Follow-up signals from superradiant instabilities of black hole merger remnants

Superradiant instabilities can trigger the formation of bosonic clouds around rotating black holes. If the bosonic field growth is sufficiently fast, these clouds could form shortly after a binary black hole merger. Such clouds are continuous sources of gravitational waves whose detection (or lack thereof) can probe the existence of ultralight bosons (such as axion-like particles) and their properties. Motivated by the binary black hole mergers seen by Advanced LIGO so far, we investigate in detail the parameter space that can be probed with continuous gravitational wave signals from ultralight scalar field clouds around black hole merger remnants with particular focus on future ground-based detectors (A+, Voyager and Cosmic Explorer). We also study the impact that the confusion noise from a putative stochastic gravitational-wave background from unresolved sources would have on such searches and we estimate, under different astrophysical priors, the number of binary black-hole merger events that could lead to an observable post-merger signal. Under our most optimistic assumptions, Cosmic Explorer could detect dozens of post-merger signals.