The X-Ray Globular Cluster Population in NGC 1399

Abstract

We report on the Chandra observations of the elliptical galaxy NGC 1399, concentrating on the X-ray sources identified with globular clusters (GCs). A large fraction of the 2-10 keV X-ray emission in the 8' × 8' Chandra image is resolved into point sources with luminosities ≥5 × 1037 ergs s-1. These sources are most likely low-mass X-ray binaries (LMXBs). In a region imaged by the Hubble Space Telescope, about 70% of the X-ray sources are located within GCs. This association suggests that in giant elliptical galaxies luminous X-ray binaries preferentially form in GCs. Many of the GC sources have super-Eddington luminosities (for an accreting neutron star), and their average luminosity is higher than the non-GC sources. The X-ray spectral properties of both GC and non-GC sources are similar to those of LMXBs in our Galaxy. Two of the brightest sources, one of which is in a GC, have an ultrasoft spectrum, similar to that seen in the high state of black hole candidates. The "apparent" super-Eddington luminosity in many cases may be due to multiple LMXB systems within individual GCs but with some of the most extremely luminous systems containing massive black holes.