Unidentified γ-Ray Sources: Hunting γ-Ray Blazars

One of the main scientific objectives of the ongoing Fermi mission is unveiling the nature of unidentified γ -ray sources (UGSs). Despite the major improvements of Fermi in the localization of γ -ray sources with respect to the past γ -ray missions, about one-third of the Fermi-detected objects are still not associated with low-energy counterparts. Recently, using the Wide-field Infrared Survey Explorer survey, we discovered that blazars, the rarest class of active galactic nuclei and the largest population of γ -ray sources, can be recognized and separated from other extragalactic sources on the basis of their infrared (IR) colors. Based on this result, we designed an association method for the γ -ray sources to recognize if there is a blazar candidate within the positional uncertainty region of a generic γ -ray source. With this new IR diagnostic tool, we searched for γ -ray blazar candidates associated with the UGS sample of the second Fermi γ -ray LAT catalog (2FGL). We found that our method associates at least one γ -ray blazar candidate as a counterpart to each of 156 out of 313 UGSs analyzed. These new low-energy candidates have the same IR properties as the blazars associated with γ -ray sources in the 2FGL catalog.