Evidence for a moderate spin from X-ray reflection of the high-mass supermassive black hole in the cluster-hosted quasar H1821+643

TL;DR

This study analyzes deep X-ray spectra of the quasar H1821+643 to measure the black hole's spin, finding a moderate spin value that suggests growth through chaotic accretion or mergers, and provides the most constrained spin for such a massive black hole.

Contribution

First measurement of black hole spin in a supermassive black hole with mass over 10^9 solar masses using high-resolution X-ray reflection spectroscopy.

Findings

Black hole spin is modest, with a credible range of 0.62^{+0.22}_{-0.37}.

Black hole mass estimated between 10^{9.2} and 10^{10.5} solar masses.

Supports theories of black hole growth via incoherent accretion or mergers.

Abstract

We present an analysis of deep $Chandra$ Low-Energy and High-Energy Transmission Grating archival observations of the extraordinarily luminous radio-quiet quasar H1821+643, hosted by a rich and massive cool-core cluster at redshift $z=0.3$. These datasets provide high-resolution spectra of the AGN at two epochs, free from contamination by the intracluster medium and from the effects of photon pile-up, providing a sensitive probe of the iron-$K$ band. At both epochs, the spectrum is well described by a power-law continuum plus X-ray reflection from both the inner accretion disc and cold, slowly-moving distant matter. Adopting this framework, we proceed to examine the properties of the inner disc and the black hole spin. Using Markov chain Monte Carlo (MCMC) methods, we combine constraints from the two epochs assuming that the black hole spin, inner disc inclination, and inner disc iron abundance are invariant. The black hole spin is found to be modest, with a 90$\%$ credible range of ${a}^{*}=0.62^{+0.22}_{-0.37}$; and, with a mass $M_\mathrm{BH}$ in the range $\log (M_\mathrm{BH}/M_\odot)\sim 9.2-10.5$, this is the most massive black hole candidate for which a well-defined spin constraint has yet been obtained. The modest spin of this black hole supports previous suggestions that the most massive black holes may grow via incoherent or chaotic accretion and/or SMBH-SMBH mergers.