The Neutron Star–Helium White Dwarf Population in the Galactic Disk

Abstract

The population of low-mass binary pulsars in the Galactic disk has grown considerably in the past few years. By low mass, we mean that the companion to the neutron star is, or is very likely to be, a helium white dwarf. The distribution of these objects in the P- plane, where P is the rotation period, suggests both that their initial periods after spin-up are near 3 ms and that the age of the population is near 5 Gyr. This initial spin-up is similar to the inferred rotation rates in low-mass X-ray binaries (LMXBs) with dual-line quasi-periodic oscillations at kilohertz frequencies. Furthermore, the age of 5 Gyr is consistent with models of the LMXB population as evolving from stars with masses just above 1 M☉, although significant uncertainties remain in observationally derived birthrates of the radio and X-ray binaries. The observed distribution of minimum masses in the low-mass binary pulsar population requires a distribution of actual masses and is not the result of a random distribution of inclination angles. This distribution further leads to the conclusion that these millisecond pulsar systems do not have random inclination angles. A preference for orthogonal magnetic and spin axes is a possible configuration.