Post-Newtonian expansions of extreme mass ratio inspirals of spinning bodies into Schwarzschild black holes

TL;DR

This paper develops a post-Newtonian expansion method for modeling the inspiral of spinning bodies into Schwarzschild black holes, improving waveform accuracy for gravitational wave detection.

Contribution

It introduces a hybrid inspiral model combining PN expansions with relativistic flux calculations, specifically including spin effects up to 5PN order for eccentric orbits.

Findings

PN approximation is sufficient for low eccentricities.

Hybrid model improves waveform accuracy.

Waveform mismatch analysis validates the approach.

Abstract

Space-based gravitational-wave detectors such as LISA are expected to detect inspirals of stellar-mass compact objects into massive black holes. Modeling such inspirals requires fully relativistic computations to achieve sufficient accuracy at leading order. However, subleading corrections such as the effects of the spin of the inspiraling compact object may potentially be treated in weak-field expansions such as the post-Newtonian (PN) approach. In this work, we calculate the PN expansion of eccentric orbits of spinning bodies around Schwarzschild black holes. Then we use the Teukolsky equation to compute the energy and angular momentum fluxes from these orbits up to the 5PN order. Some of these PN orders are exact in eccentricity, while others are expanded up to the tenth power in eccentricity. Then we use the fluxes to construct a hybrid inspiral model, where the leading part of the fluxes is calculated numerically in the fully relativistic regime, while the linear part in the small spin is analytically approximated using the PN series. We calculate LISA-relevant adiabatic inspirals and respective waveforms with this model and a fully relativistic model. Through the calculation of mismatch between the waveforms from both models we conclude that the PN approximation of the linear-in-spin part of the fluxes is sufficient for lower eccentricities.