Optical variability of AGN in the PTF/iPTF survey

TL;DR

This study analyzes optical variability in quasars from the PTF/iPTF surveys, revealing that variability amplitude correlates strongly with luminosity and that many quasar light curves are steeper than expected from simple models.

Contribution

It provides a comprehensive re-calibration of quasar light curves and links optical variability characteristics to physical parameters like luminosity and black hole mass using SF and PSD analyses.

Findings

Variability amplitude correlates with luminosity.

Many quasar light curves are steeper than a damped random walk.

Steeper PSD slopes are linked to higher black hole mass.

Abstract

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.