Astrophysical gravitational-wave echoes from galactic nuclei
Abstract:
Galactic nuclei (GNs) are dense stellar environments abundant in gravitational-wave (GW) sources for the Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo, and Kamioka Gravitational Wave Detector (KAGRA). The GWs may be generated by stellar-mass black hole (BH) or neutron star mergers following gravitational bremsstrahlung, dynamical scattering encounters, Kozai–Lidov-type oscillations driven by the central supermassive black hole (SMBH), or gas-assisted mergers if present. In this paper, we examine a smoking gun signature to identify sources in GNs: the GWs scattered by the central SMBH. This produces a secondary signal, an astrophysical GW echo, which has a very similar time–frequency evolution as the primary signal but arrives after a time delay. We determine the amplitude and time-delay distribution of the GW echo as a function of source distance from the SMBH. Between ∼10 per cent and 90 per cent of the detectable echoes arrive within ∼(1--100)M6s after the primary GW for sources between 10 and 104 Schwarzschild radius, where M6=MSMBH,z/ (106M⊙), and MSMBH, z is the observer-frame SMBH mass. The echo arrival times are systematically longer for high signal-to-noise ratio (SNR) primary GWs, where the GW echo rays are scattered at large deflection angles. In particular, ∼10 per cent--90 per cent of the distribution is shifted to ∼(5--1800)M6s for sources, where the lower limit of echo detection is 0.02 of the primary signal amplitude. We find that ∼5 per cent--30 per cent(∼1 per cent--7 per cent) of GW sources have an echo amplitude larger than 0.2–0.05 times the amplitude of the primary signal if the source distance from the SMBH is 50 (200) Schwarzschild radius. Non-detections can rule out that a GW source is near an SMBH.
A numerical study of stellar discs in galactic nuclei
Merger rates of intermediate-mass black hole binaries in nuclear star clusters
Abstract:
Repeated mergers of stellar-mass black holes in dense star clusters can produce intermediate-mass black holes (IMBHs). In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the black hole (BH) merger products, in spite of the significant recoil kicks due to anisotropic emission of gravitational radiation. These events can be detected in gravitational waves, which represent an unprecedented opportunity to reveal IMBHs. In this paper, we analyze the statistical results of a wide range of numerical simulations, which encompass different cluster metallicities, initial BH seed masses, and initial BH spins, and we compute the merger rate of IMBH binaries. We find that merger rates are in the range 0.01–10 Gpc−3 yr−1 depending on IMBH masses. We also compute the number of multiband detections in ground-based and space-based observatories. Our model predicts that a few merger events per year should be detectable with LISA, DECIGO, Einstein Telescope (ET), and LIGO for IMBHs with masses ≲1000 M⊙, and a few tens of merger events per year with DECIGO, ET, and LIGO only.
Merger rates of intermediate-mass black hole binaries in nuclear star clusters
Repeated mergers, mass-gap black holes, and formation of intermediate-mass black holes in dense massive star clusters
Abstract:
Current theoretical models predict a mass gap with a dearth of stellar black holes (BHs) between roughly 50 M⊙ and 100 M⊙, while above the range accessible through massive star evolution, intermediate-mass BHs (IMBHs) still remain elusive. Repeated mergers of binary BHs, detectable via gravitational-wave emission with the current LIGO/Virgo/Kagra interferometers and future detectors such as LISA or the Einstein Telescope, can form both mass-gap BHs and IMBHs. Here we explore the possibility that mass-gap BHs and IMBHs are born as a result of successive BH mergers in dense star clusters. In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the BH merger products after they receive significant recoil kicks due to anisotropic emission of gravitational radiation. Using for the first time simulations that include full stellar evolution, we show that a massive stellar BH seed can easily grow to ∼103–104 M⊙ as a result of repeated mergers with other smaller BHs. We find that lowering the cluster metallicity leads to larger final BH masses. We also show that the growing BH spin tends to decrease in magnitude with the number of mergers so that a negative correlation exists between the final mass and spin of the resulting IMBHs. Assumptions about the birth spins of stellar BHs affect our results significantly, with low birth spins leading to the production of a larger population of massive BHs.