Exploring the Mass Segregation Effect of X-ray Sources in Globular Clusters: The Case of 47 Tucanae

Abstract

Using archival {\it Chandra} observations with a total exposure of 510 ks, we present an updated catalog of point sources for Globular Cluster 47 Tucanae. Our study covers an area of $\sim 176.7$ arcmin${}^2$ (i.e., with $R\lesssim7.5\arcmin$) with 537 X-ray sources. We show that the surface density distribution of X-ray sources in 47 Tuc is highly peaked in cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii, with two distribution dips at $R\sim 100\arcsec$ and $R\sim 170\arcsec$ for the faint ($L_X\lesssim 5.0\times 10^{30}\mathrm{erg}{\mathrm{s}}^{-1}$) and bright ($L_X\gtrsim 5.0\times 10^{30}\mathrm{erg}{\mathrm{s}}^{-1}$) groups of X-ray sources, separately. These distribution features are similar to those of Blue Straggler Stars (BSS), where the distribution dip is located at $R\sim 200\arcsec$ \citep{ferraro2004}. By fitting the radial distribution of each group of sources with a "generalized King model", we estimated an average mass of $1.51\pm 0.17M_{\odot }$, $1.44\pm 0.15M_{\odot }$ and $1.16\pm 0.06M_{\odot }$ for the BSS, bright and faint X-ray sources, respectively. These results are consistent with the mass segregation effect of heavy objects in GCs, where more massive objects drop to the cluster center faster and their distribution dip propagates outward further. Besides, the peculiar distribution profiles of X-ray sources and BSS are also consistent with the mass segregation model of binaries in GCs, which suggests that in addition to the dynamical formation channel, primordial binaries are also a significant contributor to the X-ray source population in GCs.

arXiv comment19 pages, 7 figures 3 tables, accepted for publication in ApJ

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