NEUTRON-STAR AND BLACK-HOLE BINARIES IN THE GALAXY

We present a statistical analysis of the number and birthdate of double neutron star (NS-NS) binaries. We estimate that there are approximately 10(4.5)z0 such systems in the Galaxy, with a birthrate approximately 10(-5)z0 yr-1, where z0 (kpc) is their scale height; we expect z0 approximately few kpc. We find that mergers of NS-NS binaries through gravitational radiation losses should be occurring roughly once a year in galaxies within 200/h Mpc. Also, we estimate that one out of approximately 10(2)/z0 massive stars in close binaries ends up in a NS-NS system. Progenitor stars more massive than approximately 50 M. probably form black holes (BHs). We estimate that the number and formation rate of BH-NS binaries in the Galaxy are comparable to the corresponding estimates for NS-NS binaries, and we predict that a BH-NS binary is likely to be discovered in pulsar surveys. Such an object may be an even better general relativistic laboratory than PSR 1913 + 16 or PSR 1534 + 12. BH-NS binaries may also be strong sources of gravitational radiation.