Double and single recycled pulsars : an evolutionary puzzle ?

We investigate the statistics of isolated recycled pulsars and double neutron star bina-ries in the Galactic disk. Since recycled pulsars are believed to form through accretion and spinup in close binaries, the isolated objects presumably originate from disrupted progenitors of double neutron stars. As noted by previous authors, there are about twice as many double neutron star systems compared to isolated recycled pulsars. We find that standard evolutionary models cannot explain this observational fact, predicting just the opposite trend: twice as many isolated recycled pulsars than those in double neutron star systems. We demonstrate, through population synthesis calculations , that the velocity distribution of isolated recycled pulsars is broader than for binary systems. When this is accounted for in a model for radio pulsar survey selection effects, which include the effects of Doppler smearing for the double neutron star binaries, we find that there is a small (∼ 25%) bias towards the detection of double neutron star systems. This bias, however, is not significant enough to explain the observational discrepancy if standard (∼ 300 km s −1) neutron star natal kick velocities are invoked in binary population syntheses. Only population syntheses in which the 3D velocity dispersion of the natal kick is in the range 0–130 km s −1 are consistent with the observations. These conclusions lend support to the hypothesis that the neutron stars formed in close interacting binaries receive significantly smaller natal kicks than the velocities of Galactic single pulsars would seem to indicate.