The optical spectral index vs. luminosity relation in AGN and one of its implications

Spectroscopic optical data (∼4200-6300 Å in the rest-frame) acquired during the monitoring of some QSOs seem to confirm that AGN spectra normally harden at higher luminosities. Focusing only on the global trend (i.e. comparing different objects), the implications in deriving luminosities using optical photometric data (through the K-correction) is briefly outlined here, as well as the result of its parameterization for calculating BH masses through the M_BH ∝ L_V^γ (FWHM Hβ)2 virial relation.