Ring formation from black hole superradiance through repeated particle production on bound orbits

TL;DR

This paper proposes a novel mechanism where ultralight bosonic fields around rotating black holes form dense rings of particles through repeated resonant production, affecting gravitational-wave signals.

Contribution

It introduces a new process of bound particle formation via parametric resonance in black hole superradiance, with potential observational implications.

Findings

Dense rings of particles can form around black holes through repeated resonance.

The process influences the evolution of scalar clouds and gravitational-wave signatures.

Particle mass ratios determine the density and structure of the rings.

Abstract

Ultralight bosonic fields around a rotating black hole can extract energy and angular momentum through the superradiant instability and form a dense cloud. We investigate the scenario involving two scalar fields, $\phi$ and $\chi$, with a coupling term $\frac{1}{2}\lambda\phi\chi^2$, which is motivated by the multiple-axion framework. The ultralight scalar $\phi$ forms a cloud that efficiently produces $\chi$ particles nonperturbatively via parametric resonance, with a large mass hierarchy, $\mu_\chi \gg \mu_\phi$. Rather than escaping the system as investigated by previous studies, these $\chi$ particles remain bound, orbiting the black hole. Moreover, the particle production occurs primarily at the peak of the cloud's profile, allowing $\chi$ particles in quasicircular orbits to pass repeatedly through resonant regions, leading to cumulative amplification. This selective process naturally forms a dense ring of $\chi$ particles, with a mass ratio to the cloud fixed by $(\mu_\phi/\mu_\chi)^2$. Our findings reveal a novel mechanism for generating bound-state particles via parametric resonance, which also impacts the evolution of the cloud. This process can be probed through its imprint on binary dynamics and its gravitational-wave signatures.