Pan-STARRS1 variability of XMM-COSMOS AGN

II. Physical correlations and power spectrum analysis

¹ Max-Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, Postfach 1312, 85741 Garching, Germany
e-mail: tsimm@mpe.mpg.de
² University Observatory Munich, Ludwig-Maximilians Universitaet, Scheinerstrasse 1, 81679 Munich, Germany
³ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Dipartimento di Fisica e Astronomia Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ Department of Physics, Institute of Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland

Received: 13 September 2015
Accepted: 20 October 2015

Abstract

Aims. The goal of this work is to better understand the correlations between the rest-frame UV/optical variability amplitude of quasi-stellar objects (QSOs) and physical quantities such as redshift, luminosity, black hole mass, and Eddington ratio. Previous analyses of the same type found evidence for correlations between the variability amplitude and these active galactic nucleus (AGN) parameters. However, most of the relations exhibit considerable scatter, and the trends obtained by various authors are often contradictory. Moreover, the shape of the optical power spectral density (PSD) is currently available for only a handful of objects.

Methods. We searched for scaling relations between the fundamental AGN parameters and rest-frame UV/optical variability properties for a sample of ~90 X-ray selected AGNs covering a wide redshift range from the XMM-COSMOS survey, with optical light curves in four bands (g_P1, r_P1, i_P1, z_P1) provided by the Pan-STARRS1 (PS1) Medium Deep Field 04 survey. To estimate the variability amplitude, we used the normalized excess variance (σ²_rms) and probed variability on rest-frame timescales of several months and years by calculating σ²_rms from different parts of our light curves. In addition, we derived the rest-frame optical PSD for our sources using continuous-time autoregressive moving average (CARMA) models.

Results. We observe that the excess variance and the PSD amplitude are strongly anticorrelated 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 ~100 and ~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 ~–2 and ~–4. These findings unveil significant deviations from the simple damped random walk model that has frequently been used in previous optical variability studies. We find a weak tendency for AGNs with higher black hole mass to have steeper high-frequency PSD slopes.