Optical variability of AGN in the PTF/iPTF survey
  We characterize the optical variability of quasars in the intermediate
Palomar Transient Factory (iPTF) and Palomar Transient Factory (PTF) surveys.
We re-calibrate the $r$-band light curves for $\sim$28,000 luminous, broad-line
AGNs from the SDSS, producing a total of $\sim$2.4 million photometric data
points. We utilize both the structure function (SF) and power spectrum density
(PSD) formalisms to search for links between the optical variability and the
physical parameters of the accreting supermassive black holes that power the
quasars. The excess variance (SF$^{2}$) of the quasar sample tends to zero at
very short time separations, validating our re-calibration of the time-series
data. We find that the the amplitude of variability at a given time-interval,
or equivalently the time-scale of variability to reach a certain amplitude, is
most strongly correlated with luminosity with weak or no dependence on black
hole mass and redshift. For a variability level of SF($\tau$)=0.07 mag, the
time-scale has a dependency of $\tau \propto L^{0.4}$. This is broadly
consistent with the expectation from a simple Keplerian accretion disk model,
which provides $\tau \propto L^{0.5}$. The PSD analysis also reveals that many
quasar light curves are steeper than a damped random walk. We find a
correlation between the steepness of the PSD slopes, specifically the fraction
of slopes steeper than 2.5, and black hole mass, although we cannot exclude the
possibility that luminosity or Eddington ratio are the drivers of this effect.
This effect is also seen in the SF analysis of the (i)PTF data, and in a PSD
analysis of quasars in the SDSS Stripe 82.
