A “Horizon-adapted” Approach to the Study of Relativistic Accretion Flows onto Rotating Black Holes

We present a new geometrical approach to the study of accretion flows onto rotating black holes. Instead of Boyer-Lindquist coordinates, the standard choice in all existing numerical simulations in the literature, we employ the simplest example of a horizon-adapted coordinate system, the Kerr-Schild coordinates. This choice eliminates unphysical divergent behavior at the event horizon. Computations of Bondi-Hoyle accretion onto extreme Kerr black holes, performed here for the first time, demonstrate the key advantages of this procedure. We argue that it offers the best approach to the numerical study of the observationally increasingly, more accessible, relativistic inner region around black holes.