Gravitational Wave Phase shifts of black hole mergers in AGN Disks

TL;DR

This paper studies how gravitational wave phase shifts caused by accelerations in AGN disks can help identify the environment of black hole mergers, with implications for current and future GW detectors.

Contribution

It introduces a semi-analytical model to predict GW phase shifts from mergers in AGN disks, highlighting the significance of three-body interactions and gas-hardening effects.

Findings

GW phase shifts are likely larger in AGN environments than other channels.

A significant fraction of mergers exhibit phase shifts > 1 rad at frequencies > 10 Hz.

Some phase shifts could be detectable by current GW detectors like LIGO/Virgo/KAGRA.

Abstract

Ground-based gravitational wave (GW) detectors have discovered about 200 compact object mergers. The astrophysical origins of these events are highly debated, and it is possible that at least a fraction of them originate from dynamical environments. Among these, the disks of active galactic nuclei (AGN) are particularly interesting as promising environments, as some observed properties may be more readily produced there. When compact objects merge in these environments, acceleration from the central supermassive black hole (SMBH) or nearby companions is inevitable. Such acceleration induces a phase shift in the observed GW waveforms, which can serve as a useful tool to distinguish the underlying merging environments for each GW event. In this paper, we investigate the expected distribution of such acceleration-induced GW phase shifts, using a semi-analytical model combined with a one-dimensional AGN population synthesis code. We find significant contributions from three-body interactions involving a nearby third object. Our results indicate that the GW phase shift is likely to be larger compared to other channels, making it distinguishable by future GW facilities such as TianQin, DECIGO, Taiji, Einstein Telescope, and Cosmic Explorer. Interestingly, a notable fraction of mergers in fact exhibit a significant GW phase shift ($\gtrsim~{\rm 1\ rad}$) at frequencies above $10~{\rm Hz}$, which could even be detectable by current GW detectors such as LIGO/Virgo/KAGRA. Additionally, if gas-hardening during three-body interactions is taken into account, the GW frequency can be boosted to $\gtrsim 10~{\rm Hz}$, potentially further aiding in the detection of the phase shift.