Extreme-mass-ratio inspirals into rotating boson stars: nonintegrability, chaos, and transient resonances

Abstract: General relativity predicts that black holes are described by the Kerr metric, which has integrable geodesics. This property is crucial to produce accurate waveforms from extreme-mass-ratio inspirals. Astrophysical environments, modifications of gravity and new fundamental fields may lead to nonintegrable geodesics, inducing chaotic effects. We study geodesics around self-interacting rotating boson stars and find robust evidence of nonintegrability and chaos. We identify islands of stability around resonant orbits, where the orbital radial and polar oscillation frequency ratios, known as rotation numbers, remain constant throughout the island. These islands are generically present both in the exterior and the interior of compact boson stars. A monotonicity change of rotation curves takes place as orbits travel from the exterior to the interior of the star. Therefore, configurations with neutron-star-like compactness can support degenerate resonant islands. This anomaly is reported here for the first time and it is not present in black holes. Such configurations can also support extremely prolonged resonant islands that span from the exterior to the interior of the star and are shielded by thick chaotic layers. We adiabatically evolve inspirals using approximated post-Newtonian fluxes and find time-dependent plateaus in the rotation curves which are associated with island-crossing orbits. Crossings of external islands give rise to typical gravitational-wave glitches found in non-Kerr objects. Furthermore, when an inspiral is traversing an internal island that is surrounded by a thick chaotic layer, a new type of simultaneous multifrequency glitch occurs that may be detectable with space interferometers such as LISA, and can serve as evidence of an extreme-mass-ratio inspiral around a supermassive boson star.