Inspiral into Gargantua
  We model the inspiral of a compact object into a more massive black hole rotating very near the theoretical maximum. We find that once the body enters the near-horizon regime the gravitational radiation is characterized by a constant frequency, equal to (twice) the horizon frequency, with an exponentially damped profile. This contrasts with the usual "chirping" behavior and, if detected, would constitute a "smoking gun" for a near-extremal black hole in nature. 
Samuel E. Gralla, Scott A. Hughes, Niels Warburton
gr-qc astro-ph.HE hep-th