J ul 2 00 9 X - ray and Optical Study of Low Core Density Globular Clusters NGC 6144 and E 3

We report on the Chandra X-ray Observatory and Hubble Space Telescope observation 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 6 X-ray sources within the half-mass radius of NGC6144, among which 4 are expected to be background sources; 3 X-ray sources are also found within the half-mass radius of E3, of which 3 is expected to be background source. Therefore, we cannot exclude that all our sources are background sources. However, combining the results from X-ray and optical observations, we found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be cataclysmic variables and that 1 source in NGC6144 is an active binary, based on the Xray 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. Institute of Astronomy and Department of Physics, National Tsing Hua University 101 Section 2 Kuang Fu Road , Hsinchu, Taiwan 30013, R.O.C. Astronomical Institute, Utrecht University, P.O. Box 80000, 3508 TA, Utrecht, the Netherlands Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 SRON, Netherlands Institute for Space Research Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands Department of Astrophysics, IMAPP, Radboud University Nijmegen PO Box 9010, 6500 GL Nijmegen, The Netherlands Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 Astronomy Department, University of Wisconsin-Madison, 475 North Charter Street, Madison, WI 53706 Kenda Foundation Golden Jade Fellow