An improved method is utilized to estimate the X-ray power spectral densities ( PSD ) and the variation time scales of three quasars in the Lockman Hole field .
Five archival ROSAT PSPC data covering two year range are analyzed .
To estimate PSD from sparse and unevenly-sampled lightcurves , a forward-method approach with extensive Monte-Carlo simulations is adopted .
A broken power-law type PSD with a constant Poisson noise component is assumed with a break frequency f _ { b } .
Then , assuming the PSD slope \alpha as -2 < \alpha < -1 , 1 / f _ { b } is constrained as \gtrsim 25 days for one object , while the constraints on the other two objects are very weak .
The long time scale of the one object is consistent with the view that luminous AGNs host massive black holes .