Aims: Establish the dependence of variability properties, such as
characteristic timescales and variability amplitude, on basic quasar parameters
such as black hole mass and accretion rate, controlling for the rest-frame
wavelength of emission. Methods: Using large catalogs of quasars, we selected
the g-band light curves for 4770 objects from the Zwicky Transient Facility
archive. All selected objects fall into a narrow redshift bin, 0.6<z<0.7, but
cover a wide range of accretion rates in Eddington units (REdd) and black hole
masses (M). We grouped these objects into 26 independent bins according to
these parameters, calculated low-resolution g-band variability power spectra
for each of these bins, and approximated the power spectra with a simple
analytic model that features a break at a timescale tbβ. Results: We found a
clear dependence of the break timescale tbβ on REdd, on top of the known
dependence of tbβ on the black hole mass M. In our fits, tbββM0.65β0.55 REdd 0.35β0.3, where the ranges in the exponents
correspond to the best-fitting parameters of different power spectrum models.
Scaling tbβ to the orbital timescale of the innermost stable circular orbit
(ISCO), tISCOβ, results approximately in tbβ/tISCOββ(REdd/M)0.35. The observed values of tbβ are βΌ10 longer than the
orbital timescale at the light-weighted average radius of the disc region
emitting in the (observer frame) g-band. The different scaling of the break
frequency with M and REdd shows that the shape of the variability power
spectrum cannot be solely a function of the quasar luminosity, even for a
single rest-frame wavelength. Finally, the best-fitting models have slopes
above the break in the range -2.5 and -3. A slope of -2, as in the damped
random walk models, fits the data significantly worse.Comment: Accepted for publication in A&