# Retrieving the True Masses of Gravitational-wave Sources

## TL;DR

This paper investigates how gas-rich environments affect gravitational wave signals from binary black holes, revealing that neglecting gas effects can lead to overestimating their masses, especially for space-based detectors like LISA.

## Contribution

It introduces a model for hydrodynamic drag effects on BBH waveforms and demonstrates their significance for mass measurements in space-based gravitational wave observations.

## Key findings

- Gas effects make BBH waveforms resemble more massive vacuum BBHs.
- Neglecting gas effects can cause overestimation of chirp mass in LISA observations.
- Gas effects are negligible for LIGO/Virgo binary detections.

## Abstract

Gravitational waves (GWs) encode important information about the mass of the source. For binary black holes (BBHs), the templates that are used to retrieve the masses normally are developed under the assumption of a vacuum environment. However, theories suggest that some BBHs form in gas-rich environments. Here we study the effect of hydrodynamic drag on the chirp signal of a stellar-mass BBH and the impact on the measurement of the mass. Based on theoretical arguments, we show that the waveform of a BBH in gas resembles that of a more massive BBH residing in a vacuum. The effect is important for LISA sources but negligible for LIGO/Virgo binaries. Furthermore, we carry out a matched-filtering search of the best fitting parameters. We find that the best-fit chirp mass could be significantly greater than the real mass if the gas effect is not appropriately accounted for. Our results have important implications for the future joint observation of BBHs using both ground- and space-based detectors.