Observational evidence for quadratic quasi-normal modes in binary black hole merger events

At sufficiently late times in the post-merger regime in a binary black hole merger, the gravitational wave signal is dominated by a superposition of back hole quasi-normal modes. These modes are associated with the remnant black hole and athe mode frequencies and damping times are determined by its mass and spin in the framework of linear perturbation theory. This framework is cosistent with observations: analyses of the postmerger regime of several merger events detecten by the LIGO and Virgo detectors have found clear observational evidence for multiple quasi-normal modes.

Closer to the merger however, one might expect non-linear effects to play a role. Indeed, analysis of numerical simulations have provided evidence that quadratic quasi-normal modes in the emitted gravitational wave signal, as well as in the infalling radiation at the black hole horizon. In this talk we will discuss observational evidence for quadratic quasi-normal modes in massive binary merger events and the corresponding implications for black hole spectroscopy and other tests of general relativity.