Unveiling the Nature of Unidentified Gamma-Ray Sources. I. A New Method for the Association of Gamma-Ray Blazars

We present a new method for identifying blazar candidates by examining the locus, i.e., the region occupied by the Fermi γ -ray blazars in the three-dimensional color space defined by the WISE infrared colors. This method is a refinement of our previous approach that made use of the two-dimensional projection of the distribution of WISE γ -ray-emitting blazars (the Strip) in the three WISE color–color planes. In this paper,we define the three-dimensional locus by means of a principal component analysis of the color distribution of a large sample of blazars composed of all the ROMA-BZCAT sources with counterparts in the WISE All-Sky Catalog associated with γ -ray sources in the second Fermi-LAT catalog (2FGL; the WISE Fermi blazars sample, WFB). Our new procedure yields a total completeness of ctot ∼ 81% and a total efficiency of etot ∼ 97%. We also obtain local estimates of the efficiency and completeness as functions of the WISE colors and galactic coordinates of the candidate blazars. The catalog of all WISE candidate blazars associated with theWFB sample is also presented, complemented by archival multi-frequency information for the alternative associations. Finally, we apply the new association procedure to all γ -ray blazars in the 2FGL and provide a catalog containing all the γ -ray candidate blazars selected according to our procedure.