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A statistical analysis of the number and birthrate of double neutron star (NS-NS) binaries is presented. It is estimated that there are approximately 10 exp 4.5 z0 such systems in the Galaxy, with a birthrate of approximately 10 exp -5 z0 per year, where z0 is their scale height. It is found that merges of NS-NS binaries through gravitational radiation losses should be occurring roughly once a year in galaxies within 200/h Mpc. It is suggested that progenitor stars more massive than about 50 solar masses form black holes. The number and formation rate of BH-NS binaries in the Galaxy are estimated to be comparable to the corresponding estimates of NS-NS binaries. It is predicted that a BH-NS binary is likely to be discovered in pulsar surveys.
Narayan et al. (Sun,) studied this question.