Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-10-20
Astronomy and Astrophysics
Astrophysics
8 pages, 3 figures, accepted in PASJ
Scientific paper
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 $f_{\rm b}$. Then, assuming the PSD slope $\alpha$ as $-2<\alpha<-1$, $1/ f_{\rm b}$ 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.
Ezoe Yu-ichiro
Hasinger Gunter
Iyomoto Naoko
Makishima Kazuo.
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