Galactic gamma bays and the origin of cosmic rays

Abstract

The question of the origin of the cosmic radiation is a fundamental problem in high energy astro physics, this thesis is concerned with one particular aspect of this question namely, whether or not the cosmic ray sources are Galactic or Extragalactic. This problem is Investigated through the results of Galactic Gamma-ray astronomy. The main mechanism for gamma ray production are discussed. The observational results of Galactic Gamma ray astronomy are described. Theories about the Galactic cosmic ray sources and the distribution of these sources are described and related to the resulting distribution of cosmic rays in various propagation models. The distribution of gas in the galaxy is described. Calculations are presented which indicate that the role of Inverse Compton Scattering in producing the observed Galactic Gamma ray intensity is small. The reason for the disagreement of this result with earlier results is analysed. The distribution of cosmic rays in the inner Galaxy (0-10 kpc from the Galactic Centre) is determined from a comparison of the distribution of Gamma ray emissivity and the hydrogen distribution, this Is found to follow the distribution of sources between 6 - 10 kpc. The penetration of dense molecular clouds by cosmic rays is investigated. It is concluded that the uncertainties in the hydrogen distribution do not permit us to rule out an Extragalactic origin, from the inner Galaxy data . The distribution of gamma ray intensity in the Galactic anticentre is modelled on Galactic and Extragalactic theories and the predictions of the Galactic model are found to be in excellent agreement with observation. The cosmic ray distribution is found to be consistent with the origin in Supernovae or their remnants. It is concluded that there is rather strong evidence for the majority of cosmic ray nuclei between 1 and 10 GeV having been produced within the Galaxy.