Optical counterparts of undetermined type ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray sources above a 4$\ddot{I}ƒ$ 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 ${\hat{I}}^3$-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 ${\hat{I}}^3$-rays, providing a much better source localization potential, we focused our attention on a sample of ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray emission and a study of their redshift distribution, with the subsequent implications on the intrinsic source power. We furthermore analyze which ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray sources, we give an overview on the status of our work, and we discuss the implications of a large scale study of ${\hat{I}}^3$-ray emitting AGNs.

9 pages, 2 figures, proceedings of the 10th Serbian Conference on Spectral Line Shapes in Astrophysics. JOAA, accepted