Long-term variability of the low-luminosity active galactic nucleus of M81

Long-term X-ray variability of the low-luminosity active galactic nucleus of M81 was studied, using 16 ASCA observations spanning 5.5 yr. The object exhibits a factor of 3 variation over the 5.5 yr. 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^-8.2 -10^-4.3 Hz (period range of 0.25 d-5.5 yr), 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 ≥ 800 d and α = 1.4 ± 0.2.