Unveiling the Nature of the Unidentified Gamma-Ray Sources. V. Analysis of the Radio Candidates with the Kernel Density Estimation

Nearly one-third of the γ -ray sources detected by Fermi are still unidentified, despite significant recent progress in this area. However, all of the γ -ray extragalactic sources associated in the second Fermi-LAT catalog have a radio counterpart. Motivated by this observational evidence, we investigate all the radio sources of the major radio surveys that lie within the positional uncertainty region of the unidentified γ -ray sources (UGSs) at a 95% level of confidence. First, we search for their infrared counterparts in the all-sky survey performed by the Wide-field Infrared Survey Explorer (WISE) and then we analyze their IR colors in comparison with those of the known γ -ray blazars. We propose a new approach, on the basis of a two-dimensional kernel density estimation technique in the single [3.4] − [4.6] − [12] μm WISE color–color plot, replacing the constraint imposed in our previous investigations on the detection at 22 μm of each potential IR counterpart of the UGSs with associated radio emission. The main goal of this analysis is to find distant γ -ray blazar candidates that, being too faint at 22μm, are not detected by WISE and thus are not selected by our purely IR-based methods. We find 55 UGSs that likely correspond to radio sources with blazar-like IR signatures. An additional 11 UGSs that have blazar-like IR colors have been found within the sample of sources found with deep recent Australia Telescope Compact Array observations.