The Gamma‐Ray Blazar Content of the Northern Sky

Abstract

Using survey data, we have re-evaluated the correlation of flat-spectrum radio sources with EGRET sources in the northern sky. An analysis incorporating the radio and X-ray properties and the γ-ray source localization is used to gauge the reliability of associations and to search for counterparts of previously unidentified EGRET sources. Above |b| = 10°, where the classification is complete, we find that 70% of the northern EGRET sources have counterparts similar to the bright EGRET blazars. For several of these we identify known blazar counterparts more likely than the earlier proposed 3EG association; for ∼20 we have new identifications. Spectroscopic confirmation of these candidates is in progress, and we have found flat-spectrum radio quasars and BL Lacertae counterparts with redshifts as high as 4. We also find strong evidence for a set of 28 objects with no plausible counterpart like the known EGRET blazars. These thus represent either a new extragalactic population or a population of Galactic objects with a large scale height. The survey has been extended into the plane, where we find several new blazar candidates; the bulk of the sources are, however, Galactic. Looking ahead to the GLAST era, we predict that several of the present 3EG sources are composite and that higher resolution data will break these into multiple blazar identifications.