  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<z<3.3), in X-ray spectral slope (-1.5
<ax<2.6) and in optical-to-X-ray ratio (0.4$<aox<1.5). A correlation is found
between ax and aox; similar correlations have recently been reported in other
X-ray and optical samples. We also identify previously unreported relationships
between the optical slope (aopt) and ax (particularly at high redshifts) and
between aopt and aox. These trends show that the overall optical-to-X-ray conti
nuum changes from convex to concave as ax hardens, demonstrating a strong beha
vioural link between the optical/UV big blue bump (BBB) and the soft X-ray ex
cess, 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 ab
sorbed 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 ax=1 power
law; the column density of the cold gas ranges from 0 to ~4E21 cm-2 and the
dust-to-gas ratio is assumed to be Galactic. The `big bump' may be represented
by a T~1E6 K thermal bremsstrahlung or an accretion disk with a surrounding hot
corona. The scatter in the data can accommodate a wide range in big bump tempe
rature (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.
