A cosmic ray trap at the center of the Milky Way

The 5 H.E.S.S. telescopes (Photo H.E.S.S., MPIK/Christian Foehr)
The 5 H.E.S.S. telescopes (Photo H.E.S.S., MPIK/Christian Foehr)

An article published in the journal “Physical Review Letters” describes a new interpretation of observations made with NASA’s Fermi Gamma-ray Space Telescope and the High Energy Stereoscopic System (H.E.S.S.) observatory in Namibia. According to a team of Italian researchers led by Daniele Gaggero of the University of Amsterdam, at the center of the Milky Way there’s a kind of trap that focuses there some of the highest energy cosmic rays, the fastest particle of the galaxy.

Cosmic rays are made up of about 90% by protons and of the rest by electrons and nuclei of various atoms. These high energy particles move in space at speeds close to that of light and during these movements they’re influenced by the magnetic fields they encounter. The consequence is that their course is changed, making it impossible to understand their origin, one of the problems in studying them.

An opportunity to study cosmic rays comes from cases where they interact with matter and emit gamma rays, electromagnetic radiation at the highest energy. In March 2016, scientists of the of the H.E.S.S. collaboration reported that they found evidence of extreme activity at the center of the Milky Way just in the form of gamma rays. The widespread glow of those gamma rays reaches nearly 50 TeraElectronVolts (TeV), 50 times higher than the gamma ray energy detected by the Fermi Space Telescope’s Large Area Telescope (LAT) instrument.

The two observatories work in different wasy, as are the energies they are sensitive at. H.E.S.S. detects gamma ray emissions when the Earth’s atmosphere absorbs them, triggering a cascade of particles that in turn produce a blue light called Cherenkov light. Fermi detects gamma ray emissions when the enter LAT.

The scientists who conducted this new research on data collected in 2016 combined the high energy emission data of H.E.S.S. with those of Fermi’s lower energies. The result is that they obtained a gamma ray spectrum emitted from the center of the Milky Way from a few GeV (GigaElectronVolts).

For the researchers it was surprising to find such an agreement between the data collected by two instruments built to detect gamma rays at different energies in different ways. Their conclusion is that gamma rays were emitted by the same population of cosmic rays, probably protons. They become more energetic as they approach the center of the Milky Way but that energy boost is a mystery.

After this discovery, the researchers modified the models that explain the gamma ray movement within the Milky Way, as they didn’t keep that into account. After the update, the simulations show a better agreement with the data collected by the Fermi Space Telescope.

The collisions that generate gamma rays should also produce neutrinos, particles that interact very little with matter and therefore reach the Earth directly. Detecting them is one of the goals of experiments such as IceCube, in Antarctica. The center of the Milky Way could be identified as a source of neutrons.

This research on cosmic rays is complex because it requires instruments that are specific and very diverse to detect the effects of the highest-energy particles activity. Something’s happening in the central area of ​​the Milky Way and further research is needed to try to solve this new mystery tied to cosmic rays.

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