High-frequency gravitational waves from supermassive black holes: Prospects for ligo-virgo detections

Abstract

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 GWs to leak to higher frequencies. In particular, if the high-frequency spectral tail asymptotes to h̃(f)∝ f-α, where α ≲ 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: (1) gravitational bremsstrahlung of ultrarelativistic objects in the vicinity of an SMBH, (2) ringdown modes excited by an external process that has a high-frequency component or terminates abruptly, and (3) gravitational lensing echoes and diffraction. We estimate the order of magnitude of the detection signal-to-noise ratio for each mechanism (1, 2, and 3) as a function of the waveform parameters. In particular for (3), SMBHs produce GW echoes 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 both amplified if the binary is within a ∼10 deg (r/100M)-1/2 cone behind the SMBH relative to the line of sight at a 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. © 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.