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Aims: Establish the dependence of variability properties, such ascharacteristic timescales and variability amplitude, on basic quasar parameterssuch as black hole mass and accretion rate, controlling for the rest-framewavelength of emission. Methods: Using large catalogs of quasars, we selectedthe g-band light curves for 4770 objects from the Zwicky Transient Facilityarchive. All selected objects fall into a narrow redshift bin, $0.6<z<0.7$, butcover a wide range of accretion rates in Eddington units (REdd) and black holemasses ($M$). We grouped these objects into 26 independent bins according tothese parameters, calculated low-resolution $g$-band variability power spectrafor each of these bins, and approximated the power spectra with a simpleanalytic model that features a break at a timescale $t_b$. Results: We found aclear dependence of the break timescale $t_b$ on REdd, on top of the knowndependence of $t_b$ on the black hole mass $M$. In our fits, $t_b\proptoM^{0.65 - 0.55}$ REdd $^{0.35 - 0.3}$, where the ranges in the exponentscorrespond to the best-fitting parameters of different power spectrum models.Scaling $t_b$ to the orbital timescale of the innermost stable circular orbit(ISCO), $t_{\rm ISCO}$, results approximately in $t_{b}/t_{\rm ISCO} \propto($REdd$/M)^{0.35}$. The observed values of $t_b$ are $\sim 10$ longer than theorbital timescale at the light-weighted average radius of the disc regionemitting in the (observer frame) $g$-band. The different scaling of the breakfrequency with $M$ and REdd shows that the shape of the variability powerspectrum cannot be solely a function of the quasar luminosity, even for asingle rest-frame wavelength. Finally, the best-fitting models have slopesabove the break in the range -2.5 and -3. A slope of -2, as in the dampedrandom walk models, fits the data significantly worse.

The universal power spectrum of Quasars in optical wavelengths: Break timescale scales directly with both black hole mass and accretion rate