Optical Counterparts of Undetermined Type γ -Ray Active Galactic Nuclei with Blazar-Like Spectral Energy Distributions.

During its first four years of scientific observations, the Fermi Large Area Telescope (Fermi-LAT) detected 3033 γ -ray sources above a 4σ significance level. Although most of the extra-galactic sources are active galactic nuclei (AGN) of the blazar class, other families of AGNs are observed too, while a still high fraction of detections (~30%) remains with uncertain association or classification. According to the currently accepted interpretation, the AGN γ -ray emission arises from inverse Compton (IC) scattering of low energy photons by relativistic particles confined in a jet, which, in the case of blazars, is oriented very close to our line-of-sight. Taking advantage of data from radio and X-ray wavelengths, which we expect to be produced together with γ-rays, providing a much better source localization potential, we focused our attention on a sample of γ -ray Blazar Candidates of Undetermined type (BCUs), starting a campaign of optical spectroscopic observations. The main aims of our investigation include a census of the AGN families that contribute to γ -ray emission and a study of their redshift distribution, with the subsequent implications on the intrinsic source power.We furthermore analyze which γ -ray properties can better constrain the nature of the source, thus helping in the study of objects not yet associated with a reliable low frequency counterpart. Here we report on the instruments and techniques used to identify the optical counterparts of γ -ray sources, we give an overview on the status of our work, and we discuss the implications of a large scale study of γ -ray emitting AGNs. [ABSTRACT FROM AUTHOR]