A cosmic gamma ray detected by the Milky Way has broken the record for the most energetic we have ever found, with a whopping 957 trillion electron volts (tera-electron volts, or TeV).
Not only does it double more than the previous record, it also brings us close to the range of petoelectron volts (that is, a quadrillion electron volts), which ultimately confirms the existence of cosmic super accelerators that can amplify photons of these energies in the Milky Way.
Such a super-accelerator is called a PeVatron, and if we find it, it can help us figure out what the high-energy gamma rays that line the galaxy produce.
“This groundbreaking work opens a new window for exploration of the extreme universe,” said physicist Jing Huang of the Chinese Academy of Sciences in China. “Observational evidence is an important milestone in the revelation of cosmic radiation, which has surprised mankind for more than a century.”
The detection was the most energetic in 23 gamma rays with ultra-high gamma radiation detected by the team, more than 398 TeV, at ASgamma, which has been managed jointly by China and Japan in Tibet since 1990.
Interestingly enough, and unlike the previous record holder, which was traced back to the scratch nebula, it seemed as if these 23 gamma rays did not point back to a source, but spread in a diffuse way across the galactic disk.
Distribution of gamma rays. (HEASARC / LAMBDA / NASA / GFSC)
Above: gamma ray distribution. The galactic plane is the glow in the center; the gray areas are outside ASgamma’s field of vision.
However, they can still tell us where to look for PeVatrons in the Milky Way – which can lead us again to finally discover where the universe’s most powerful cosmic rays are born.
First, we need to distinguish between cosmic and gamma rays. Cosmic rays are particles such as protons and atomic nuclei that flow constantly through space at almost the speed of light.
Cosmic rays with ultra-high energy 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 establish these ideas with observations, because cosmic rays carry an electric charge; this means that they change direction as they move through a magnetic field – with which the galaxy is absolutely charged.
But! These powerful little particles do not just zoom around sequentially. They can interact with the interstellar medium – gas and dust that hang in space between the stars – which in turn produce high-energy gamma-ray photons, with about 10 percent of the energy of their cosmic ray parents.
This happens near the PeVatron – and gamma rays do not have an electric charge, so they just zoom straight through space from A to B, completely unobstructed by magnetic fields.
The Tibetan Air Shower Series is 4,300 m above sea level. (Institute of High-Energy Physics)
If we are happy, B is the earth; the gamma ray collides with our atmosphere and causes an avalanche of harmless particles. This is the shower that takes up ASgamma’s Surface Air Shower range.
Underground water Cherenkov detectors were added in 2014 to detect muons produced by cosmic rays, enabling scientists here on earth to extract the cosmic ray data from the background in order to detect and reconstruct the gamma ray showers.
This is how the collaboration locates their record-breaker Crab Nebula gamma-ray; and now, how they found their 23 ultra-high-energy gamma rays, including the even more record-breaking gamma ray of the PeV series.
Cherenkov-type muzzle detectors were added in 2014. (Institute of High Energy Physics)
Their existence and diffuse distribution imply the existence of protons that accelerate to even the 10 PeV range – indicating ubiquitous PeVatrons distributed across the Milky Way, the researchers said.
The next step will be to try to locate it. It is possible that at least some of them became extinct, and are no longer active, leaving only cosmic rays and gamma rays as evidence.
“Of dead PeVatrons, which became extinct like dinosaurs, we can only see the footprint – the cosmic rays they produced over several million years spread across the galactic disk,” said astrophysicist Masato Takita of the University of Tokyo in Japan said.
“If we can detect real, active PeVatrons, we can still study many questions. What type of star do we emit sub-PeV gamma rays and related cosmic rays? How can a star accelerate cosmic rays to PeV energy? How do the rays disperse? inside 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 emerging detectors, such as the large high-altitude airborne observation center, the Cherenkov telescope series and the southern large-field gamma-ray observatory, may finally help us find it.
The research was published in Physical overview letters.