Scientists created a model that explains high-energy cosmic rays in our Galaxy, detailing where they come from, how they behave, and how they interact with space between stars. This model includes different processes, like supernova explosions and active galactic nuclei, that help speed up these cosmic particles to almost the speed of light.
Understanding these cosmic rays is an exciting step towards unraveling the mysteries of the universe, as they provide valuable insights into the fascinating conditions and events happening in distant galaxies. It can even positively influence atmospheric chemistry on Earth.
From the MOSCOW INSTITUTE OF PHYSICS AND TECHNOLOGY
A team of scientists from Russia and China has developed a model that explains the nature of high-energy cosmic rays (CRs) in our Galaxy. These CRs have energies exceeding those produced by supernova explosions by one or two orders of magnitude. The model focuses mainly on the recent discovery of giant structures called Fermi bubbles.
Credit: Third Party Reference
One of the key problems in the theory of the origin of cosmic rays (high-energy protons and atomic nuclei) is their acceleration mechanism. The issue was addressed by Vitaly Ginzburg and Sergei Syrovatsky in the 1960s when they suggested that CRs are generated during supernova (SN) explosions in the Galaxy. A specific mechanism of charged particle acceleration by SN shock waves was proposed by Germogen Krymsky and others in 1977. Due to the limited lifetime of the shocks, it is estimated that the maximum energy of the accelerated particles cannot exceed 1014-1015 eV (electronvolts).
The question of how to explain particles with energies above 10^15 eV arose when the Fermi Gamma-ray Space Telescope found two large structures emitting gamma-ray radiation in the Galaxy’s center in November 2010.
These structures, known as Fermi bubbles, are elongated and symmetrically positioned in the Galactic plane, extending 50,000 light-years, which is about half the diameter of the Milky Way. Later, the Planck telescope team also detected their emission in the microwave band.
Credit: Third Party Reference
The Fermi bubbles’ nature is still not fully understood, but their location suggests they are linked to activities in the galaxy’s center, where there is a central black hole of 106 solar masses.
Current models suggest the bubbles may be connected to star formation or energy released from stars being pulled apart as they get close to the black hole. These bubbles are not unique to the Milky Way; similar structures can also be found in other galaxies with active centers.
Dmitry Chernyshov, Vladimir Dogiel, and their colleagues have studied Fermi bubbles. They have revealed that X-ray and gamma-ray emissions are due to relativistic electrons accelerated by shock waves from black holes.
These shock waves also accelerate protons, which, unlike electrons, can retain energy throughout the galaxy. The authors propose that the shock fronts of giant Fermi bubbles can significantly re-accelerate protons from supernovae to energies over 15 eV.
Credit: Third Party Reference
The proposed model explains the spectral distribution of the observed CR flux. It can be said that the processes we described are capable of re-accelerating galactic cosmic rays generated in supernova explosions. Unlike electrons, protons have a significantly greater lifetime, so when accelerated in Fermi bubbles, they can fill up the volume of the Galaxy and be observed near the Earth.
“Our model suggests that the cosmic rays containing high-energy protons and nuclei with energy lower than 1015 eV (below the energy range of the observed spectrum’s “knee”), were generated in supernova explosions in the Galactic disk. Such CRs are re-accelerated in Fermi bubbles to energies over 1015 eV (above the “knee”). The final cosmic ray distribution is shown on the spectral diagram,” says Vladimir Dogiel.
The researchers have proposed an explanation for the peculiarities in the CR spectrum in the energy range from 3×1015 to 1018 eV
Credit: Third Party Reference
Scientists have shown that particles from SN explosions with energies below 3×10^15 eV undergo re-acceleration in Fermi bubbles as they transition from the galactic disk to the halo. The model used to describe this acceleration can account for the cosmic ray spectrum above 3×10^15 eV, while the spectrum below this threshold remains unchanged. It results in a spectral distribution consistent with observations.
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