Space and Velocity Distributions of Galactic Isolated Old Neutron Stars

Abstract

ABSTRACTI present the results of Monte Carlo orbital simulations of Galactic Neutron Stars (NSs). The simulations take into account the up-to-date observed NS space and velocity distributions at birth, and account for their formation rate. I simulate two populations of NSs. NSs in the first population were born in the Galactic disk at a constant rate, in the past 12 Gyr. Those in the second population were formed simultaneously 12 Gyr ago in the Galactic bulge. I assume that the NSs born in the Galactic disk comprise 40% of the total NS population. Since the initial velocity distribution of NSs is not well known, I run two sets of simulations, each containing 3 × 106 simulated NSs. One set utilizes a bimodal initial velocity distribution and the other a unimodal initial velocity distribution, both are advocated based on pulsars observations. In light of recent observational results, I discuss the effect of dynamical heating by Galactic structure on NS space and velocity distributions and show it can be neglected. I present catalog of simulated NS space and velocity vectors in the current epoch, and catalog of positions, distances and proper motions of simulated NSs, relative to the Sun. Assuming there are 109 NSs in the Galaxy, I find that in the solar neighborhood the density of NSs is about 2 - 4 × 10-4 pc-3, and their scale height is about 0.3–0.6 kpc (depending on the adopted initial velocity distribution). Moreover, I estimate that about 60–70% of the NSs born in the Milky Way are gravitationally unbound to the Galaxy. These catalogs can be used to test the hypothesis that some radio transients are related to these objects.