The variability of active galaxies – I. Broad-band noise X-ray power spectra from <i>XMM–Newton</i> and <i>Swift</i>

ABSTRACT Accreting supermassive black holes at the centres of galaxies are the engine of active galactic nuclei (AGNs). X-ray light curves of unabsorbed AGNs show dramatic random variability on time-scales ranging from seconds to years. The power spectrum of the fluctuations is usually well modelled with a power law that decays as $1/f$ at low frequencies, and which bends to $1/f^{2-3}$ at high frequencies. The time-scale associated with the bend correlates well with the mass of the black hole and may also correlate with bolometric luminosity in the ‘X-ray variability plane’. Because AGN light curves are usually irregularly sampled, the estimation of AGN power spectra is challenging. In a previous paper, we introduced a new method to estimate the parameters of bending power law power spectra from AGN light curves. We apply this method to a sample of 56 variable and unabsorbed AGNs, observed with XMM–Newton and Swift in the 0.3–1.5 keV band over the past two decades. We obtain estimates of the bends in 50 sources, which is the largest sample of X-ray bends in the soft band. We also find that the high-frequency power spectrum is often steeper than 2. We update the X-ray variability plane with new bend time-scale measurements spanning from 7 min to 62 d. We report the detections of low-frequency bends in the power spectra of five AGN, three of which are previously unpublished: 1H 1934-063, Mkn 766 and Mkn 279.