A Combined Optical and X-ray Study of Unobscured Type 1 AGN. II. Relation Between X-ray Emission and Optical Spectra

TL;DR

This study investigates the relationship between optical spectra and hard X-ray emission in unobscured Type 1 AGNs, revealing strong correlations especially with broad line regions, and introduces the Correlation Spectrum Technique (CST).

Contribution

It develops the CST method to quantify correlations between X-ray luminosity and optical spectral features in Type 1 AGNs, highlighting the link between X-ray emission and broad line regions.

Findings

Strong correlation between X-ray luminosity and optical emission lines in Seyfert 1s.

The broad component of Balmer lines correlates best with X-ray emission.

Different physical conditions are indicated by Balmer decrement variations.

Abstract

In this paper we study the properties of the optical spectra of Type 1 active galactic nuclei (AGNs) by using the unobscured hard X-ray emission as a diagnostic. We develop the `Correlation Spectrum Technique' (CST) and use this to show the strength of correlation between the hard X-ray luminosity and each wavelength of the optical spectrum. This shows that for Broad Line Seyfert 1s all the strong emission lines (broad component of H\alpha and H\beta, [NeIII] \lambda\lambda 3869/3967, [OI] \lambda\lambda 6300/6364, [OII] \lambda\lambda 3726/3729, [OIII] \lambda\lambda 4959/5007) and the optical underlying continuum all strongly correlate with the hard X-ray emission. But the NLS1s appear to be somewhat different. Among the various Balmer line components and the broadband SED components, the best correlation exists between the hard X-ray component and broad component (BC) of the Balmer lines, which supports the view that broad line region (BLR) has the closest link with the AGN's compact X-ray emission. The equivalent widths of Balmer line IC and BC are found to correlate with L$_{2-10keV}$, $\kappa_{2-10keV}^{-1} = L_{bol}/L_{2-10keV}$, Balmer line FWHM and black hole mass. There is a non-linear dependence of the Balmer line IC and BC luminosities with L$_{2-10keV}$ and L$_{5100}$, which suggests that a second-order factor such as the 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 BLR and 4.8 in ILR. This suggests different physical conditions in these regions. The [OIII] line is composed of a narrow core together with a blue-shifted component with average outflow velocity of $130^{+230}_{-80} km s^{-1}$. The total luminosity of [OIII] \lambda 5007 well correlates with the hard X-ray luminosity.