Ringdown of a dynamical spacetime

The gravitational waves emitted (some time) after two black holes merge are well described by the theory of linear perturbations on a spacetime characterized by the mass and spin of the remnant. However, in the very early stages right after merger, both the mass and spin are changing. In this work we explore, in a setup based on Vaidya's spacetime, the dynamical consequences of a change of mass in the spacetime due to the accretion of null matter (for example, gravitational waves). We show that accretion imprints time-dependent frequencies and amplitude to a ringdown waveform, and we show how to model accurately this effect in certain regimes. We also comment on the direct emission of gravitational waves due to perturbations in the infalling matter, which is of relevance for black holes embedded in astrophysical environments.