A combined optical and X-ray study of unobscured type 1 active galactic nuclei - III. Broad-band SED properties

In this third paper in a series of three, we present a detailed study of the broad-band spectral energy distribution (SED) of active galactic nuclei (AGNs) based on a nearby unobscured type 1 AGN sample. We perform a systematic cross-correlation study of several key parameters, i.e. Γ_{2-10 keV}, L_{2-10 keV}, L_bol/L_Edd = λ_Edd, L_bol/L_{2-10 keV} = κ_{2-10 keV}, L bol /L5100 Å=κ5100 Å, FWHM_Hβ, M_BH, α_ox, α_X and α_UV. The well-defined spectral properties of the sample enable us to improve existing relations and to identify new correlations among these parameters. We confirm a break region around FWHM_Hβ ≃ 4000 km s^-1 in the Γ_{2-10 keV} versus FWHM_Hβ correlation and log(M_BH) ≃ 8.0 in the Γ_{2-10 keV} versus M_BH correlation, where these correlations appear to change form. Beyond the break point, the intrinsic Γ_{2-10 keV} index is dispersed around 1.8. Several new correlations are also reported in this paper, e.g. strong correlations in κ₅₁₀₀ versus λ_Edd, κ₅₁₀₀ versus κ_{2-10 keV} and κ_{2-10 keV} versus M_BH. The principal component analysis (PCA) is performed on the correlation matrix of the above parameters. This 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.

For each key parameter, we split the AGNs into three subsamples, binned based on increasing value of that parameter. We co-add the model SEDs for each object in the subsample to see how the SED changes with that parameter. Most parameters, except L_bol, show similar systematic changes in the SED such that the temperature at which the disc peaks is correlated with the ratio of power in the disc versus the Comptonized components and the hard X-ray spectral index. This underlying change in SED shape shows that AGNs do exhibit intrinsically different spectral states. This is superficially similar to the SED differences in black hole binary (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 AGNs 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 ultraviolet line-driven disc wind, which complicates simple mass scaling between stellar and supermassive black holes.