A Combined Optical and X-ray Study of Unobscured Type 1 AGN. I. Optical Spectra and SED Modeling

TL;DR

This study models the broadband spectra of 51 unobscured Type 1 AGN using optical, UV, and X-ray data, revealing intrinsic properties and differences between NLS1 and BLS1 AGN.

Contribution

It introduces a comprehensive broadband SED model combining optical, UV, and X-ray data, and analyzes the intrinsic properties of unobscured Type 1 AGN with high-quality spectra.

Findings

Narrow Line Seyfert 1s have lower black hole masses and higher Eddington ratios.

The broadband SED fitting requires an additional optical continuum component.

Balmer continuum edges indicate higher electron densities than in BLR clouds.

Abstract

We present modeling and interpretation of the continuum and emission lines for a sample of 51 unobscured Type 1 active galactic nuclei (AGN). All of these AGNs have high quality spectra from both XMM-Newton and Sloan Digital Sky Survey (SDSS). We extend the wavelength coverage where possible by adding simultaneous UV 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. 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. We perform multi-component spectral fitting for strong optical emission lines and the whole optical spectra. We fit the combined optical, UV and X-ray data by applying a new broadband SED model which comprises the accretion disc emission, low temperature optically thick Comptonisation and a hard X-ray tail by introducing the a corona radius (Done et al. 2011). 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 Photo-recombination edge of Balmer continuum shifts and broadens beyond the standard limit of 3646{\AA}, implying an electron number density which is far higher than that in the broad line region clouds. Our results indicate that the Narrow Line Seyfert 1s 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 \alpha_{ox}, compared with typical broad line Seyfert 1s (BLS1), although their bolometric luminosities are similar. We illustrate these differences in properties by forming an average SED for three subsamples, based on the FWHM velocity width of the H{\beta} emission line.