A combined Optical and X-ray Spectra Study for Type 1 AGN. III. Broadband SED Properties

Abstract

In this third paper in a series of three, we present a detailed study of the AGN broadband SED based on a nearby unobscured Type 1 AGN sample. We perform a systematic cross-correlation study of the following key parameters: $Γ_{2-10keV}$, $L_{2-10keV}$, $L_{bol}$, $L_{bol}/L_{Edd}$, $κ_{2-10keV}$, $κ_{5100A}$, FWHM$_{Hβ}$, M$_{BH}$, $α_{ox}$, $α_{X}$ and $α_{UV}$, and identify various strong correlations among these parameters. The principal component analysis (PCA) is performed on the correlation matrix of the above parameters, which shows that the three physical parameters, i.e. black hole mass, mass accretion rate and Eddington ratio, drive the majority of the correlations. This is consistent with PCA results found from previous optical spectral studies. We produce various mean SEDs classified by each of the key parameters. Most parameters, except L$_{bol}$, show similar systematic changes in the mean SEDs such that the temperature at which the disc peaks is correlated with the ratio of power in the disc versus the Comptonised components and the hard X-ray spectral index. This underlying change in SED shape shows that AGN do exhibit intrinsically different spectral states. This is superficially similar to the SED differences in BHB seen as $λ_{Edd}$ increases, but the analogy does not hold in detail. Only objects with the highest $λ_{Edd}$ appear to correspond to a BHB spectral state (the disc dominated high/soft state). The AGN with typical mass accretion rates have spectra which do not match well with any state observed in BHB. We speculate that this could be due to the presence of a powerful UV line driven disc wind, which complicates simple mass scaling between stellar and supermassive black holes.