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High Frequency Gravitational Waves from Supermassive Black Holes: Prospects for LIGO-Virgo Detections

It is commonly assumed that ground-based gravitational wave (GW) instruments will not be sensitive to supermassive black holes (SMBHs) because the characteristic GW frequencies are far below the ~ 10 - 1000 Hz sensitivity bands of terrestrial detectors. Here, however, we explore the possibility of SMBH gravitational waves to leak to higher frequencies. In particular, if the high frequency spectral tail asymptotes to h(f) ~ f^{-alpha}, where alpha<=2, then the spectral amplitude is a constant or increasing function of the mass M at a fixed frequency f>>c^3/GM. This will happen if the time domain waveform or its derivative exhibits a discontinuity. Ground based instruments could search for these universal spectral tails to detect or rule out such features irrespective of their origin. We identify the following processes which may generate high frequency signals: (i) gravitational bremsstrahlung of ultrarelativistic objects in the vicinity of a SMBH, (ii) ringdown modes excited by an external process that has a high frequency component or terminates abruptly, (iii) gravitational lensing echos and diffraction. In particular for (iii), SMBHs produce GW echos of inspiraling stellar mass binaries in galactic nuclei with a delay of a few minutes to hours. The lensed primary signal and GW echo are amplified if the binary is within a ~10 deg (r/100M)^{-1/2} cone behind the SMBH relative to the line of sight at distance r from the SMBH. For the rest of the binaries near SMBHs, the amplitude of the GW echo is ~0.1 (r/100M)^{-1} of the primary signal on average.