Distinguishing compact objects in extreme-mass-ratio inspirals by gravitational waves

TL;DR

This study investigates how gravitational waves from extreme-mass-ratio inspirals can reveal the spin and quadrupole moments of compact objects, enabling differentiation between black holes, neutron stars, and white dwarfs.

Contribution

The paper introduces simulations of EMRI gravitational wave signals that include the effects of spin and mass quadrupole moments of the compact objects, improving parameter estimation accuracy.

Findings

Spin, tidal quadrupole, and spin quadrupole can be measured with high precision.

Detectable waveform deviations caused by spin across all object types.

Tidal quadrupoles are significant mainly for white dwarfs near intermediate mass ratios.

Abstract

Extreme-mass-ratio inspirals (EMRIs) are promising gravitational-wave (GW) sources for space-based GW detectors. EMRI signals typically have long durations, ranging from several months to several years, necessitating highly accurate GW signal templates for detection. In most waveform models, compact objects in EMRIs are treated as test particles without accounting for their spin, mass quadrupole, or tidal deformation. In this study, we simulate GW signals from EMRIs by incorporating the spin and mass quadrupole moments of the compact objects. We evaluate the accuracy of parameter estimation for these simulated waveforms using the Fisher Information Matrix (FIM) and find that the spin, tidal-induced quadruple, and spin-induced quadruple can all be measured with precision ranging from $10^{-2}$ to $10^{-1}$, particularly for a mass ratio of $\sim$$10^{-4}$. Assuming the ``true'' GW signals originate from an extended body inspiraling into a supermassive black hole, we compute the signal-to-noise ratio (SNR) and Bayes factors between a test-particle waveform template and our model, which includes the spin and quadrupole of the compact object. Our results show that the spin of compact objects can produce detectable deviations in the waveforms across all object types, while tidal-induced quadrupoles are only significant for white dwarfs, especially in cases approaching an intermediate-mass ratio. Spin-induced quadrupoles, however, have negligible effects on the waveforms. Therefore, our findings suggest that it is possible to distinguish primordial black holes from white dwarfs, and, under certain conditions, neutron stars can also be differentiated from primordial black holes.