Magnetic Reconnection as a Mechanism for Energy Extraction from Rotating Black Holes

TL;DR

This paper analytically demonstrates that magnetic reconnection in the ergosphere of high-spin black holes can extract energy efficiently, leading to black hole spindown and bursty electromagnetic emissions.

Contribution

It introduces a novel analytical model showing how magnetic reconnection enables energy extraction from rotating black holes in specific plasma conditions.

Findings

Energy extraction is possible for high black hole spin and strong magnetization.

Maximum power extracted scales with plasma magnetization and black hole parameters.

The process can cause significant black hole spindown and produces bursty emissions.

Abstract

Spinning black holes store rotational energy that can be extracted. When a black hole is immersed in an externally supplied magnetic field, reconnection of magnetic field lines within the ergosphere can generate negative energy (relative to infinity) particles that fall into the black hole event horizon while other particles escape stealing energy from the black hole. We show analytically that energy extraction via magnetic reconnection is possible when the black hole spin is high (dimensionless spin $a\sim1$) and the plasma is strongly magnetized (plasma magnetization $\sigma_0>1/3$). The parameter space region where energy extraction is allowed depends on the plasma magnetization and the orientation of the reconnecting magnetic field lines. For $\sigma_0 \gg 1$, the asymptotic negative energy at infinity per enthalpy of the decelerated plasma that is swallowed by a maximally rotating black hole is found to be $\epsilon^\infty_- \simeq - \sqrt{\sigma_0/3}$. The accelerated plasma that escapes to infinity and takes away black hole energy asymptotes the energy at infinity per enthalpy $\epsilon^\infty_+ \simeq \sqrt{3\sigma_0}$. We show that the maximum power extracted from the black hole by the escaping plasma is $P_{\rm extr}^{\rm max} \sim 0.1 M^2\sqrt{\sigma_0}\,w_0$ (here, $M$ is the black hole mass and $w_0$ is the plasma enthalpy density) for the collisionless plasma regime and one order of magnitude lower for the collisional regime. Energy extraction causes a significant spindown of the black hole when $a \sim 1$. The maximum efficiency of the plasma energization process via magnetic reconnection in the ergosphere is found to be $\eta_{\rm max} \simeq 3/2$. Since fast magnetic reconnection in the ergosphere should occur intermittently in the scenario proposed here, the associated emission within a few gravitational radii from the black hole is expected to display a bursty nature.