DISCRETE SOURCE INTERPRETATION OF RECENT HIGH ENERGY COSMIC GAMMA RAY MEASUREMENTS

RECENT data from the high energy gamma ray detector on the OSO-3 spacecraft are the first convincing measurement of cosmic gamma rays around 100 MeV. The angular distribution of the events indicates that a large fraction of the flux originates in the galactic disk. One possible interpretation is in terms of secondary gamma rays produced by the interactions of cosmic rays with the ambient interstellar gas. Detailed calculations of the expected flux due to this process have been done by various authors using gas density data from 21 cm hydrogen line measurements, and assuming that cosmic rays pervade the entire galaxy at the intensity level measured locally2-8. The predicted fluxes fall short by more than an order of magnitude in explaining the observed line intensity of 5 × 10-4 photons cm-2 s-1 rad-1 towards the galactic centre as well as the weaker anticentre intensity1. Modification of the predicted intensity by increasing the gas density or the cosmic ray flux cannot be clearly ruled out, but would imply a drastic modification of the accepted magnitudes and their galactic distribution. © 1969 Nature Publishing Group.