A cosmic gamma ray detected through the Milky Way has broken the record for the most energetic we have found so far, with a whopping 957 trillion electron volts (teraelectron volts or TeV).
This not only doubles the previous record, but brings us closer to the range of petaelectronvolts (i.e., a quadrillion electron electrons), finally confirming the existence of cosmic superaccelerators that can propel photons to these energies in the Milky Way.
A superaccelerator like this is called PeVatron and finding them could help us find out what is producing the high-energy gamma rays that are being released across the galaxy.
“This pioneering work opens a new window for the exploration of the extreme universe,” said physicist Jing Huang of the Chinese Academy of Sciences of China. “Observational evidence marks an important milestone toward the revelation of the origins of cosmic rays, which have baffled humanity for more than a century.”
The detection was the most energetic in a journey of 23 ultra-high energy gamma rays detected by the team, above 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 dates back to the Crab Nebula, these 23 gamma rays did not appear to point to a source, but spread fuzzily across the galactic disk.
Distribution of gamma rays. (HEASARC / LAMBDA / NASA / GFSC)
Above: gamma ray distribution. The galactic plane is the radiance of the center; the gray areas are outside the visual field of Asgamma.
However, they could still tell us where we might try to look for PeVatrons in the Milky Way, which in turn could lead us to discover where the most powerful cosmic rays in the Universe are born.
First, we must make a distinction 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.
High-energy cosmic rays are believed to come from sources such as supernova and supernova remnants, star-forming regions, and supermassive black holes, where powerful magnetic fields can accelerate particles. But it has been difficult to pinpoint 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 not only zoom without consequences. They can interact with the interstellar medium (gas and dust that hangs in space between stars), which in turn produces high-energy gamma-ray photons, with about 10% of their parents’ energy. cosmic rays.
This happens near the PeVatron, and the gamma rays have no electric charge, so they only zoom directly through space A to B, totally annoyed by the magnetic fields.
The Tibetan air shower set located at 4,300 m above sea level. (Institute of High Energy Physics)
If we are lucky, B is the Earth; gamma ray collides with our atmosphere, producing a cascading rain of harmless particles. It is this shower that collects the set of ASgamma surface air showers.
Cherenkov groundwater detectors were added in 2014 to detect muons produced by cosmic rays, which allowed Earth scientists to extract data from cosmic rays from the bottom in order to more clearly detect and reconstruct the showers of cosmic rays. gamma rays.
This is how the collaboration detected its gamma-ray record of the Crab Nebula; and now, how have they found their 23 ultra-high-energy gamma rays, including even more record-breaking PeV gamma rays.
Cherenkov-type muon detectors were added in 2014. (Institute of High Energy Physics)
Its existence and diffuse distribution implies the existence of accelerated protons up to perhaps up to 10 PeVs, suggesting 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 are extinct and no longer active, leaving only cosmic and gamma ray tests.
“From the dead PeVatrons, which have become extinct like dinosaurs, we can only see the footprint: the cosmic rays they produced over several million years, scattered across the galactic disk,” said astrophysicist Masato Takita of the University of Tokyo in Japan.
“If we can locate active and real PeVatrons, we can study many more questions. What kind 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 even possible, as with so many things, that there is more than one answer to all these questions.
Future work, both by ASgamma and future detectors such as the High Altitude Air Shower Observatory, the Cherenkov Telescope, and the Southern Wide Field Gamma Ray Observatory, could finally help us find them.
The research has been published in Physical review letters.