Gamma-ray evidence for cosmic-ray sources

Abstract

The origin of cosmic-rays is one of the long-standing problems in astro physics. In recent years, strong evidence has been found that certain classes of object contain and are able to accelerate particles to high energies. In this thesis the origin problem is addressed in two different ways. Firstly, two different regions of the Galaxy are studied using γ-ray observations from the COSB satellite combined with atomic and molecular gas measurements. The Vela region contains a pulsar and a supernova remnant and is particularly valuable location for cosmic-ray studies because of its proximity, the association of the two objects, and the intensity of the γ -ray flux it produces. At greater longitudes, the region around the peculiar object η Carinae is also studied. It is rich in potential sources of cosmic rays including active stars and a spiral arm seen at a tangent at l ~ 282º .Analysis of the Vela region reveals strong evidence for cosmic ray production at all energies observed by COSB. The supernova remnant seems the most likely candidate, but the possibility of the pulsar itself producing some of the particles cannot be ruled out. The excess γ -ray emission from around η Carinae does not appear correlated with the active stars but seems to becoming predominantly from the spiral arm. This is the first time evidence has been presented for cosmic-ray acceleration by the spiral shock in a particular, known spiral arm which is observed as a feature in the gas. The γ -rays are produced in the gas clouds associated with this arm. The second approach to the cosmic-ray origin problem involves a model for cosmic-ray production in supernova remnants and is used in association with a Monte-Cailo simulation of their occurrence in the Galaxy. Unlike earlier models (Bhat et al 1987), the motion of the Sun is also taken into account and the supernova explosions occur mainly in spiral arms. The results are in the form of a time sequence of energy density values and tire compared in detail with (^10)Be results. It is found that the model accounts for the long-term rise in the concentration of this radioisotope and does not predict large excursions from the mean energy density that beset older models. Thus the cosmic ray production by supernova remnants seems to be consistent with the radioisotope data.