The Simultaneous Optical-to-X-ray Spectral Energy Distribution of Soft X-ray Selected AGN observed by Swift

TL;DR

This study presents simultaneous optical/UV and X-ray observations of 92 bright soft X-ray selected AGN, revealing correlations between spectral slopes and accretion rates, and providing insights into their spectral energy distributions and bolometric corrections.

Contribution

It offers the largest sample of simultaneous optical/UV and X-ray data for AGN, analyzing spectral energy distributions and their relation to accretion properties, especially distinguishing NLS1s from BLS1s.

Findings

Strong correlation between alpha-x and alpha-UV in AGN with X-ray slopes <1.6

NLS1s are generally fainter in UV and hard X-ray energies than BLS1s

Alpha-UV can be used to estimate L/Ledd, especially in high-redshift QSOs

Abstract

We report Swift observations of a sample of 92 bright soft X-ray selected active galactic nuclei (AGN). This sample represents the largest number of AGN observed to study the spectral energy distribution (SED) of AGN with simultaneous optical/UV and X-ray data. The principal motivation of this study is to understand the SEDs of AGN in the optical/UV to X-ray regime and to provide bolometric corrections which are important in determining the Eddington ratio L/Ledd. In particular, we rigorously explore the dependence of the UV-EUV contribution to the bolometric correction on the assumed EUV spectral shape. We find strong correlations of the spectral slopes alpha-x and alpha-UV with L/Ledd. Although Narrow-Line Seyfert 1 galaxies (NLS1s) have steeper alpha-x and higher L/Ledd than Broad-Line Seyfert 1 galaxies (BLS1s), their optical/UV to X-ray spectral slopes alpha-ox and optical/UV slopes alpha-UV are very similar. The mean SED of NLS1s shows that in general this type of AGN appears to be fainter in the UV and at hard X-ray energies than BLS1s. We find a strong correlation between alpha-x and alpha-UV for AGN with X-ray spectral slopes alpha-x<1.6. For AGN with steeper X-ray spectra, both this relation and the relation between alpha-x and L/Ledd break down. At alpha-x$\approx$1.6, L/Ledd reaches unity. We note an offset in the alpha-UV - L/Ledd relation between NLS1s and BLS1s. We argue that alpha-UV is a good estimator of L/Ledd and suggest that alpha-UV can be used to estimate L/Ledd in high-redshift QSOs. Although NLS1s appear to be highly variable in X-rays they only vary marginally in the UV.