Extragalactic origin of low energy gamma rays

Abstract

The existence of a finite flux of diffuse cosmic photons in the energy region of about 1 keV to about 1 MeV is well established. Furthermore, from observations on the isotropy of the radiation in the energy region of 1-100 keV, it is generally believed that a dominant fraction of the radiation in this energy band, if not all of it, is of extragalactic origin. On the other hand, at energies above ~ 200 keV, there is, as yet, no observational evidence for the isotropy or otherwise of the diffuse radiation, although some authors have advanced indirect arguments in favour of their extragalactic nature. More recently there have been attempts to interpret the diffuse cosmic gamma rays in the neighbourhood of 1 MeV as of galactic origin. From a study of the directionality of the diffuse cosmic gamma rays between about 200 keV and 4 MeV, we present in this communication the first observational evidence of their extragalactic nature.