The optical structure of X-ray globular clusters

CTIO 4-m prime-focus plates of eight globular clusters containing bright X-ray sources have been obtained. Plates in several colors (principally U and B) of each cluster have been digitized. A maximum symmetry method for determining the center of a globular cluster image is described and applied to each cluster in order to determine the cluster center with an accuracy of above 1.0 arc sec. Surface brightness profiles have been derived for seven clusters and fitted with King (1966, 1980) models to yield core radii within about 10 percent. These determinations of the cluster centers and core radii are sufficiently accurate to allow Grindlay et al. (1984) to statistically measure the mass of the globular cluster X-ray sources. Two clusters (NGC 6624 and M15) show significant excesses of light in their cores over the best-fit King model, in agreement with the results of Djorgovski and King (1984). The significance of these excesses, as well as the lack of any dependence of globular cluster structural parameters on color, is discussed.