X-RAY AND OPTICAL STUDY OF LOW CORE DENSITY GLOBULAR CLUSTERS NGC6144 AND E3

We report on the Chandra X-ray Observatory and Hubble Space Telescope (HST) observations of two low core-density globular clusters, NGC6144 and E3. By comparing the number of X-ray sources inside the half-mass radius to those outside, we found six X-ray sources within the half-mass radius of NGC6144, of which four are expected to be background sources; three X-ray sources are also found within the half-mass radius of E3, of which three are expected to be background sources. Therefore, we cannot exclude the possibility that all our sources are background sources. However, combining the results from X-ray and optical observations, we found that one to two sources in NGC6144 and one source in E3 are likely to be cataclysmic variables and that one source in NGC6144 is an active binary, based on the X-ray and optical properties. The number of faint X-ray sources in NGC6144 and E3 found with Chandra and HST is higher than a prediction based on collision frequency, but is closer to that based on mass. Our observations strongly suggest that the compact binary systems in NGC6144 and E3 are primordial in origin.