Magnetic reconnection as a mechanism for energy extraction from rotating black holes

Luca Comisso and Felipe A. Asenjo
Phys. Rev. D 103, 023014 – Published 13 January 2021

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 the other accelerated 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 a1) and the plasma is strongly magnetized (plasma magnetization σ0>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 σ01, 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 εσ0/3. The accelerated plasma that escapes to infinity and takes away black hole energy asymptotes the energy at infinity per enthalpy ε+3σ0. We show that the maximum power extracted from the black hole by the escaping plasma is Pextrmax0.1M2σ0w0 (here, M is the black hole mass and w0 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 a1. The maximum efficiency of the plasma energization process via magnetic reconnection in the ergosphere is found to be ηmax3/2. 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.

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  • Received 8 November 2020
  • Accepted 17 December 2020

DOI:https://doi.org/10.1103/PhysRevD.103.023014

© 2021 American Physical Society

Physics Subject Headings (PhySH)

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Luca Comisso1,* and Felipe A. Asenjo2,†

  • 1Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA
  • 2Facultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Santiago 7941169, Chile

  • *luca.comisso@columbia.edu
  • felipe.asenjo@uai.cl

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Vol. 103, Iss. 2 — 15 January 2021

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