The intensity of cosmic rays in the Galactic centre region

Cosmic rays are energetic particles that reach the Earth from outer space. Most cosmic rays are accelerated within our Galaxy, and remain trapped in the Galactic magnetic field for about 10 million years before escaping in the intergalactic space.

Cosmic rays interact with the matter that constitutes the interstellar medium. Cosmic rays characterised by particle energies in the MeV domain ionise the interstellar gas, while particles in the GeV and TeV domain (or above) collide with interstellar matter producing neutral pions that then decay into gamma rays. Therefore, gamma-ray observations, as well as measurements of the ionisation rate of the interstellar gas can be used to infer the intensity of the cosmic radiation throughout the Galaxy. The picture emerging from observations reveals that the intensity of cosmic rays is roughly constant at any location in the Galaxy. However, data from the Galactic centre region are puzzling.

On one side, gamma-ray observations of a 200 pc region surrounding the centre of the Galaxy show that the intensity of GeV and TeV cosmic rays in that region is slightly enhanced (a factor of a few) with respect to the local one. On the other hand, the ionisation rate measured in the Galactic centre region largely exceeds the local one, suggesting that the intensity of MeV cosmic rays must largely exceed (a factor of 100 or even more!) the local one.

Why is the intensity of MeV cosmic rays so large in the Galactic centre region, while that of GeV and TeV cosmic rays is not?

The goal of the internship will be to develop models for the acceleration and transport of cosmic ray particles in the Galactic centre region, and search for a scenario that could explain the puzzling difference in the behaviour of low and high energy cosmic rays.


Stefano Gabici






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