X-Ray Sources in Globular Clusters

Summary

The total mass of the globular cluster system of our Galaxy makes only ∼10⁻³ the mass of the Galactic disk. It contains, however, ∼20% of all known low-mass binary X-ray sources, about one half of all binary pulsars, and more than a half of the millisecond pulsars in the Galaxy. Close binary systems containing neutron stars should thus form much more easily in the dense stellar environment of globular clusters than elsewhere in the Galaxy. In these lectures we first review the formation mechanism of neutron stars. Then, we present the evolutionary scenarios leading to the formation of binary X-ray sources and binary and millisecond pulsars in the Galactic disk and the Galactic bulge. We later discuss the specific mechanisms to form neutron star binaries in globular clusters. We end by discussing the open issues concerning the origin and evolution of X–ray sources and millisecond pulsars in globular clusters, and their relationship with the structure, dynamics and evolution of the clusters themselves.

Low–mass binary X–ray sources and millisecond pulsars

An early, unexpected result of X–ray astronomy was the discovery of several bright X–ray sources in globular clusters. Later on, searches for radio pulsars have produced many detections, especially of short period pulsars. We begin these lectures with a very schematic presentation of those two kinds of objects.

X–ray binaries

Luminous Galactic binary X–ray sources provided the first evidence of neutron star binaries, that is binary star systems containing neutron stars (Giacconi et al. 1971; Lewin et al. 1971; Schreier et al. 1972; Tananbaum et al. 1972).