Physical properties of gravitationally lensed Active Galactic Nuclei

Abstract

When lensed AGNs are discovered, the primary focus is usually on their lensing properties rather than their physical properties. This Master thesis aims at finding new estimates of the physical properties of a sample of ten gravitationally lensed quasars using a uniform method based on spectral modelling and scaling relations. The sample is comprised of doubly and quadruply lensed quasars at redshifts roughly between 1.5 and 2.6 showing various spectral properties. The physical properties computed here are the black hole mass, the Eddington ratio and the size of the BLR. The latter allows an estimation of the sensitivity of the source to the microlensing effect assuming a theoretical lens mass. The basis on which the determination of the physical properties rests is the optical/UV spectra obtained from various instruments such as the MUSE (Multi-Unit Spectroscopic Explorer) or the FORS (FOcal Reducer and low dispersion Spectrograph) instruments of the VLT. The methodology can be summarized as follows. The first step is the modelling of the considered spectrum and the identification of known broad and narrow emission lines. Secondly, the black hole mass estimate and the size of the BLR are computed using known scaling relations based on the virial theorem. These involve the calibrated intrinsic luminosity of the continuum either at 1450 or 3000 Å depending on the chosen emission line, either C IV at 1549Å or Mg II at 2798 Å. Finally, the accretion efficiency is evaluated by the ratio between the Eddington luminosity, which is the upper limit on the luminosity an AGN can theoretically have, and the bolometric luminosity. The sensitivity to the microlensing effect is then evaluated by the ratio of the size of the Broad Line Region to the projected Einstein radius of the microlens. The uncertainties of each of the physical quantity are also computed. Finally, a discussion about the implications of this method and its limitation is given.