Low-frequency gravitational waves from black hole macho binaries

Nakamura et al. have recently estimated the initial distribution of binary MACHOs in galactic halos, assuming that the MACHOs are primordial black holes of mass similar to 0.5 M., and considered their coalescence as a possible source for ground-based interferometer gravitational wave detectors such as LIGO. Evolving their binary distribution forward in time to the present, the low-frequency (10(-5) Hz < f < 10(-1) Hz) spectrum of gravitational waves associated with such a population of compact binaries is calculated. The resulting gravitational waves would form a strong stochastic background in proposed space interferometers such as LISA and the OMEGA. Low-frequency gravitational waves are likely to become a key tool for determining the properties of binaries within the dark MACHO population.