X-Ray Variability of SDSS Quasars Based on the SRG/eROSITA All-Sky Survey

We examine the long-term (rest-frame time scales from a few months to ∼ 20 years) X-ray variability of a sample of 2344 X-ray bright quasars from the SDSS DR14Q Catalogue, based on the data of the SRG/eROSITA All-Sky Survey complemented for ∼ 7 Serendipitous Source Catalogue. We characterise variability by a structure function, SF^2(Δ t). We confirm the previously known anti-correlation of the X-ray variability amplitude with luminosity. We also study the dependence of X-ray variability on black hole mass, M_ BH, and on an X-ray based proxy of the Eddington ratio, λ_ X. Less massive black holes prove to be more variable for given Eddington ratio and time scale. X-ray variability also grows with decreasing Eddington ratio and becomes particularly strong at λ_ X of less than a few per cent. We confirm that the X-ray variability amplitude increases with increasing time scale. The SF^2(Δ t) dependence can be satisfactorily described by a power law, with the slope ranging from ∼ 0 to ∼ 0.4 for different (M_ BH, λ_ X) subsamples (except for the subsample with the lowest black hole mass and lowest Eddington ratio, where it is equal to 1.1± 0.4)