion electron volts (tera electron volts or TeV).
Not only is this more than a doubling of the previous record, it brings us close to the range of peta electron volts (that’s a trillion electron voltage) – ultimately 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 if we find them, we can figure out what produces the high-energy gamma rays that flow through the galaxy.
“This groundbreaking work opens a new window for the exploration of the extreme universe,” said physicist Jing Huang of the Chinese Academy of Sciences in China. “The observational evidence marks an important milestone toward revealing the origins of cosmic rays that have puzzled mankind for more than a century.”
The detection was the most energetic in a series of 23 ultra-high energy gamma rays detected by the team, above the range of 398 TeV, at ASgamma, a facility jointly operated by China and Japan in Tibet since 1990.
Interestingly, and unlike the previous record holder, which was traced back to the Crab Nebula, these 23 gamma-rays did not appear to point to a source, but were diffused across the galactic disk.
Gamma radiation distribution. (HEASARC / LAMBDA / NASA / GFSC)
Above: gamma ray distribution. The galactic plane is the glow in the center; the gray areas are outside the field of view of ASgamma.
However, they could still tell us where we could try to look for PeVatrons in the Milky Way – which could lead us to finally discover where the universe’s most powerful cosmic rays are born.
First, we must 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.
Ultra-high-energy cosmic rays are believed 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 record these ideas with observations, because cosmic rays have an electric charge; this means that their direction changes when they travel through a magnetic field – which the galaxy is absolutely burdened with.
But! These powerful little particles don’t just zoom around without consequences. They can interact with the interstellar medium – gas and dust that hangs in space between the stars – which in turn produces high-energy gamma-ray photons, with about 10 percent of their parent’s energy from cosmic rays.
This happens close to the PeVatron – and gamma rays have no electric charge, so they just zoom across space from A to B, completely undisturbed by magnetic fields.
The Tibet air shower setup is located 4,300 m above sea level. (Institute of High Energy Physics)
If we’re lucky, B is Earth; the gamma ray collides with our atmosphere and produces a cascade of harmless particles. It is this shower that incorporates ASgamma’s Surface Air Shower array.
Underground water Cherenkov detectors were added in 2014 to detect muons produced by cosmic rays, allowing scientists here on Earth to extract the cosmic rays from the background to detect and reconstruct the gamma ray showers more cleanly.
This is how the collaboration discovered their record-breaking Gamma Rays from the Crab Nebula; and now how they found their 23 ultra-high energy gamma rays, including the even more record-breaking PeV series gamma rays.
Cherenkov-type muon detectors added in 2014. (Institute of High Energy Physics)
Their existence and diffuse dispersion implies the existence of protons that have accelerated to perhaps even the 10 PeV range – suggesting that ubiquitous PeVatrons are scattered across the Milky Way, the researchers said.
The next step is to try to find them. It is possible that at least some of them are extinct and no longer active, leaving only cosmic rays and gamma rays as evidence.
“Of dead PeVatrons, which became extinct as dinosaurs, we can only see the footprint – the cosmic rays they produced over a few million years, spread across 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 is emitting our sub-PeV gamma rays and related cosmic rays? How can a star accelerate cosmic rays to PeV energies? How do the rays spread? spawns within our galactic disk?
It is even possible – as with so many things – that there is more than one answer to all these questions.
Future work, from both ASgamma and future detectors such as the Large High Altitude Air Shower Observatory, the Cherenkov Telescope Array, and the Southern Wide-field Gamma-ray Observatory, could finally help us find them.
The research is published in Physical Review Letters