Variability of active galaxies. I.

X-ray light curves of unabsorbed AGN show dramatic random variability on timescales 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 timescale 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.