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

Understand

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.