Improved gravitational radiation time-scales II: spin-orbit contributions and environmental perturbations

TL;DR

This paper refines gravitational wave inspiral time-scale estimates for highly eccentric and relativistic binaries by including spin-orbit effects, environmental perturbations, and post-Newtonian corrections, crucial for accurate LISA event predictions.

Contribution

It provides analytical fits and expressions that incorporate spin-orbit couplings and environmental effects, improving upon Peters' formula for better accuracy in extreme orbital regimes.

Findings

Analytical corrections for inspiral time-scales including 1.5PN hereditary fluxes and spin-orbit effects.

Quantitative assessment of environmental perturbations like third-body interactions and dynamical friction.

Identification of phase space regions where environmental effects dominate or are comparable to PN corrections.

Abstract

Peters' formula is an analytical estimate of the time-scale of gravitational wave (GW)-induced coalescence of binary systems. It is used in countless applications, where the convenience of a simple formula outweighs the need for precision. However, many promising sources of the Laser Interferometer Space Antenna (LISA), such as supermassive black hole binaries and extreme mass-ratio inspirals (EMRIs), are expected to enter the LISA band with highly eccentric ($e \gtrsim$ 0.9) and highly relativistic orbits. These are exactly the two limits in which Peters' estimate performs the worst. In this work, we expand upon previous results and give simple analytical fits to quantify how the inspiral time-scale is affected by the relative 1.5 post-Newtonian (PN) hereditary fluxes and spin-orbit couplings. We discuss several cases that demand a more accurate GW time-scale. We show how this can have a major influence on quantities that are relevant for LISA event-rate estimates, such as the EMRI critical semi-major axis. We further discuss two types of environmental perturbations that can play a role in the inspiral phase: the gravitational interaction with a third massive body and the energy loss due to dynamical friction and torques from a surrounding gas medium ubiquitous in galactic nuclei. With the aid of PN corrections to the time-scale in vacuum, we find simple analytical expressions for the regions of phase space in which environmental perturbations are of comparable strength to the effects of any particular PN order, being able to qualitatively reproduce the results of much more sophisticated analyses.