Long-term X-ray variability of the low-luminosity active galactic nucleus of M81 was studied , using 16 ASCA observations spanning 5.5 years .
The object exhibits a factor three variation over the 5.5 years .
The source intensity was relatively constant within each observation which lasted typically for one day , but intra-day variability by 30 % was detected on the 15th observation .
The power-spectral density ( PSD ) was estimated in “ forward ” manner , over a frequency range of 10 ^ { -8.2 } – 10 ^ { -4.3 } Hz ( period range of 1/4 day–5.5 years ) , by utilizing 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 \propto f ^ { - \alpha } above f _ { b } , where f is frequency and \alpha is a positive parameter , the M81 light curve is well described with 1/ f _ { b } \geq 800 days and \alpha = 1.4 \pm 0.2 .