SPECTRAL DEPENDENCE OF THE BROAD EMISSION-LINE REGION IN ACTIVE GALACTIC NUCLEI

We derive a theoretical relation between R-BLR, the size of the broad emission line region (BLR) of active galactic nuclei (AGNs), and the o bserved soft X-ray luminosity and spectrum. We show that in addition t o the well-known R(BLR)similar to L-1/2 scaling, R-BLR should depend a lso on the soft X-ray spectral slope, and we derive the expected relat ion between R-BLR and the X-ray luminosity and spectral index. Applyin g this relation to calculate a predicted BLR radius for 10 AGNs with r everberation data, we show that including the dependence on the spectr um improves the agreement between the calculated BLR radius and the ra dius independently determined from reverberation mapping. Similarly, w e evaluate an expression for the line width and show that including th e dependence on the spectrum significantly improves the agreement betw een the calculated BLR velocity dispersion and the observed FWHM of th e H beta line. The theoretical expression for the line width also prov ides a physical explanation to the anticorrelation between the soft X- ray slope and the emission-line width observed in narrow-line Seyfert galaxies.