Double and single recycled pulsars: an evolutionary puzzle?
  We investigate the statistics of isolated recycled pulsars and double neutron
star binaries 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. There are a
comparable number of double neutron star systems compared to isolated recycled
pulsars. We find that standard evolutionary models cannot explain this fact,
predicting several times the number of 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 (sigma = 265 km/s) neutron star natal kick velocities are invoked in
binary population syntheses. Population syntheses in which the 1D Maxwellian
velocity dispersion of the natal kick is sigma=170 km/s are consistent with the
observations. These conclusions further support earlier findings the neutron
stars formed in close interacting binaries receive significantly smaller natal
kicks than the velocities of Galactic single pulsars would seem to indicate.
