  The observed relation between the soft X-ray and the optical-ultraviolet
emission in active galactic nuclei (AGN) is non-linear and it is usually
parametrized as a dependence between the logarithm of the monochromatic
luminosity at 2500 {\AA} and at 2 keV. Previous investigations have found that
the dispersion of this relation is rather high (~0.35-0.4 in log units), which
may be caused by measurement uncertainties, variability, and intrinsic
dispersion due to differences in the AGN physical properties (e.g. different
accretion modes). We show that, once optically-selected quasars with
homogeneous SED and X-ray detection are selected, and dust reddened and/or gas
obscured objects are not included, the measured dispersion drops to
significantly lower values (i.e. ~0.21-0.24 dex). We show that the residual
dispersion is due to some extent to variability, and to remaining measurement
uncertainties. Therefore, the real physical intrinsic dispersion should be
<0.21 dex. Such a tight relation, valid over 4 decades in luminosity, must be
the manifestation of an intrinsic (and universal) physical relation between the
disk, emitting the primary radiation, and the hot electron corona emitting
X-rays.
