Floating orbits and energy extraction from magnetized Kerr black holes

TL;DR

This paper investigates electromagnetic radiation reaction on charged particles around magnetized Kerr black holes, revealing the existence of floating orbits where particles extract energy from the black hole and outspiral, with results supported by numerical and analytical methods.

Contribution

It demonstrates that magnetic fields around Kerr black holes can produce floating orbits through superradiance, a novel insight into energy extraction mechanisms.

Findings

Energy fluxes on the horizon are negative due to superradiance.

Floating orbits occur for orbital radii greater than approximately 9M.

Particles on these orbits gain energy and outspiral to infinity.

Abstract

We study electromagnetic radiation reaction on a charged particle around a weakly magnetized Kerr black hole. We solve numerically the Teukolsky equation to find 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, 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 $r_0 \gtrsim 9M$ -- floating orbits are a generic outcome of having magnetic fields around black holes. Particles on these orbits extract energy from the BH 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.