Extragalactic γ -rays

Only a few extragalactic objects have been studied in the γ -ray region of the spectrum. At high energies the COS-B experiment detected emission from the quasar 3C273 while at lower energies the results indicate that the emission from the Seyfert galaxy NGC 4151 is variable. A similar variability may also account for the conflicting reports of line emission from the radio galaxy Cen A. The implication of these and other observations in relation to the possible physical conditions in the nuclei of active galaxies.