Astrophysical Journal Letters 380: L17-L21 (1991)

The Rate of Neutron Star Binary Mergers in the Universe:

Minimal Predictions for Gravity Wave Detectors

E.S. Phinney

Theoretical Astrophysics, 130-33 California Institute of Technology, Pasadena, California 91125

Of the many sources which gravitational wave observatories might see, merging neutron star binaries are the most predictable. Their waveforms at the observable frequencies are easy to calculate. And three systems which will merge in less than a Hubble time have already been observed as binary pulsars: two in the disk of our Galaxy, and one in a globular cluster. From the lifetimes and positions of these, we infer with confidence a lower limit to the merger rate in our Galaxy and globular cluster system. Taking the merger rate in other galaxies to scale with the star formation rate, we compute the merger rate expected in the local universe. An ultra-conservative lower limit to the rate gives 3 per year within 1 Gpc. Our best estimate, still conservative in that it considers only systems like those already observed, gives 3 per year within 200 Mpc. An upper limit of 3 mergers per year within 23/h Mpc is set by the rate of type Ib supernovae. The rates of black hole binary mergers and black hole -- neutron star binary mergers are model-dependent, but could be comparable to the given rate of neutron-star binary mergers.