The Neutron Star Census

The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity 0 ≤ V ≤ 550 km s-1. The spin-down, induced by dipole losses and by the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: ejector, propeller, accretor, and georotator. Taking from the ROSAT All Sky Survey an upper limit of ~10 accreting neutron stars within ~140 pc from the Sun, we infer a lower bound for the mean kick velocity, V 200-300 km s-1, corresponding to a velocity dispersion σV 125-190 km s-1. The same conclusion is reached for both a constant magnetic field (B ~ 1012 G) and a magnetic field decaying exponentially with a timescale ~109 yr. Such high velocities are consistent with those derived from radio pulsar observations. Present results, moreover, constrain the fraction of low-velocity stars, which could have escaped pulsar statistics, to less than 1%.