Black hole energy extraction via stationary scalar clouds

TL;DR

This paper investigates stationary scalar field configurations around Kerr black holes, demonstrating their potential to mimic energy extraction mechanisms similar to the Blandford-Znajek process through superradiance.

Contribution

It introduces a new model of scalar clouds with semi-permeable boundaries that can extract energy from rotating black holes, paralleling electromagnetic energy extraction.

Findings

Scalar clouds can be sustained with semi-permeable boundaries allowing energy radiation.

The energy flux behavior matches predictions from magnetohydrodynamic simulations of the BZ mechanism.

Stationary scalar configurations can serve as proxies for force-free magnetospheres in energy extraction studies.

Abstract

We study scalar field configurations around Kerr black holes with a time-independent energy-momentum tensor. These stationary `scalar clouds', confined near the black hole (BH) by their own mass or a mirror at fixed radius, exist at the threshold for energy extraction via superradiance. Motivated by the electromagnetic Blandford-Znajek (BZ) mechanism, we explore whether scalar clouds could serve as a proxy for the force-free magnetosphere in the BZ process. We find that a stationary energy-extracting scalar cloud solution exists when the reflecting mirror is replaced by a semi-permeable surface which allows the cloud to radiate some energy to infinity while maintaining self-sustained superradiance. The radial energy flux displays the same behaviour for rapidly rotating holes as magnetohydrodynamic simulations predict for the BZ mechanism.