Optical counterparts of undetermined type $γ$-ray Active Galactic Nuclei with blazar-like Spectral Energy Distributions

Abstract

During its first four years of scientific observations, the Fermi Large Area Telescope (Fermi-LAT) detected 3033 $γ$-ray sources above a 4$σ$ 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 ($\sim 30\%$) 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 that, 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. In this communication 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.