A selection effect boosting the contribution from rapidly spinning black holes to the Cosmic X-ray Background

TL;DR

This paper reveals that rapidly spinning black holes are over-represented in the Cosmic X-ray Background due to their higher radiative efficiency, significantly impacting our understanding of AGN contributions and survey interpretations.

Contribution

It demonstrates how black hole spin biases influence the CXB and flux-limited surveys, highlighting the importance of spin distribution in X-ray background modeling.

Findings

Rapidly spinning black holes can produce 50% of the CXB with only 15% of AGN.

A small population of maximally-spinning black holes can explain the CXB peak.

Hard X-ray reflection enhances detectability of high-spin black holes.

Abstract

The Cosmic X-ray Background (CXB) is the total emission from past accretion activity onto supermassive black holes in active galactic nuclei (AGN) and peaks in the hard X-ray band (30 keV). In this paper, we identify a significant selection effect operating on the CXB and flux-limited AGN surveys, and outline how they must depend heavily on the spin distribution of black holes. We show that, due to the higher radiative efficiency of rapidly-spinning black holes, they will be over-represented in the X-ray background, and therefore could be a dominant contributor to the CXB. Using a simple bimodal spin distribution, we demonstrate that only 15 per cent maximally-spinning AGN can produce 50 per cent of the CXB. We also illustrate that invoking a small population of maximally-spinning black holes in CXB synthesis models can reproduce the CXB peak without requiring large numbers of Compton-thick AGN. The spin bias is even more pronounced for flux-limited surveys: 7 per cent of sources with maximally-spinning black holes can produce half of the source counts. The detectability for maximum spin black holes can be further boosted in hard (>10 keV) X-rays by up to ~60 per cent due to pronounced ionised reflection, reducing the percentage of maximally spinning black holes required to produce half of the CXB or survey number counts further. A host of observations are consistent with an over-representation of high-spin black holes. Future NuSTAR and ASTRO-H hard X-ray surveys will provide the best constraints on the role of spin within the AGN population.