Ergoregions in magnetized black hole spacetimes (Q2841464)

This article considers the spacetimes generated by applying Ernst's procedure to add an external magnetic field to the Kerr-Newman space time of charged rotating black holes. Usually it is assumed that they are asymptotic to the Melvin metric. This paper shows that this is not always the case. When the charge, angular momentum and magnetic field satisfy certain conditions, there exists an ergoregion that extends all the way from the black hole to infinity. Also, an ambiguity in the choice of asymptotically timelike Killing field can occur. For certain choices the metric is asymptotically static, with an ergoregion confined close to the horizon. With other choices, for sufficiently large magnetic field there is no ergoregion at all, and for smaller values a toroidal ergoregion develops.