The WISE Gamma-Ray Strip Parameterization: The Nature of the Gamma-Ray Active Galactic Nuclei of Uncertain Type

Despite the large number of discoveries made recently by Fermi, the origins of the so-called unidentified γ -ray sources remain unknown. The large number of these sources suggests that there could be a population among them that significantly contributes to the isotropic gamma-ray background and it is therefore crucial to understand their nature. The first step toward a complete comprehension of the unidentified γ -ray source population is to identify those that can be associated with blazars, the most numerous class of extragalactic sources in the γ -ray sky. Recently, we discovered that blazars can be recognized and separated from other extragalactic sources using the infrared (IR) WISE satellite colors. The blazar population delineates a remarkable and distinctive region of the IR color–color space, the WISE blazar strip. In particular, the subregion delineated by the γ -ray emitting blazars is even narrower and we named it the WISE Gamma-ray Strip (WGS). In this paper, we parameterize the WGS on the basis of a single parameter s that we then use to determine if γ -ray active galactic nuclei of the uncertain type (AGUs) detected by Fermi are consistent with the WGS and can be considered blazar candidates. We find that 54 AGUs out of a set of 60 analyzed have IR colors consistent with the WGS; only 6 AGUs are outliers. This result implies that a very high percentage (i.e., in this sample about 90%) of the AGUs detected by Fermi are indeed blazar candidates.