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

TL;DR

This study analyzes Suzaku X-ray data of the quasar H1821+643, revealing details about its environment, accretion disk, and feeding mechanism from the intracluster medium, with implications for black hole growth.

Contribution

It provides the first detailed spectral separation of the AGN and ICM in this system, identifying reflection features and constraining the black hole's accretion state.

Findings

Detection of X-ray reflection from cold, slow-moving material near the AGN

Sub-solar iron abundance indicating feeding from the ICM

Estimated black hole mass of 3-6 billion solar masses

Abstract

We present a deep Suzaku observation of H1821+643, an extremely rare example of a powerful quasar hosted by the central massive galaxy of a rich cooling-core cluster of galaxies. Informed by previous Chandra studies of the cluster, we achieve a spectral separation of emission from the active galactic nucleus (AGN) and the intracluster medium (ICM). With a high degree of confidence, we identify the signatures of X-ray reflection/reprocessing by cold and slowly moving material in the AGN's immediate environment. The iron abundance of this matter is found to be significantly sub-solar (Z~0.4Zsun), an unusual finding for powerful AGN but in line with the idea that this quasar is feeding from the ICM via a Compton-induced cooling flow. We also find a subtle soft excess that can be described phenomenologically (with an additional black body component) or as ionized X-ray reflection from the inner regions of a high inclination (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 to constrain the Eddington fraction of the source. Applying these arguments to our spectrum implies an Eddington fraction of 0.25-0.5, with an associated black hole mass of 3-6x10^9Msun.