Galactic neutron stars I. Space and velocity distributions in the disk and in the halo

TL;DR

This study uses Monte Carlo simulations to analyze the space and velocity distributions of neutron stars in the Milky Way, revealing most are in the halo and providing models for their density and motion.

Contribution

It offers new phase space distribution models for neutron stars in the Galactic disk and halo based on detailed orbital simulations.

Findings

~80% of neutron stars are in bound orbits.

Less than 20% of neutron stars are in the Galactic disk.

Provides sky maps and distribution fits for neutron star populations.

Abstract

Aims. Neutron stars (NSs) produced in the Milky Way are supposedly ten to the eighth - ten to the ninth, of which only $\sim 2 \times 10^{3}$ are observed. Constraining the phase space distribution of NSs may help to characterize the yet undetected population of stellar remnants. Methods. We perform Monte Carlo simulations of NS orbits, under different assumptions concerning the Galactic potential and the distribution of progenitors and birth velocities. We study the resulting phase space distributions, focusing on the statistical properties of the NS populations in the disk and in the solar neighbourhood. Results. It is shown that $\sim 80$ percent of NSs are in bound orbits. The fraction of NSs located in a disk of radius 20 kpc and width 0.4 kpc is $\lesssim 20$ percent. Therefore the majority of NSs populate the halo. Fits for the surface density of the disk, the distribution of heights on the Galactic plane and the velocity distribution of the disk, are given. We also provide sky maps of the projected number density in heliocentric Galactic coordinates (l, b). Our results are compared with previous ones reported in the literature. Conclusions. Obvious applications of our modelling are in the revisiting of accretion luminosities of old isolated NSs, the issue of the observability of the nearest NS and the NS optical depth for microlensing events. These will be the scope of further studies.