The simultaneous optical-to-X-Ray spectral energy distribution of soft X-Ray selected active galactic nuclei observed by Swift

We report Swift observations of a sample of 92 bright soft X-ray selected active galactic nuclei (AGNs). This sample represents the largest number of AGNs observed to study the spectral energy distribution (SED) of AGNs with simultaneous optical/UV and X-ray data. The principal motivation of this study is to understand the SEDs of AGNs in the optical/UV to X-ray regime and to provide bolometric corrections which are important in determining the Eddington ratio L/L Edd. 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 αX and αUV with L/L Edd. Although narrow-line Seyfert 1 galaxies (NLS1s) have steeper αX and higher L/L Edd than broad-line Seyfert 1 galaxies (BLS1s), their optical/UV to X-ray spectral slopes αox and optical/UV slopes αUV are very similar. The mean SED of NLS1s shows that in general this type of AGNs appears to be fainter in the UV and at hard X-ray energies than BLS1s. We find a strong correlation between αX and αUV for AGNs with X-ray spectral slopes αX<1.6. For AGNs with steeper X-ray spectra, both this relation and the relation between αX and L/L Edd break down. At αX≈1.6, L/L Edd reaches unity. We note an offset in the αUV-L/L Edd relation between NLS1s and BLS1s. We argue that αUV is a good estimator of L/L Edd and suggest that αUV can be used to estimate L/L Edd in high-redshift QSOs. Although NLS1s appear to be highly variable in X-rays, they only vary marginally in the UV.