The nonlinear X‐ray/ultraviolet relation in active galactic nuclei: Contribution of instrumental effects on the X‐ray variability

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.