A cosmic gamma ray detected zipping across the Milky Way has broken the record for the most energetic we have yet found, looking into a giant 957 trillion electron volts (teraelectron volts or TeV).
This not only doubles the previous record, it brings us close to the range of petaelectron volts (it is a quadrillion electron volts) – finally confirms the existence of cosmic super accelerators that can boost photons to these energies in the Milky Way.
Such a super accelerator is called a PeVatron, and finding them could help us figure out what produces high-energy gamma rays streaking across the galaxy.
“This groundbreaking work opens a new window on the exploration of the extreme universe,”
The detection was the most energetic in a series of 23 ultra-high-energy gamma rays detected by the team, over the 398 TeV range, at ASgamma, a plant operated by China and Japan in Tibet since 1990.
Interestingly, and unlike the previous record holder, which was traced back to the Crab Yoke, these 23 gamma rays did not appear to point back to a source, but were scattered in a diffuse manner across the galactic disk.
Above: Gamma ray distribution. The galactic plane is the light in the center; the gray areas are out of sight of ASgamma.
However, they could still tell us where we could try to look for PeVatrons within the Milky Way – which in turn could lead us to finally discover where the universe’s most powerful cosmic rays are born.
First, we must distinguish between cosmic and gamma rays. Cosmic rays are particles such as protons and atomic nuclei that constantly flow through space at almost the speed of light.
Cosmic rays with ultra-high energy are thought 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 has been difficult to establish these ideas with observations, because cosmic rays carry an electric charge; this means that their direction changes as they move through a magnetic field – which the galaxy is absolutely filled with.
But! These strong small particles do not just zoom around consequently. They can interact with the interstellar medium – gas and dust that hangs around the space between the stars – which in turn produces high-energy gamma-ray photons with approx. 10 percent of the energy from their cosmic ray parents.
This happens close to PeVatron – and gamma rays do not have an electric charge, so they just zoom right through space from A to B, completely uncomfortable by magnetic fields.
If we are lucky, B is the earth; the gamma ray collides with our atmosphere and produces a cascading shower of harmless particles. It is this shower that ASgamma’s surface Air Shower array picks up.
Underwater 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 to more purely detect and reconstruct the gamma-ray bursts.
That’s how the collaboration discovered their record-breaking gamma ray with Crab Nebula; and now how they have found their 23 ultra-high energy gamma rays, including the even more record-breaking PeV range of gamma rays.
Their existence and diffuse distribution imply the existence of protons accelerated to perhaps even the 10 PeV range – suggesting ubiquitous PeVatrons scattered across 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 rays and gamma rays as evidence.
“From dead PeVatrons, which are extinct as dinosaurs, we can only see footprints – the cosmic rays they produced over a few million years, scattered 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 emits our sub-PeV gamma rays and related cosmic rays? How can a star accelerate cosmic rays up to PeV energies? How do the 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 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.