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

We study the stellar binary black holes (BBHs) inspiraling/merging in galactic nuclei based on our numerical method GNC. We find that 3%-40% of all newborn BBHs will finally merge due to various dynamical effects. In a five-year mission, up to 10(4), 10(5), and similar to 100 of BBHs inspiraling/merging in galactic nuclei can be detected with signal-to-noise ration >8 in Advanced LIGO (aLIGO), Einstein/DECIGO, and TianQin/LISA/TaiJi, respectively. Roughly 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 have e ( i ) > 0.1 when they enter into aLIGO, Einstein, or space observatories, respectively. Such high eccentricities provide a possible explanation for that of GW190521. Most highly eccentric BBHs are not detectable in LISA/Tianqin/TaiJi before entering into aLIGO/Einstein, as their strain becomes significant only at f (GW) greater than or similar to 0.1 Hz. DECIGO becomes 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 less than or similar to 10(3) r (SW) from the massive black hole, with significant accelerations, such that the Doppler phase drift of similar to 10-10(5) of them can be detected with signal-to-noise ratio >8 in space observatories. The energy density of the gravitational-wave backgrounds (GWBs) contributed by these BBHs deviates from the power-law slope of 2/3 at f (GW) less than or similar to 1 mHz. The high eccentricity, significant accelerations, and the different profile of the GWB of these sources make them distinguishable, and thus interesting for future gravitational-wave detections and tests of relativities.

Automated summary

Research suggests that 3%-40% of stellar binary black holes in galactic nuclei will eventually merge, and space observatories can detect thousands to hundreds of these merging events. The unique characteristics of these black holes, such as high eccentricities and significant accelerations, make them interesting subjects for future gravitational-wave detections and tests of relativities.