A Monopole Near a Black Hole

Claudio Bunster, Marc Henneaux

Published 2007-03-16, updated 2007-07-14Version 2

We study an electric charge held at rest outside a magnetically charged black hole. We find that even if the electric charge is treated as a perturbation on a spherically symmetric magnetic Reissner-Nordstrom hole, the geometry at large distances is that of a magnetic Kerr-Newman black hole. When the charge approaches the horizon and crosses it, the exterior geometry becomes that of a Kerr-Newman hole with electric and magnetic charges and with total angular momentum given by the standard value for a charged monopole pair. Thus, in accordance with the "no-hair theorem", once the charge is captured by the black hole, the angular momentum associated with the charge monopole system, looses all traces of its exotic origin and it is perceived from the outside as common rotation. It is argued that a similar analysis performed on Taub-NUT space should give the same result, namely, if one holds an ordinary mass outside of the horizon of a Taub-NUT space with only magnetic mass, the system, as seen from large distances, is endowed with an angular momentum proportional to the product of the two kinds of masses. When the ordinary, electric, mass reaches the horizon, the exterior metric becomes that of a rotating Taub-NUT space.