Exploring the Mass Segregation Effect of X-ray Sources in Globular
  Clusters: The Case of 47 Tucanae
  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 ($L_{X}\lesssim 5.0\times 10^{30} {\rm\ erg\,s^{-1}}$) and bright
($L_{X}\gtrsim 5.0\times 10^{30} {\rm\ erg\,s^{-1}}$) 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
$1.51\pm0.17\ M_{\odot}$, $1.44\pm0.15\ M_{\odot}$ and $1.16\pm0.06\ M_{\odot}$
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.
