Imprints of accretion on gravitational waves from black holes

Abstract

We explore the subtleties that may emerge if mass accretion events increase significantly the mass of a black hole horizon during its gravitational wave (GW) emission. We find that horizon growth extends the phenomenology of black hole ring-down in a well defined manner. The resulting GW signals are best described as amplitude and frequency (AM-FM) modulated versions of the simple damped sinusoidal wave forms. Any appreciable increase in the horizon mass during emission reflects on the instantaneous signal frequency, which shows a prominent negative branch in the $\mathrm{f\ifmmode \dot{}\else \.{}\fi{}}(f)$ evolution diagram. The features of the frequency evolution pattern reveal key properties of the accretion event, such as the total accreted mass and the accretion rate. For slow accretion rates the frequency evolution follows verbatim the accretion rate, as expected from dimensional arguments.