EMRIs in Kerr Spacetime: The Role of Inclination Diffusion

Extreme mass ratio inspirals (EMRIs) are expected to be a major source of gravitational waves (GWs) for the future Laser Interferometer Space Antenna (LISA). An EMRI occurs when a compact object, driven by the emission of gravitational radiation, slowly inspirals into a massive black hole (MBH). The classical formation channel involves repeated two-body interactions within dense nuclear star clusters. However, both EMRI event rates and the expected distribution of their orbital parameters remain highly uncertain. A major source of this uncertainty is the poorly constrained ratio between gradually inspiraling EMRIs and direct plunges, which do not produce detectable GWs. While most studies of astrophysical EMRI production focus on the Schwarzschild limit, previous work examining the Kerr metric found that the spin of the central MBH can significantly affect the EMRI-to-plunge ratio. This enhancement depends sensitively on orbital inclination, yet earlier studies held inclination fixed. In this work, we develop the mathematical framework describing inclination evolution driven by two-body encounters evolving along Kerr geodesics. By implementing 3D diffusion into Monte Carlo simulations, we show that initially prograde orbits may evolve into retrograde states, thereby converting potential EMRIs into plunges or vice versa. We estimate the EMRI-to-plunge ratio and find that substantial MBH spin enhances the EMRI rate by a factor of about 2.5. Additionally, even for a non-spinning MBH, modeling the orbits as relativistic geodesics rather than Newtonian trajectories increases the number of EMRIs by a similar factor of about 2.5. The distribution of Kerr EMRI inclinations has a substantial prograde bias that may aid in identifying the subset of loss cone EMRIs within large future EMRI samples.