2002ApJ...565...86F

We present the high-resolution X-ray spectrum of the low-redshift quasar H1821+643 and its surrounding hot cluster observed with the Chandra High-Energy Transmission Grating Spectrometer. An iron emission line attributed to the quasar at ∼6.43 keV (rest frame) is clearly resolved, with an equivalent width of ∼100 eV. Although we cannot rule out contributions to the line from a putative torus, the DISKLINE model provides an acceptable fit to this iron line. We also detect a weak emission feature at ∼6.9 keV (rest frame). We suggest that both lines could originate in an accretion disk composed of a highly ionized optically thin atmosphere sitting atop a mostly neutral disk. We search for absorption features from a warm/hot component of the intergalactic medium (IGM) along the ∼1.5h^–1 Gpc line of sight to the quasar. No absorption features are detected at or above the 3 σ level, while a total of six O VI intervening absorption systems have been detected with the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer. Based on the lack of O VII and O VIII absorption lines and by assuming collisionally ionization, we constrain the gas temperature of a typical O VI absorber to 10⁵K<T<10⁶ K, which is consistent with the results from hydrodynamic simulations of the IGM. The zeroth-order image reveals the extended emission from the surrounding cluster. We have been able to separate the moderate CCD X-ray spectrum of the surrounding cluster from the central quasar and find that this is a hot cluster with a temperature of ∼10 keV and a metal abundance of ∼0.3 Z_☉. We also independently obtain the redshift of the cluster, which is consistent with the optical results. We estimate that the cluster makes negligible contributions to the 6.9 keV iron K-line flux.