... Murray

Canada Research Chair in Astrophysics.

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... parameter

There are some objects which can still be modeled adequately with only one ionization parameter, e.g., Mrk 509 (Yaqoob et al. 2003).

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... drop

An alternative explanation for this drop (based on XMM-Newton data) is that it is the blue edge of a relativistically broadened O VIII Ly$\alpha$ line (Sako et al. 2003; Branduardi-Raymont et al. 2001). However, this interpretation has been challenged from both observational (Lee et al. 2001) and theoretical (Ballantyne et al. 2002) points of view.

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