OPTICAL AND X-RAY-PROPERTIES OF THE RIXOS ACTIVE GALACTIC NUCLEI .1. THE CONTINUA

We present measurements of the optical and X-ray continua of 108 AGN ( Seyfert Is and quasars) from the ROSAT International X-ray/Optical Sur vey (RIXOS). The sample covers a wide range in redshift (0 < z < 3.3), in X-ray spectral slope (- 1.5 < alpha(x) < 2.6) and in optical-to-X- ray ratio (0.4 < alpha(ox) < 1.5). A correlation is found between ct, and a,; similar correlations have recently been reported in other X-ra y and optical samples. We also identify previously unreported relation ships between the optical slope (alpha(opt)) and alpha(x) (particularl y at high redshifts) and between alpha(opt) and alpha(ox). These trend s show that the overall optical-to-X-ray continuum changes from convex to concave as alpha(x) hardens, demonstrating a strong behavioural li nk between the optical/UV big blue bump (BBB) and the soft X-ray exces s, which is consistent with them being part of the same spectral compo nent. By constructing models of the optical-to-X-ray continuum, we dem onstrate that the observed correlations are consistent with an intrins ic spectrum which is absorbed through different amounts of cold gas an d 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 similar to 4 x 10(21) cm(-2) and the dust-to-gas ratio is assumed to be Galactic. The 'big bump' may be re presented by a T-brem similar to 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 ma ss) and strength. A source for the absorbing gas may be the dusty, mol ecular torus which lies beyond the broad-line-emitting regions, althou gh with a much lower column density than observed in Seyfert 2 galaxie s. Alternatively, it may be the bulge of a spiral host galaxy or an el liptical host galaxy.