BL LAC POPULATION STUDY AT HIGH ENERGIES

Abstract

In the framework of Active Galactic Nuclei (AGN) unification, BL Lacs and their parent population would share the same intrinsic characteristics, the observational differences being due to the orientation of the relativistic jet compared to the line of sight. BL Lacs would be the objects whose jet is oriented towards us, their emission being amplified by the relativistic Doppler boosting. Constraints arising from fast variability and/or large optical depth to pair production commonly imply large Lorentz factors. The growing number of BL Lacs detected at HE (> 100 MeV) and VHE (> 100 GeV) is a challenge for this unification scheme. Indeed, the high values of Doppler factor needed in the simplest radiative model to explain the emission of these sources imply a large density for the parent population. A possible solution to this Doppler factor crisis lies in considering different geometries for the jet. In this study, we use the BL Lacs detected at HE and VHE to investigate the intrinsic properties of the associated parent population. Using the results presented in Fermi's second AGN catalog and performing MC simulations of the parent population, we constrain the jet parameters: its intrinsic luminosity, Lorentz factor and geometric opening angle. The simulated density of parent population and Doppler factors of the objects detectable at HE within this population are presented according to the jet parameters.