Powerful particle accelerators float through the Milky Way

04/05/2021 at 08:00 CEST

A huge complex of telescopes in Tibet has discovered 23 gamma rays of unprecedented ultra-high energy, which are scattered across the Milky Way producing spectacular glows.

These gamma rays come from the regions of the Milky Way where the greatest number of stars and planets are concentrated (galactic disk) and contain energies between approximately 0.1 and 1 PeV.

The highest energy observed in some of them is almost 1 PeV (1015 eV), a record that represents a world record for gamma ray photons detected anywhere.

The electron volt (eV) is a unit of energy that represents the change in energy that an electron experiences when moving from one point to another.

In high energy physics multiples of eV are used, such as megaelectronvolt (MeV or 106 eV) or gigaelectronvolt (GeV or 109 eV). In particle accelerators, such as the Large Hadron Collider (LHC), up to 14 teraelectronsvolts have been reached (one TeV equals 1012 eV).

In the universe, energy accelerators are even more powerful, but the gamma rays detected now are three orders of magnitude greater than any known cosmic ray-induced gamma ray, or any particle that humans have accelerated in state-of-the-art labs in the world. Earth, the researchers highlight in a statement.

They add that this discovery shows that there are powerful energy accelerators in our galaxy that have not been detected until now: they have been floating throughout the Milky Way for millions of years. The results of this research are published today in the journal Physical Review Letters.

Origin of gamma rays

Origin of gamma raysScientists believe that these gamma rays are produced by the nuclear interaction between cosmic rays escaping from the most powerful galactic sources and interstellar gas distributed by the Milky Way galaxy.

Cosmic rays are highly energetic particles, mostly protons, that travel through space. As they travel through the gas of the interstellar medium, some of the cosmic rays interact and emit gamma rays: unlike cosmic rays, they do not undergo magnetic deflections and therefore their origin can be traced.

The origin of cosmic rays has puzzled humanity for more than a century, particularly the origin of gamma rays that are above 100 TeV.

Astronomers have assumed that they come from cosmic accelerators known as PeVatrons, because they are capable of spewing cosmic rays with an energy of up to a petaelectronvolt (PeV).

Those possible PeVatrons include supernova explosions, star-forming regions, and the supermassive black hole at the center of our galaxy, but as such they have not been detected until now.

The new study has obtained the first evidence of their existence: cosmic rays have left clear traces of the origin of the bright gamma rays scattered throughout the galaxy, a discovery long awaited for decades.

Dead or alive?

Dead or alive?What researchers don’t know yet is whether those PeVatrons are active or dead. If they were dead, the cosmic rays detected now would be the trace of a phenomenon that took place a few million years ago.

But if some of the PeVatrons identified now are still active, astronomers could discover what kind of star emits ultra-high-energy gamma rays, how it achieves such acceleration, and how those rays spread through the galactic environment.

The discovery has been made in the Tibet ASĪ³ experiment, a joint research project between China and Japan to observe cosmic rays.

As the experiment took place in Tibet, at 4,300 meters above sea level, the researchers plan to search for PeVatron footprints in the southern hemisphere and confirm the gamma-ray results using neutrino detectors in Antarctica and beyond.

The research could also prove useful in the search for dark matter because it opens a new window for exploration of the extreme universe, the researchers conclude.


ReferenceFirst detection of sub-PeV diffuse gamma rays from the Galactic disk: Evidence for ubiquitous galactic cosmic rays beyond PeV energies. M. Amenomori et al. (The Tibet ASĪ³Collaboration). Phys. Rev. Lett.

Top photo: Very high energy scattered gamma rays (yellow dots) are distributed across the galactic disk. Credits: Heasarc / Lambda / Nasa / Gfsc