The galactic population of binaries containing neutron stars

Abstract

The research presented herein is a theoretical investigation of the formation, evolution, and ultimate fate of low-, intermediate-, and high-mass X-ray binaries (L/I/HMXBs). The primary theoretical tool used throughout is binary population synthesis. Results of these calculations are used to account for the numbers and properties of observed X-ray binaries and their descendants, as well as to direct future observational and theoretical work. Combining binary population synthesis and binary stellar evolution calculations, I present a systematic population study of L/IMXBs in the Galactic plane. Since full stellar evolution calculations are used to model the X-ray binary phase, it is possible to make detailed comparisons between the theoretical models and observations. It is demonstrated quantitatively that IMXBs probably play a crucial role in shaping the population of LMXBs observed at the current epoch, as well as their descendant binary millisecond radio pulsars. Recently, a new class of HMXBs has emerged, distinguished from other HMXBs by their wide, nearly circular orbits. I show that the discovery of a significant number of such systems is at odds with the conventional wisdom that most neutron stars receive very large "kick" speeds at birth. This problem may be rectified in a self-consistent way if the kick speed depends on the rotation rate of the pre-collapse core, which I propose is strongly influenced by the evolution of the neutron-star progenitor in a binary system. The reasonable suggestion that certain globular clusters contain nearly 1000 neutron stars conflicts with the large mean kick speeds estimated from observations of isolated radio pulsars, which are 5 to 10 times the present cluster escape speeds.

(cont.) Therefore, most neutron stars born from single progenitors should have been ejected from their host clusters. I show that many more neutron stars are retained if a significant fraction are formed with massive stellar companions, but that the retained fraction is still too small to account for the inferred large numbers of neutron stars at the current epoch. Several alternative hypotheses are discussed, including the intriguing possibility that globular clusters we see today were ten times more massive in the distant past. The Chandra X-ray Observatory has revealed hundreds of previously undetected point sources in a small field about the Galactic center. I show that the majority of these sources may be neutron stars accreting from the winds of unevolved companion stars. Infrared observations are proposed to search for the stellar counterparts of the X-ray sources.