Optical and X-ray properties of the RIXOS active galactic nuclei - I. The continua

We present measurements of the optical and X-ray continua of 108 AGN (Seyfert 1s and quasars) from the ROSAT International X-ray/Optical Survey (RIXOS). The sample covers a wide range in redshift (0<<e1>z<</e1>3.3), in X-ray spectral slope (-1.5f<alpha_x<2.6) and in optical-to-X-ray ratio (0.4<alpha_ox<1.5). A correlation is found between alpha_x and alpha_ox similar correlations have recently been reported in other X-ray and optical samples. We also identify previously unreported relationships between the optical slope (alpha_opt) and alpha_x (particularly at high redshifts) and between alpha_opt and alpha_ox. These trends show that the overall optical-to-X-ray continuum changes from convex to concave as alpha_x hardens, demonstrating a strong behavioural link between the optical/UV big blue bump (BBB) and the soft X-ray excess, which is consistent with them being part of the same spectral component. By constructing models of the optical-to-X-ray continuum, we demonstrate that the observed correlations are consistent with an intrinsic spectrum which is absorbed through different amounts of cold gas and dust. The intrinsic spectrum is the sum of an optical-to-soft X-ray `big bump' component and an alpha_x=1 power law; the column density of the cold gas ranges from 0 to ~4x10^21 cm^-2 and the dust-to-gas ratio is assumed to be Galactic. The `big bump' may be represented by a T_brem~10^6 K thermal bremsstrahlung or an accretion disc with a surrounding hot corona. The scatter in the data can accommodate a wide range in big bump temperature (or black hole mass) and strength. A source for the absorbing gas may be the dusty, molecular torus which lies beyond the broad-line-emitting regions, although with a much lower column density than observed in Seyfert 2 galaxies. Alternatively, it may be the bulge of a spiral host galaxy or an elliptical host galaxy.