THE LUMINOSITY-DEPENDENT BROAD-LINE REGION IN ACTIVE GALACTIC NUCLEI

We investigate the effect of non-linear line response on the observed variability characteristics of the broad emission lines in AGN. The li ne response is intrinsically non-linear due to the dependence of the t hermal and ionization structure of the line-emitting gas on the incide nt continuum flux. The BLR models we use include the effect of emissio n-line anisotropy and incorporate a mixture of optically thick and thi n 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 pro file. We discuss the observational consequences of these dependences o n attempts to determine the BLR structure. Non-linear response may als o influence the observed AGN mass-luminosity relation, and must be all owed for when trying to constrain variability of the BLR structure wit h time or luminosity.