A combined optical and X-ray study of unobscured type 1 active galactic nuclei - II. Relation between X-ray emission and optical spectra

Abstract

In this second paper in a series of three, we study the properties of the various emission features and underlying continuum in the optical spectra of type 1 active galactic nuclei (AGNs) by using the unobscured hard X-ray emission as a diagnostic. We introduce the use of the ‘correlation spectrum technique’ (CST) for the first time. We use this to show the strength of the correlation between the hard X-ray luminosity and each wavelength of the optical spectrum. This shows that for broad-line Seyfert 1 galaxies all the strong emission lines (the broad component of Hα and Hβ, [Ne III] λλ3869/3967, [O I] λλ6300/6364, [O II] λλ3726/3729 and [O III] λλ4959/5007) and the optical underlying continuum all strongly correlate with the hard X-ray emission. In contrast, the narrow-line Seyfert 1 galaxies show a stronger correlation in the optical continuum but a weaker correlation in the lines. A cross-correlation with luminosity between the various Balmer line components and the broad-band spectral energy distribution (SED) components shows that the best correlation exists between the hard X-ray component and the broad component (BC) of the Balmer lines. Such a correlation is weaker for the intermediate (IC) and narrow components, which supports the view that the broad-line region (BLR) has the closest link with the AGN’s compact X-ray emission. The equivalent widths of the Balmer line IC and BC are found to correlate with Graphic, Graphic, Balmer line full width at half-maximum (FWHM) and black hole mass. There is a non-linear dependence of the Balmer line IC and BC luminosities with Graphic and L5100, which suggests that a second-order factor such as the intermediate-line region (ILR) and BLR covering factors affect the Balmer line component luminosities. The Balmer decrement is found to decrease from ∼5 in the line core to ∼2 in the extended wings, with mean decrements of 2.1 in the BLR and 4.8 in the ILR. This suggests different physical conditions in these regions, such as variations in electron density and dust abundance. The [O III] line is composed of a narrow core together with a blueshifted component with an average outflow velocity of Graphic. The total luminosity of [O III] λ5007 shows the best correlation with the luminosity of hard X-ray emission, and so can be used to estimate the intrinsic X-ray luminosity of obscured AGNs. We use the CST to show the correlation of the [O III] λ5007 luminosity with each wavelength of the full continuum SED. This shows as before that not only does the [O III] λ5007 luminosity strongly correlate with a power-law tail, but it also correlates almost as strongly with the optical continuum from the disc, but not with the soft excess.