A cosmic gamma ray detected by waiting over the Milky Way broke the record for the most energetic we have found so far, with 957 trillion electron volts (teraelectron volts or TeV).
This not only doubles the previous record, but brings us closer to the range of petaelectronvolts (ie one billion electron volts) – finally confirming the existence of cosmic superaccelerators that can enhance photons to these energies in the Milky Way.
Such a superaccelerator is called PeVatron, and finding them could help us figure out what produces high-energy gamma rays that creep across the galaxy.
“This pioneering work opens a new window for exploring the extreme universe,” said physicist Jing Huang of the Chinese Academy of Sciences in China. “Observational evidence marks an important step toward revealing the origins of cosmic rays, which have puzzled mankind for more than a century.”
The detection was the strongest of a series of 23 ultra-high-energy gamma rays detected by the team, over the range of 398 TeV, at ASgamma, a facility jointly managed 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 ray distribution. (HEASARC / LAMBDA / NASA / GFSC)
Above: gamma ray distribution. The galactic plane is the brightness in the middle; the gray areas are outside the field of view of ASgamma.
However, they could still tell us where we might try to look for PeVatrons in the Milky Way – which in turn could eventually lead us to discover where the strongest cosmic rays of the Universe are born. .
First of all, we must distinguish between cosmic rays and gamma rays. Cosmic rays are particles such as protons and atomic nuclei that flow constantly in space at almost the speed of light.
Ultra-high-energy cosmic rays are thought to come from sources such as supernovae and supernova remnants, star-forming regions and supermassive black holes, where strong magnetic fields can accelerate particles. But it was difficult to identify 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 charged.
But! These small powerful particles do not just zoom around 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, about 10% of their parents’ cosmic-ray energy.
This happens close to PeVatron – and gamma rays do not have an electric charge, so they only zoom directly through space from A to B, completely disturbed by magnetic fields.
The Tibetan air shower device located at 4,300 m above sea level. (Institute of High Energy Physics)
If we are lucky, B is the Earth; the gamma ray collides with our atmosphere, producing a cascading shower of harmless particles. This is the shower that the ASgamma surface air shower matrix lifts.
Cherenkov groundwater detectors were added in 2014 to detect muons produced by cosmic rays, allowing scientists here on Earth to extract cosmic ray data from the background to detect and reconstruct gamma-ray showers cleaner.
This is how the collaboration detected the record-breaking gamma ray of the Crab Nebula; and now, how they found their 23 ultra-high-energy gamma rays, including even more gamma rays with a record PeV range.
Cherenkov type muon detectors added in 2014. (Institute of High Energy Physics)
Their existence and diffuse distribution imply the existence of accelerated protons up to the range of 10 PeVs – suggesting ubiquitous PeVatrons scattered on 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 evidence of cosmic rays and gamma rays.
“From the dead PeVatrons, extinct like dinosaurs, we can only see the imprint – the cosmic rays they produced over millions of years, scattered 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 other questions. What kind of star does our sub-PeV gamma rays and related cosmic rays emit? How can a star accelerate cosmic rays to PeV energies? How do rays propagate inside our galactic disk? “
It is even possible – as in so many things – to have more answers to all these questions.
Future work on both ASgamma and future detectors, such as the High Altitude Air Shower Observatory, the Cherenkov Telescope, and the Southern Broadband Gamma Observatory, could eventually help us to we find them.
The research was published in Physical review letters.