Cosmic Rays in the Galactic Center Region

Abstract

EGRET data on the Gamma ray emission from the inner Galaxy have shown a rather flat spectrum, extending to about 50 GeV. It is usually assumed that these gamma-rays arise from the interactions of cosmic ray nuclei with ambient matter. Cosmic Ray particles have been observed up to 3 10^20 eV, with many arguments suggesting, that up to about 3 10^18 eV they are of Galactic origin. Cosmic ray particles get injected by supernova explosions. Their injected spectrum is steepened by diffusive losses from the Galaxy to yield the observed spectrum. As cosmic ray particles roam around in the Galactic disk, they encounter molecular clouds and through p-p collisions produce gamma rays from pion decay. The flux and spectrum of these gamma rays is then a clear signature of cosmic rays throughout the Galaxy. Star formation activity peaks in the central region of the Galaxy, around the Galactic Center. Looking then at the gamma ray spectrum of the central region of our Galaxy yields clues as to where the cosmic ray particles interact, and with what spectrum. Using the FLUKA Monte-Carlo, we have modelled this spectrum, and find a best fit for a powerlaw spectral index of 2.34, rather close to the suggested injection spectrum for supernovae which explode into their own winds. This suggests that most cosmic ray interaction happens near the sources of injection; this is consistent with the spectrum of cosmic ray nuclei derived from spallation. One consequence is that cosmic ray heating and ionization should be strong in the Galactic Center region.