The luminosity-dependent broad-line region in active galactic nuclei

Abstract

We investigate the effect of non-linear line response on the observed variability characteristics of the broad emission lines in AGN. The line response is intrinsically non-linear due to the dependence of the thermal and ionization structure of the line-emitting gas on the incident continuum flux. The BLR models we use include the effect of emission-line anisotropy and incorporate a mixture of optically thick and thin gas. We find that non-linear response causes a luminosity dependence in the form of the response function, the line variability time-scale, the strength of the line relative to the continuum, and the line profile. We discuss the observational consequences of these dependences on attempts to determine the BLR structure. Non-linear response may also influence the observed AGN mass-luminosity relation, and must be allowed for when trying to constrain variability of the BLR structure with time or luminosity.