Neutron star and black hole binaries in the Galaxy

We present a statistical analysis of the number and birthrate of double neutron star (NS-NS) binaries. We estimate that there are ̃10^4.5z₀ such systems in the Galaxy, with a birthrate ̃10^-5z₀ yr^-1, where z₀ (kpc) is their scale height; we expect z₀ ̃ 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 ̃10²/z₀ massive stars in close binaries ends up in a NS-NS system. Progenitor stars more massive than ̃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.