Abstract
We report in this study of 47 Tucanae the largest number of optical identifications of X-ray sources yet obtained in a single globular cluster. Using deep Chandra ACIS-I imaging and extensive Hubble Space Telescope studies with Wide Field Planetary Camera 2 (WFPC2; including a 120 orbit program giving superb V and I images), we have detected optical counterparts to at least 22 cataclysmic variables (CVs) and 29 chromospherically active binaries (BY Dra and RS CVn systems) in 47 Tuc. These identifications are all based on tight astrometric matches between X-ray sources and objects with unusual (non-main-sequence [non-MS]) optical colors and/or optical variability. Several other CVs and active binaries have likely been found, but these have marginal significance because of larger offsets between the X-ray and optical positions, or colors and variability that are not statistically convincing. These less secure optical identifications are not subsequently discussed in detail. In the U versus U-V color-magnitude diagram (CMD), where the U band corresponds to either F336W or F300W, the CVs all show evidence for blue colors compared with the MS, but most of them fall close to the main sequence in the V versus V-I CMD, showing that the secondary stars dominate the optical light. The X-ray-detected active binaries have magnitude offsets above the MS (in both the U versus U-V or V versus V-I CMDs) that are indistinguishable from those of the much larger sample of optical variables (eclipsing and contact binaries and BY Dra variables) detected in the recent WFPC2 studies of Albrow et al. We also present the results of a new, deeper search for optical companions to millisecond pulsars (MSPs). One possible optical companion to an MSP (47 Tuc T) was found, adding to the two optical companions already known. Finally, we study several blue stars with periodic variability from Albrow et al. that show little or no evidence for X-ray emission. The optical colors of these objects differ from those of 47 Tuc (and field) CVs. An accompanying paper will present time series results for these optical identifications and will discuss X-ray-to-optical flux ratios, spatial distributions, and an overall interpretation of the results.
Highlights
The study of globular cluster binaries is motivated by their profound impact on the dynamical evolution of globular clusters (Hut et al 1992)
In this study of 47 Tucanae, the largest number of optical identifications of X-ray sources yet obtained in a single globular cluster
In this two–paper series, we report: (1) full details of the cataclysmic variables (CVs) and active binary optical identifications first reported by GHE01a, (2) a large number of new optical identifications using deeper photometric and time series analysis of the extensive GO-8267 dataset, plus a general search for optical companions to millisecond pulsars (MSPs) and qLMXBs and (3) analysis of archival F300W and V Wide Field Planetary Camera 2 (WFPC2) images obtained in HST program GO-7503
Summary
The study of globular cluster binaries is motivated by their profound impact on the dynamical evolution of globular clusters (Hut et al 1992). The sample of active binaries reported in this paper includes the X-ray detection of several red stragglers ( known as subsubgiants), objects that are found just below the subgiant branch in globular and open clusters Six of these objects were found in 47 Tuc by Albrow et al (2001; hereafter AGB01), and others are known in NGC 6397 (GHE01b) and M67 (Mathieu et al 2002). The 47 Tuc U binary companion is a He WD and the companion to 47 Tuc W is consistent with a heated main sequence star showing large amplitude orbital variations In this two–paper series, we report: (1) full details of the CV and active binary optical identifications first reported by GHE01a, (2) a large number of new optical identifications using deeper photometric and time series analysis of the extensive GO-8267 dataset, plus a general search for optical companions to MSPs and qLMXBs and (3) analysis of archival F300W and V WFPC2 images obtained in HST program GO-7503 A detailed analysis section will be given in Paper II, including a study of the spatial distribution of the sources and their X-ray to optical flux ratios
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