A combined optical and X-ray study of unobscured type 1 active galactic nuclei - I. Optical spectra and spectral energy distribution modelling

Abstract

We present modelling and interpretation of the continuum and emission lines for a sample of 51 unobscured type 1 active galactic nuclei (AGNs). All of these AGNs have high-quality spectra from both XMM–Newton and the Sloan Digital Sky Survey. We extend the wavelength coverage where possible by adding simultaneous ultraviolet data from the OM onboard XMM–Newton. Our sample is selected based on low reddening in the optical and low gas columns implied by their X-ray spectra, except for one case, the broad absorption line quasar PG 1004+130. They also lack clear signatures for the presence of a warm absorber. Therefore, the observed characteristics of this sample are likely to be directly related to the intrinsic properties of the central engine. To determine the intrinsic optical continuum, we subtract the Balmer continuum and all major emission lines (including Fe II). We also consider possible effects of contamination from the host galaxy. The resulting continuum is then used to derive the properties of the underlying accretion disc. We constrain the black hole masses from spectral fits of the Balmer emission lines and determine the best-fitting value from the modelling of broad-band spectral energy distributions (SEDs). In addition to the disc component, many of these SEDs also exhibit a strong soft X-ray excess, plus a power law extending to higher X-ray energies. We fit these SEDs by applying a new broad-band SED model which comprises accretion disc emission, low-temperature optically-thick Comptonization and a hard X-ray tail by introducing the concept of a corona radius. We find that in order to fit the data, the model often requires an additional long-wavelength optical continuum component, whose origin is discussed in this paper. We also find that the photorecombination edge of the Balmer continuum shifts and broadens beyond the standard limit of 3646 Å, implying an electron number density which is far higher than that in the broad-line-region clouds. Our results indicate that the narrow-line type 1 Seyfert galaxies in this sample tend to have lower black hole masses, higher Eddington ratios, softer 2–10 keV band spectra, lower 2–10 keV luminosities and higher αox, compared with typical broad-line type 1 Seyfert galaxies, although their bolometric luminosities are similar. We illustrate these differences in properties by forming an average SED for three subsamples, based on the full width at half-maximum velocity width of the Hβ emission line.