Galactic neutron stars

I. Space and velocity distributions in the disk and in the halo

¹ Dipartimento di Fisica e Matematica, Università dell'Insubria, via Valleggio 11, 22100, Como, Italy e-mail: nicola.sartore@gmail.com
² Dipartimento di Fisica, Università di Milano Bicocca, Piazza delle Scienza 3, 20126, Milano, Italy
³ Dipartimento di Fisica, Università di Padova, via Marzolo 8, 35131, Padova, Italy
⁴ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK

Received: 28 March 2009
Accepted: 10 August 2009

Abstract

Aims. Neutron stars (NSs) produced in the Milky Way are supposedly ten to the eighth – ten to the ninth – of which only ~2 10³ are observed. Constraining the phase space distribution of NSs may help to characterize the yet undetected population of stellar remnants.

Methods. We performed 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 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 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 modeling 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.