The Eccentric and Accelerating Stellar Binary Black Hole Mergers in Galactic Nuclei: Observing in Ground and Space Gravitational Wave Observatories

TL;DR

This paper investigates the properties and detectability of stellar binary black hole mergers in galactic nuclei, highlighting their high eccentricities and accelerations, and discusses their implications for current and future gravitational wave observatories.

Contribution

It introduces a numerical method to study BBHs in galactic nuclei, revealing their high eccentricities, accelerations, and unique gravitational wave signatures, enhancing understanding of their observability.

Findings

Up to 10^5 BBH mergers detectable in 5 years with advanced observatories.

A significant fraction of BBHs exhibit high eccentricities, especially in space-based detectors.

Detectable Doppler phase drifts in BBHs near massive black holes due to accelerations.

Abstract

We study the stellar binary black holes (BBHs) inspiralling/merging in galactic nuclei based on our numerical method GNC. We find that $3-40\%$ of all new born BBHs will finally merge due to various dynamical effects. In a five year's mission, up to $10^4$, $10^5$, $\sim100$ of BBHs inspiralling/merging in galactic nuclei can be detected with SNR$>8$ in aLIGO, Einstein/DECIGO, TianQin/LISA/TaiJi, respectively. About tens are detectable in both LISA/TaiJi/TianQin and aLIGO. These BBHs have two unique characteristics: (1) Significant eccentricities. $1-3\%$, $2-7\%$, or $30-90\%$ of them is with $e_i>0.1$ when they enter into aLIGO, Einstein, or space observatories, respectively. Such high eccentricities provide a possible explanation for that of GW 190521. Most highly-eccentric BBHs are not detectable in LISA/Tianqin/TaiJi before entering into aLIGO/Einstein as their strain become significant only at $f_{\rm GW}\gtrsim0.1$ Hz. DECIGO become an ideal observatory to detect those events as it can fully cover the rising phase. (2) Up to $2\%$ of BBHs can inspiral/merge at distances $\lesssim10^3 r_{\rm SW}$ from the massive black hole (MBH), with significant accelerations, such that the Doppler phase drift of $\sim10-10^5$ of them can be detectable with SNR$>8$ in space observatories. The energy density of the gravitational wave backgrounds (GWB) contributed by these BBHs deviate from the powerlaw slope of $2/3$ at $f_{\rm GW}\lesssim 1$mHz. The high eccentricity, significant accelerations and different profile of GWB of these sources make them distinguishable, thus interesting for future GW detections and tests of relativities.