Pan-STARRS1 variability of XMM-COSMOS AGN. II. Physical correlations and power spectrum analysis

TL;DR

This study analyzes the UV/optical variability of approximately 90 X-ray selected AGNs from the XMM-COSMOS survey using Pan-STARRS1 data, revealing correlations with physical parameters and detailed power spectrum characteristics.

Contribution

It provides new insights into the physical drivers of AGN variability, especially highlighting the role of accretion rate and detailed PSD features, challenging simpler variability models.

Findings

Variability amplitude anti-correlates with wavelength, luminosity, and Eddington ratio.

Optical PSDs show a broken power law with a bend at 100-300 days.

No significant correlation between break timescale and fundamental AGN parameters.

Abstract

[Abbreviated] We search for scaling relations between the fundamental AGN parameters and rest-frame UV/optical variability properties for a sample of $\sim$90 X-ray selected AGNs covering a wide redshift range from the XMM-COSMOS survey, with optical light curves in four bands provided by the Pan-STARRS1 (PS1) Medium Deep Field 04 survey. To estimate the variability amplitude we utilize the normalized excess variance ($\sigma_{\mathrm{rms}}^{2}$) and probe variability on rest-frame timescales of several months and years by calculating $\sigma_{\mathrm{rms}}^{2}$ from different parts of our light curves. In addition, we derive the rest-frame optical PSD for our sources using continuous-time autoregressive moving average (CARMA) models. We observe that the excess variance and the PSD amplitude are strongly anti-correlated with wavelength, bolometric luminosity and Eddington ratio. There is no evidence for a dependency of the variability amplitude on black hole mass and redshift. These results suggest that the accretion rate is the fundamental physical quantity determining the rest-frame UV/optical variability amplitude of quasars on timescales of months and years. The optical PSD of all of our sources is consistent with a broken power law showing a characteristic bend at rest-frame timescales ranging between $\sim$100 and $\sim$300 days. The break timescale exhibits no significant correlation with any of the fundamental AGN parameters. The low frequency slope of the PSD is consistent with a value of $-1$ for most of our objects, whereas the high frequency slope is characterized by a broad distribution of values between $\sim-2$ and $\sim-4$. These findings unveil significant deviations from the simple "damped random walk" model, frequently used in previous optical variability studies. We find a weak tendency for AGNs with higher black hole mass having steeper high frequency PSD slopes.