UNVEILING THE NATURE OF THE UNIDENTIFIED GAMMA-RAYS SOURCES VIII: COMPUTING THE ASSOCIATION PROBABILITY

Abstract

Despite the significant improvements of the Fermi satellite on the source localization with respect to the previous γ-ray missions, the positional uncertainties of the Fermi sources are still large, making the search for potential low-energy counterparts a challenging task. In the Fermi source catalogs (i.e., 1FGL and 2FGL, respectively) for each counterpart associated with an high-energy source a corresponding value of the association probability was provided. Thus several methods based on the source position or on the logN-logS distribution of potential counterparts were developed to derive the association probabilities. Recently, we discovered a tight connection between the infrared (IR) surveys and the γ-ray sky that allowed us to create several lists of γ-ray blazar-like sources, potential counterparts of Fermi objects. Here we complete our previous analyses presenting a new approach based on Montecarlo simulations to determine the association probability for γ-ray blazarlike sources selected on the basis of their peculiar IR colors. We also describe a different version of the likelihood ratio technique with some improvements based on the IR-γ-ray connection. Both methods are compared with the 2FGL associations to asses their reliability. We found reliable counterparts for 39 previously unidentified γ-ray sources listed in the 2FGL and 5 new γ-ray blazar candidates out of 20 sources associated for a subsample of the 1FGL not detected in the 2FGL. Both methods are also able to associate radio loud narrow line Seyfert 1 showing blazar-like IR colors