The QUEST-La Silla AGN variability survey: connection between AGN
  variability and black hole physical properties
  We present our statistical analysis of the connection between active galactic
nuclei (AGN) variability and physical properties of the central supermassive
black hole (SMBH). We constructed optical light curves using data from the
QUEST-La Silla AGN variability survey. To model the variability, we used the
structure function, among the excess variance and the amplitude from Damp
Random Walk (DRW) modeling. For the measurement of SMBH physical properties, we
used public spectra from the Sloan Digital Sky Survey (SDSS). Our analysis is
based on an original sample of 2345 sources detected in both SDSS and QUEST-La
Silla. For 1473 of these sources we could perform a proper measurement of the
spectral and variability properties, and 1348 of these sources were classified
as variable ($91.5\%$). We found that the amplitude of the variability ($A$)
depends solely on the rest frame emission wavelength and the Eddington ratio,
where $A$ anti-correlates with both $\lambda_{rest}$ and $L/L_{\text{Edd}}$.
This suggests that AGN variability does not evolve over cosmic time, and its
amplitude is inversely related to the accretion rate. We found that the
logarithmic gradient of the variability ($\gamma$) does not correlate
significantly with any SMBH physical parameter, since there is no statistically
significant linear regression model with an absolute value of the slope higher
than 0.1. Finally, we found that the general distribution of $\gamma$ measured
for our sample differs from the distribution of $\gamma$ obtained for light
curves simulated from a DRW process. For 20.6\% of the variable sources in our
sample, a DRW model is not appropriate to describe the variability, since
$\gamma$ differs considerably from the expected value of 0.5.
