Astrophysical Gravitational-Wave Echoes from Galactic Nuclei

László Gondán; Bence Kocsis

doi:10.1093/mnras/stac1985

paper

Astrophysical Gravitational-Wave Echoes from Galactic Nuclei

arXiv:2110.09540 · doi:10.1093/mnras/stac1985

Abstract

Galactic nuclei (GNs) are dense stellar environments abundant in gravitational-wave (GW) sources for LIGO, VIRGO, and 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 $\sim 10% - 90%$ of the detectable echoes arrive within $\sim (1 - 100) M_{6} sec$ after the primary GW for sources between $10 - 1 0^{4}$ Schwarzschild radius, where $M_{6} = M_{SMBH, z} / (1 0^{6} M_{⊙})$ , and $M_{SMBH, z}$ 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, $\sim 10% - 90%$ of the distribution is shifted to $\sim (5 - 1800) M_{6} sec$ for sources, where the lower limit of echo detection is $0.02$ of the primary signal amplitude. We find that $\sim 5% - 30%$ ( $\sim 1% - 7%$ ) 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.

arXiv comment20 pages, 8 figures, accepted by MNRAS

Astrophysical Gravitational-Wave Echoes from Galactic Nuclei

Abstract

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