Galactic distribution of merging neutron stars and black holes – prospects for short gamma-ray burst progenitors and LIGO/VIRGO

We have performed a detailed population synthesis on a large number (2 × 10⁷) of binary systems in order to investigate the properties of massive double degenerate binaries. We have included new important results in our input physics in order to obtain more reliable estimates of the merging time-scales and relative formation rates. These improvements include refined treatment of the binding energy in a common envelope, helium star evolution and reduced kicks imparted to new-born black holes. The discovery and observations of gamma-ray burst afterglows and the identification of host galaxies have allowed comparisons of theoretical distributions of merger sites with the observed distribution of afterglow positions relative to host galaxies. To help investigate the physical nature of short- and long-duration gamma-ray bursts, we compute the distances of merging neutron stars (NS) and/or black holes (BH) from the centres of their host galaxies, as predicted by their formation scenario combined with motion in galactic potentials. Furthermore, we estimate the formation rate and merging rate of these massive double degenerate binaries. The latter is very important for the prospects of detecting gravitational waves with LIGO/VIRGO. We find that the expected detection rate for LIGO II is ∼850 yr⁻¹ for galactic field sources and that this rate is completely dominated by merging double black hole (BHBH) binaries. Even LIGO I may detect such an event (∼0.25 yr⁻¹). Our preferred model estimates the Galactic field double neutron star (NSNS) merger rate to be ∼1.5 × 10⁻⁶ yr⁻¹. For BHBH systems this model predicts a merger rate of ∼9.7 × 10⁻⁶ yr⁻¹. Our studies also reveal an accumulating numerous population of very wide-orbit BHBH systems which never merge (τ≫τHubble).