Superradiant instability of black holes immersed in a magnetic field

Richard Brito, Vitor Cardoso, and Paolo Pani
Phys. Rev. D 89, 104045 – Published 27 May 2014

Abstract

Magnetic fields surrounding spinning black holes can confine radiation and trigger superradiant instabilities. To investigate this effect, we perform the first fully-consistent linear analysis of the Ernst spacetime, an exact solution of the Einstein–Maxwell equations describing a black hole immersed in a uniform magnetic field B. In the limit in which the black-hole mass vanishes, the background reduces to the marginally stable Melvin spacetime. The presence of an event horizon introduces a small dissipative term, resulting in a set of long-lived—or unstable—modes. We provide a simple interpretation of the mode spectrum in terms of a small perfect absorber immersed in a confining box of size 1/B and show that rotation triggers a superradiant instability. By studying scalar perturbations of a magnetized Kerr–Newman black hole, we are able to confirm and quantify the details of this instability. The instability time scale can be orders of magnitude shorter than that associated to massive bosonic fields. The instability extracts angular momentum from the event horizon, competing against accretion. This implies that strong magnetic fields set an upper bound on the black-hole spin. Conversely, observations of highly-spinning massive black holes impose an intrinsic limit to the strength of the surrounding magnetic field. We discuss the astrophysical implications of our results and the limitations of the Ernst spacetime to describe realistic astrophysical configurations.

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  • Received 8 April 2014

DOI:https://doi.org/10.1103/PhysRevD.89.104045

© 2014 American Physical Society

Authors & Affiliations

Richard Brito1,2,*, Vitor Cardoso1,2,3, and Paolo Pani1

  • 1CENTRA, Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais 1, 1049 Lisboa, Portugal
  • 2Perimeter Institute for Theoretical Physics Waterloo, Ontario N2J 2W9, Canada
  • 3Department of Physics and Astronomy, The University of Mississippi, University, Mississippi 38677, USA

  • *richard.brito@tecnico.ulisboa.pt

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Vol. 89, Iss. 10 — 15 May 2014

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