The role of the supermassive black hole spin in the estimation of the EMRI event rate

One of the main channels of interactions in galactic nuclei between stars and the central massive black hole (MBH) is the gradual inspiral of compact remnants into the MBH due to the emission of gravitational radiation. This process is known as an \Extreme Mass Ratio Inspiral" (EMRI). Previous works about the estimation of how many events space observatories such as LISA will be able to observe during its operational time dier in orders of magnitude, due to the complexity of the problem. Nevertheless, a common result to all investigations is that the possibility that a compact object merges with the MBH after only one intense burst of GWs is much more likely than a slow adiabatic inspiral, an EMRI. The later is referred to as a \plunge" because the compact object dives into the MBH, crosses the horizon and is lost as a probe of strong gravity for eLISA. The event rates for plunges are orders of magnitude larger than slow inspirals. On the other hand, nature MBH’s are most likely Kerr and the magnitude of the spin has been sized up to be high. We calculate the number of periapsis passages that a compact object set on to an extremely radial orbit goes through before being actually swallowed by the Kerr MBH and we then translate it into an event rate for a LISA-like observatory, such as the proposed ESA mission eLISA/NGO. We prove that a \plunging" compact object is conceptually indistinguishable from an adiabatic, slow inspiral; plunges spend on average up to hundred of thousands of cycles in the bandwidth of the detector for a two years mission. This has an important impact on the event rate, enhancing in some cases signicantly,