Parameterizing black hole orbits for adiabatic inspiral

Abstract

Adiabatic binary inspiral in the small mass ratio limit treats the small body as moving along a geodesic of a large Kerr black hole, with the geodesic slowly evolving due to radiative backreaction. Up to initial conditions, geodesics are typically parameterized in two ways: using the integrals of motion energy $E$, axial angular momentum $L_z$, and Carter constant $Q$; or, using orbit geometry parameters semi-latus rectum $p$, eccentricity $e$, and (cosine of ) inclination $x_I \equiv \cos I$. The community has long known how to compute orbit integrals as functions of the orbit geometry parameters, i.e., as functions expressing $E(p, e, x_I)$, and likewise for $L_z$ and $Q$. Mappings in the other direction -- functions $p(E, L_z, Q)$, and likewise for $e$ and $x_I$ -- have not yet been developed in general. In this note, we develop generic mappings from ($E$, $L_z$, $Q$) to ($p$, $e$, $x_I$). The mappings are particularly simple for equatorial orbits ($Q = 0$, $x_I = \pm1$), and can be evaluated efficiently for generic cases. These results make it possible to more accurately compute adiabatic inspirals by eliminating the need to use a Jacobian which becomes singular as inspiral approaches the last stable orbit.