Spin Demographics of Active Supermassive Black Holes: Updated Estimates from X-ray reflection and Future opportunities

TL;DR

This paper updates the compilation of SMBH spin measurements from X-ray reflection, discusses current limitations, and highlights future prospects with next-generation observatories and Bayesian inference methods.

Contribution

It provides an updated heterogeneous SMBH spin dataset, analyzes its limitations, and emphasizes future opportunities with new X-ray observatories and statistical frameworks.

Findings

No clear correlation between Eddington ratio and spin observed.

Current sample limitations hinder testing of SMBH growth models.

Next-generation X-ray observatories will improve spin measurements.

Abstract

Understanding the growth of supermassive black holes (SMBHs) requires observational constraints on how their angular momentum (or spin) varies with mass, since the relative importance of coherent accretion, chaotic accretion, and mergers will be reflected in SMBH spin populations. Here we present an updated compilation of reflection-based SMBH spin measurements from the literature and assemble a set of ancillary quantities of interest for each SMBH (including redshift, Eddington ratio, and X-ray luminosity). No obvious apparent correlation between the Eddington-scaled accretion rate and the black hole spin is seen, noting that formal statistical tests are beyond the scope of this review. We discuss the limitations of using this heterogeneous mass--spin sample to test predictions of SMBH growth from semi-analytic models and cosmological simulations, emphasizing the need for a more uniform sample. We then highlight the encouraging prospects enabled by the next-generation NewAthena X-ray flagship observatory. Finally, we summarize how hierarchical Bayesian population inference applied to observed SMBH mass--spin populations will constitute a powerful framework for confirming tentative mass--spin trends in future samples.