2001MNRAS.321..767I

Long-term X-ray variability of the low-luminosity active galactic nucleus of M81 was studied, using 16 ASCA observations spanning 5.5yr. The object exhibits a factor of 3 variation over the 5.5yr. The source intensity was relatively constant within each observation which lasted typically for one day, but intra-day variability by 30 per cent was detected on the 15th observation. The power-spectral density (PSD) was estimated in a `forward' manner, over a frequency range of 10<sup>–8.2</sup>-10<sup>–4.3</sup>Hz (period range of 0.25d-5.5yr), by utilizing the structure function and extensive Monte Carlo simulations in order to overcome the very sparse and uneven data samplings. When the PSD is assumed to be white below a `break frequency' f_b and falls off as ∝f^–α above f_b, where f is frequency and α is a positive parameter, the M81 light curve is well described with 1/f_b≥800d and α=1.4±0.2.