On the galactic and cosmic merger rate of double neutron stars

Previous calculations of the merging rate of double neutron star systems similar to the Hulse-Taylor binary pulsar B1913 + 16 have assumed lifetimes based on the sum of the radio pulsar spin-down age and the time-scale on which the binary system merges as a result of gravitational radiation losses. Here we demonstrate that this method underestimates the merging rate, and that a more reliable calculation can be made from the radio lifetimes of these systems which are shorter by a factor of about 3. Using the latest estimates for the number of double neutron star systems in the Galaxy, we find the rate of such mergers to be similar to 8 x 10(-6) yr(-1). Following earlier extrapolations made by Curran & Lorimer for all galaxies out to 100 Mpc, we find the lower limit to the event rate of neutron star mergers detectable by the advanced LIGO gravitational wave detector to be approximately 0.3 per year.