  We have recently demonstrated that the non-linear relation between
ultraviolet and X-ray luminosity in quasars is very tight (with an intrinsic
dispersion of ~0.2 dex), once contaminants (e.g. dust reddening, X-ray
absorption), variability, and differences in the active galactic nuclei (AGN)
physical properties are taken into account. This relation has thus the great
potential to advance our understanding in both supermassive black hole
accretion physics and observational cosmology, by targeting a single class of
objects. Here we focus on the various contributions to the observed X-ray
variability in a homogenous sample of 791 quasars selected from SDSS-DR7 with
X-ray data from the 3XMM-DR7 source catalogue. The 250 quasars in this cleaned
data set with at least two X-ray observations typically vary with a standard
deviation of fractional variation of 15-30% on timescales of weeks/years. Yet,
when the count rates are computed at progressively smaller off-axis values, the
same quantity is reduced to roughly 10-25%. This suggests that, when estimating
variability indicators, part of the quoted variability amplitude could be due
to instrumental/calibration issues rather than true variations in the quasar
emission.
