Splitting the third hair: constraints on Kerr-Newman black holes from merger-ringdown gravitational-wave observations

Gravitational waves are providing increasingly stringent constraints on the properties of black holes. Dynamical processes involving such compact objects also represent engines of discovery for new particles and modifications of the Einstein-Hilbert action. In this regard, we discuss a search for a U(1) charge in the post-merger signal of binary black hole coalescences observed by LIGO and Virgo. Besides a “standard” electric charge, this "third hair” can be also identified with a hidden electromagnetic charge in models of minicharged dark matter, the vector charge in theories mediated by a gravitational vector field, or a topologically induced charge. We model the remnant object sourcing the post-merger signal as a Kerr-Newman black hole, and calculate the corresponding dominant quasi-normal mode frequencies for arbitrary values of charge and angular momentum. Then, we construct a template to describe the ringdown of a charged remnant following a binary black hole coalescence. By applying this model to LIGO-Virgo detections, we find no evidence for or against the Kerr-Newman hypothesis, and obtain upper bounds on the maximum amount of charge present in these mergers. Finally, we outline the prospects to detect black hole charge with LIGO and Virgo at their design sensitivity.