Sub-Solar Mass Intermediate Mass Ratio Inspirals: Waveform Systematics and Detection Prospects with Gravitational Waves

TL;DR

This study evaluates the detectability and waveform modeling accuracy of sub-solar mass intermediate mass-ratio inspirals for gravitational wave detectors, highlighting the importance of higher-order modes and the need for improved waveform models.

Contribution

It assesses the performance of phenomenological waveform models against perturbation theory surrogates in high mass-ratio regimes, revealing significant systematic uncertainties and biases.

Findings

Higher-order modes are essential for accurate detection.

Current models show low match and fitting factors, indicating modeling inaccuracies.

Systematic biases in parameter estimation can exceed statistical errors.

Abstract

We investigate the detectability and waveform systematics of sub-solar mass intermediate mass-ratio inspirals (SSM-IMRIs), characterized by mass ratios $q \sim 10^2-10^4$. Using the black hole perturbation theory surrogate model \textsc{BHPTNRSur1dq1e4} as a reference, we assess the performance of the \textsc{IMRPhenomX} phenomenological family in the high-mass-ratio regime. We find that the inclusion of higher-order gravitational wave modes is critical; their exclusion may degrade the signal-to-noise ratio by factors of $\sim3-5$ relative to quadrupole-only templates. With optimal mode inclusion, SSM-IMRIs are observable out to luminosity distances of $\sim575$ Mpc ($z\sim0.12$) with Advanced LIGO and $\sim10.5$ Gpc ($z\sim1.4$) with the Einstein Telescope. However, we identify substantial systematic uncertainties in current phenomenological approximants. Matches between \textsc{IMRPhenomX} and the reference surrogate model \textsc{BHPTNRSur1dq1e4} degrade to values as low as 0.2 for edge-on inclinations, and fitting factors consistently fall below 0.9, indicating a significant loss of effectualness in template-bank searches. Bayesian parameter estimation reveals that these modeling discrepancies induce systematic biases that exceed statistical errors by multiple standard deviations, underscoring the necessity for waveform models calibrated to perturbation theory in the intermediate mass-ratio regime for robust detection and inference.