Second and higher-order quasi-normal modes in binary black hole mergers

Understand

Black hole (BH) oscillations known as quasi-normal modes (QNMs) are one of the most important gravitational wave (GW) sources.

• We propose that higher perturbative order of QNMs, generated by nonlinear gravitational interaction near the BHs, are detectable and worth searching for in observations and simulations of binary BH mergers.
• We calculate the metric perturbations to second-order and explicitly regularize the master equation at the horizon and spatial infinity.
• We find that the second-order QNMs have frequencies twice the first-order ones and the GW amplitude is up to ~10% that of the first-order one.