We report the detection of X-ray emission from the host cluster of the unusual radio-quiet quasar H 1821+643 using the ROSAT HRI, and the non-detection of X-ray emission from the host cluster of the radio-loud quasar 3C 206 (3 sigma upper limit of 1.63x10(44) ergs s(-1)) using the EINSTEIN HRI. The host cluster of H 1821+643 is one of the most X-ray luminous clusters known, with a rest-frame 0.1-2.4 keV luminosity of 3.74+/-0.57h(50)(-2)X10(45) ergs s(-1), 38% of which is from a barely resolved cooling flow component. The cluster emission complicates interpretation of previous X-ray spectra of this field. In particular, the observed Fe K alpha emission can probably be attributed entirely to the cluster and either the quasar is relatively X-ray quiet for its optical luminosity or the cluster has a relatively low temperature for its luminosity. We combine these data with the recent detection of X-ray emission from the host cluster of the ''buried'' radio-quiet quasar IRAS 09104+4109 [Fabian & Crawford, MNRAS, 274, L63 (1995)], our previous upper limits for the host clusters of two z similar to 0.7 radio-loud quasars, and literature data on FR II radio galaxies. We compare this dataset to the predictions of three models for the presence and evolution of powerful AGN in clusters: the low-velocity-dispersion model, the low-ICM-density model, and the cooling flow model. Neither the low-ICM-density model nor the cooling flow model can explain all the observations. We suggest that strong interactions with gas-containing galaxies mag. be the only mechanism needed to explain the presence and evolution of powerful AGN in clusters, a scenario consistent with the far-IR and optical properties of the host galaxies studied here, all of which show some evidence for past interactions. However, the cooling flow model cannot be ruled out for at least some objects, and it is likely that both processes are at work in creating and fueling powerful AGN in clusters. Each scenario makes testable predictions for future X-ray and optical observations which can test the relative importance of each process. (C) 1997 American Astronomical Society.
