Gravitational-Wave Signatures and Periodic Orbits of Charged Particles Around a Five-Dimensional Rotating Black Hole: Comparison with the Kerr Black Hole

Abstract

We study periodic orbits of charged particles in the equatorial plane of a five-dimensional ($D=5$) rotating Myers--Perry black hole (BH) immersed in a uniform magnetic field, and the gravitational waves emitted by these orbits in an extreme mass-ratio inspiral. All results are compared with the Kerr BH in the uniform magnetic field. We find that a charged particle is much more weakly bound to the $D=5$ dimensional BH, i.e. its energy at the innermost stable circular orbit is close to unity, $0.95$--$0.99$, compared with $0.78$--$0.94$ for the Kerr BH, and the depth of the potential well is several times smaller. As a result, the periodic orbits of the $D=5$ dimensional BH lie much closer to the separatrix. The magnetic field can make the azimuthal frequency of the orbit smaller than the radial one and produces loops in the orbits far from the BH. Using the numerical kludge method, we compute the gravitational waveforms for a compact object of $10M_\odot$ orbiting a BH of $10^6M_\odot$. At the same magnetic field, the $D=5$ dimensional signal has a longer period, lower frequencies, and an amplitude smaller by about $18\%$. We show that for the magnetic parameter $βM=0.02$ the strongest spectral line is at $7.9$\,mHz, compared with $9.9$\,mHz for the Kerr BH. An estimate of the radiation reaction shows that the periodic orbits survive from less than an hour to a few days in the presence of an external magnetic field. Over these lifetimes, the signal-to-noise ratio for LISA is $7$--$31$ for the rotating BH in $D=5$ dimensions, compared to the Kerr BH where it is about $15$--$54$.