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We present a deep Suzaku observation of H1821+643, an extremely rare exampleof a powerful quasar hosted by the central massive galaxy of a richcooling-core cluster of galaxies. Informed by previous Chandra studies of thecluster, we achieve a spectral separation of emission from the active galacticnucleus (AGN) and the intracluster medium (ICM). With a high degree ofconfidence, we identify the signatures of X-ray reflection/reprocessing by coldand slowly moving material in the AGN's immediate environment. The ironabundance of this matter is found to be significantly sub-solar (Z~0.4Zsun), anunusual finding for powerful AGN but in line with the idea that this quasar isfeeding from the ICM via a Compton-induced cooling flow. We also find a subtlesoft excess that can be described phenomenologically (with an additional blackbody component) or as ionized X-ray reflection from the inner regions of a highinclination (i=57 degrees) accretion disk around a spinning (a>0.4) black hole.We describe how the ionization state of the accretion disk can be used toconstrain the Eddington fraction of the source. Applying these arguments to ourspectrum implies an Eddington fraction of 0.25-0.5, with an associated blackhole mass of 3-6x10^9Msun.

The X-ray spectrum of the cooling-flow quasar H1821+643 : A massive black hole feeding off the intracluster medium