Astrophysical Gravitational-Wave Echoes from Galactic Nuclei

Abstract

Galactic nuclei (GNs) are dense stellar environments abundant in gravitational-wave (GW) sources for LIGO, VIRGO, and KA-GRA. 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 gasassisted 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% − 90% of the detectable echoes arrive within (1 − 100) 𝑀 6 sec after the primary GW for sources between 10 − 10 4 𝑟 S , where 𝑟 S = 2𝐺 𝑀/𝑐 2 , 𝑀 is the observer-frame SMBH mass, and 𝑀 6 = 𝑀/(10 6 M ). 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% − 90% of the distribution is shifted to (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 5% − 30% (1% − 7%) of GW sources have an echo amplitude larger than 0.2 − 0.05 times the amplitude of primary signal if the source distance from the SMBH is 𝑟 = 50 𝑟 S (200 𝑟 S ). Non-detections can rule out that a GW source is near an SMBH.

Spin Variations of Black Hole Binaries in AGN Disks

“…In addition, the final obliquities show a bimodal distribution centers around 20 • and 40 • respectively, and this can serve as a smoking gun for the spin-orbit resonance. Moreover, GW echos can identify sources situated in the vicinity of a SMBH and further constrain whether the obliquity increases could be due to spin-orbit resonance (Gondán & Kocsis 2021).…”

Detectability of a spatial correlation between stellar-mass black hole mergers and Active Galactic Nuclei in the Local Universe

“…Similarly to what happens in the case of the emission of an EM counterpart, a companion GW signal can be originated from the same source of a detected event in the case of mergers happening near a SMBH. These events could therefore be identified by the independent detection of an associated gravitational echo (Kocsis 2013;Gondán & Kocsis 2021).…”