Abstract
We observe the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-Ray Observatory to study its faint X-ray populations. Five X-ray sources are found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A. The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2 hr. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources are found within the half-mass radius, reaching to a limiting luminosity of -->1.5 × 1030 ergs s−1 (0.3-8 keV). From a radial distribution analysis, we find that -->18 ± 6 of these 29 sources are associated with M71, somewhat more than predicted, and that -->11 ± 6 are background sources, both Galactic and extragalactic. M71 appears to have more X-ray sources in the range -->LX = 1030–1031 ergs s−1 than expected by extrapolating from other studied clusters using either mass or collision frequency. We explore the spectra and variability of these sources and describe the results of ground-based optical counterpart searches.
Highlights
Globular cluster X-ray sources are of interest for many reasons
Dense globular clusters bring stars into close dynamical encounters that lead to the production of X-ray binaries (e.g., Hut et al 1991), and studies of globular clusters with different structural parameters can elucidate the details of these mechanisms
Two relatively sparse nearby clusters have been carefully studied with Chandra: M4 (Bassa et al 2004) and NGC 288 (Kong et al 2006). Those studies have indicated that active binaries (ABs) are prevalent in sparse clusters, and that cataclysmic variables (CVs) are few in number, but possibly larger in number than predicted by empirical extrapolations with the density and mass of the cluster core
Summary
Globular cluster X-ray sources are of interest for many reasons. Dense globular clusters bring stars into close dynamical encounters that lead to the production of X-ray binaries (e.g., Hut et al 1991), and studies of globular clusters with different structural parameters can elucidate the details of these mechanisms. Globular clusters provide concentrations of faint X-ray sources for the study of X-ray populations at a known distance, age, and metallicity. Observations of globular clusters may provide science unique to individual X-ray sources that is enabled by knowledge of the X-ray source’s distance, reddening, and other properties An ensemble of such results for many clusters may shed light on the evolution of globular clusters and their binary populations. Two relatively sparse nearby clusters have been carefully studied with Chandra: M4 (Bassa et al 2004) and NGC 288 (Kong et al 2006) Those studies have indicated that ABs are prevalent in sparse clusters, and that CVs are few in number, but possibly larger in number than predicted by empirical extrapolations with the density and mass of the cluster core. Ransom et al (2003, 2005) and Hessels et al (2007) reported the presence of a binary MSP in M71, with PPSR 1⁄4 4:89 ms and Porb 1⁄4 4:24 hr, and the presence at 20 cm of eclipses which last roughly 20% of the orbital period
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