Optical Counterparts of X-Ray Point Sources Observed by Chandra in NGC 5128: 20 New Globular Cluster X-Ray Sources

Very Large Telescope images in BVI are used to identify the optical counterparts to bright Chandra X-ray points sources discovered by Kraft and coworkers. Of a total of 111 X-ray point sources with L_X>2×10³⁶ ergs s^-1 present in a 56 arcmin² field centered on this galaxy, 58 have optical counterparts. On the basis of the sizes, optical magnitudes, and colors, 20 new globular cluster counterparts of X-ray sources are identified, and three are identified on the basis of their sizes. This brings the total number of globular cluster X-ray sources in this galaxy to 33 and establishes that 30% of the X-ray point sources in NGC 5128 are associated with globular clusters. These X-ray globular clusters occupy the brightest end of the globular cluster luminosity function, indicating that bright low-mass X-ray binaries are preferentially found in massive clusters. Most of the globular clusters with X-ray sources have red colors, with 1.0<V-I<1.5, indicating that low-mass X-ray binaries are preferentially formed in metal-rich clusters. The NGC 5128 X-ray globular cluster sources are brighter compared with the Milky Way sources: there are 24 globular clusters with X-ray sources of L_X>10³⁷ ergs s^-1. There is, however, no globular cluster X-ray source in NGC 5128 as bright as expected for an accreting black hole. In addition, 31 optical counterparts of X-ray point sources that are not associated with globular clusters are identified. Finally, 53 X-ray point sources (48% of the population) do not have any optical counterparts down to the faintest magnitude limits (B=25).

Based on observations collected with the Very Large Telescope of the European Southern Observatory, the Magellan I Baade Telescope of the Carnegie Institution, and the Cerro Tololo Inter-American Observatory 0.9 m telescope, on images from the Very Large Telescope obtained from the ESO Archive, and on images obtained from the NASA/ESA Hubble Space Telescope Archive.