Optical Counterparts of X‐Ray Point Sources Observed byChandrain NGC 5128: 20 New Globular Cluster X‐Ray Sources

VLT images in $BVI$ are used to identify the optical counterparts to brightCHANDRA X-ray points sources discovered by Kraft et al. (2001, ApJ, 560, 675)in NGC5128. Of a total of 111 X-ray point sources with L_X>2*10^{36} ergs/spresent in a 56 arcmin^2 field centered on this galaxy, 58 have opticalcounterparts. Based on the sizes, optical magnitudes and colors, 20 newglobular cluster counterparts of X-ray sources are identified, plus 3identified based on their sizes. This brings the total number of globularcluster X-ray sources in this galaxy to 33, and establishes that 30% of theX-ray point sources in NGC5128 are associated with globular clusters. TheseX-ray globular clusters occupy the brightest end of the globular clusterluminosity function, indicating that bright low-mass X-ray binaries arepreferentially found in massive clusters. Most of the globular clusters withX-ray sources have red colors, with 1.010^{37} erg/sec. Thereis, however, no globular cluster X-ray source in NGC5128 as bright as expectedfor an accreting black hole. In addition, 31 optical counterparts of X-raypoint sources that are not associated with globular clusters are identified.Finally, 53 X-ray point sources (48% of the population), do not have anyoptical counterparts down to the faintest magnitude limits (B=25).Comment: 32 pages, 8 figures; replaced with the accepted version; to appear in ApJ, January 200