ULTRA-LOW-FREQUENCY GRAVITATIONAL-RADIATION FROM MASSIVE BLACK-HOLE BINARIES

For massive black hole binaries produced in galactic mergers, we exami ne the possibility of inspiral induced by interaction with field stars . We model the evolution of such binaries for a range of galaxy core a nd binary parameters, using numerical results from the literature to c ompute the binary's energy and angular momentum loss rates due to stel lar encounters and including the effect of back-action on the field st ars. We find that only a small fraction of binary systems can merge wi thin a Hubble time via unassisted stellar dynamics. External perturbat ions may, however, cause efficient inspiral. Averaging over a populati on of central black holes and galaxy mergers, we computed the expected background of gravitational radiation with periods P-w similar to 1-1 0 yr. Comparison with sensitivities from millisecond pulsar timing sug gests that the strongest sources may be detectable with modest improve ments to present experiments.