Floating orbits and energy extraction from magnetized Kerr black holes

We study the electromagnetic radiation reaction on a charged particle around a weakly magnetized Kerr black hole by numerically solving the Teukolsky equation to find the energy fluxes in electromagnetic radiation at the horizon and at spatial infinity. We also employ analytical methods in the low-frequency limit, finding excellent agreement with the numerical results. For a wide range of parameters, the energy fluxes on the horizon are negative for all orbital radii: the modes are amplified through superradiance. More interestingly, the flux on the horizon is larger (in absolute value) than the flux at infinity for orbits with orbital radius ${r}_{0}\ensuremath{\gtrsim}9M$---``floating orbits'' are a generic outcome of having magnetic fields around black holes. Particles on these orbits extract energy from the black hole through their radiation field; the net effect is an increase in the particles' kinetic energy so that they outspiral to infinity. Although in realistic accretion disks viscous forces are expected to dominate, floating orbits remain an interesting feature of the existence of ergoregions in black hole spacetimes.