Unidentified ?-Ray Sources: Hunting ?-Ray Blazars

One of the main scientific objectives of the ongoing Fermi mission is unveiling the nature of unidentified ?-ray sources (UGSs). Despite the major improvements of Fermi in the localization of ?-ray sources with respect to the past ?-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 ?-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 ?-ray sources to recognize if there is a blazar candidate within the positional uncertainty region of a generic ?-ray source. With this new IR diagnostic tool, we searched for ?-ray blazar candidates associated with the UGS sample of the second Fermi ?-ray LAT catalog (2FGL). We found that our method associates at least one ?-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 ?-ray sources in the 2FGL catalog.