Based on archival Chandra data, we have carried out an X-ray survey of 69, or nearly half the known population of, Milky Way globular clusters (GCs), focusing on weak X-ray sources, mainly cataclysmic variables (CVs) and coronally active binaries (ABs). Using the cumulative X-ray luminosity per unit stellar mass (i.e., X-ray emissivity) as a proxy of the source abundance, we demonstrate a paucity (lower by 41% ± 27% on average) of weak X-ray sources in most GCs relative to the field, which is represented by the Solar neighborhood and Local Group dwarf elliptical galaxies. We also revisit the mutual correlations among the cumulative X-ray luminosity (L X ), cluster mass (M ) and stellar encounter rate (Γ), finding L X ∝ M 0.74±0.13 , L X ∝ Γ 0.67±0.07 and Γ ∝ M 1.28±0.17 . The three quantities can further be expressed as L X ∝ M 0.64±0.12 Γ 0.19±0.07 , which indicates that the dynamical formation of CVs and ABs through stellar encounters in GCs is less dominant than previously suggested, and that the primordial formation channel has a substantial contribution. Taking these aspects together, we suggest that a large fraction of primordial, soft binaries have been disrupted in binary-single or binary-binary stellar interactions before they can otherwise evolve into X-ray-emitting close binaries, whereas the same interactions also have led to the formation of new close binaries. No significant correlations between L X /L K and cluster properties, including dynamical age, metallicity and structural parameters, are found.

There is a remarkable correlation between the spin periods of the accreting neutron stars (NSs) in Be/X-ray binaries (BeXBs) and their orbital periods. Recently, Knigge et al. showed that the distribution of the spin periods contains two distinct subpopulations peaked at ∼10 s and ∼200 s, respectively, and suggested that they may be related to two types of supernovae for the formation of the NSs, i.e., core-collapse and electron-capture supernovae. Here we propose that the bimodal spin period distribution is likely to be ascribed to different accretion modes of the NSs in BeXBs. When the NS tends to capture material from the warped, outer part of the Be star disk and experiences giant outbursts, a radiatively cooling dominated disk is formed around the NS, which spins up the NS and is responsible for the short-period subpopulation. In BeXBs that are dominated by normal outbursts or are persistent, the accretion flow is advection-dominated or quasi-spherical. The spin-up process is accordingly inefficient, leading to longer periods of the neuron stars. The potential relation between the subpopulations and the supernova mechanism is also discussed.

Using archival 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 ∼ 176.7 arcmin 2 (i.e., with R 7.5 ′ ) 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 ∼ 100 ′′ and R ∼ 170 ′′ for the faint (L X 5.0 × 10 30 erg s −1 ) and bright (L X 5.0 × 10 30 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 ∼ 200 ′′ ). By fitting the radial distribution of each group of sources with a "generalized King model", we estimated an average mass of 1.51 ± 0.17 M ⊙ , 1.44 ± 0.15 M ⊙ and 1.16 ± 0.06 M ⊙ 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.

Using archival Chandra observations with a total effective exposure of 323 ks, we derive an updated catalog of point sources in the bulge globular cluster M28. The catalog contains 502 Xray sources within an area of ∼ 475 arcmin 2 , and more than 90% of these sources are first detected in this cluster. We find significant dips in the radial distribution profiles of X-ray sources in M28, with the projected distance and width of the distribution dip for bright (L X 4.5×10 30 erg s −1 ) X-ray sources are larger than the faint (L X 4.5 × 10 30 erg s −1 ) sources. The "generalized King model" fitting give a slightly larger average mass for the bright sources (1.30 ± 0.15 M ⊙ ) than the faint sources (1.09 ± 0.14 M ⊙ ), which support a universal mass segregation delay between heavy objects in GCs. Compared with 47 Tuc and Terzan 5, we show that the dynamical age of M28 is comparable to Terzan 5 and much smaller than 47 Tuc, but it is evolving more fast (i.e., with smaller two-body relaxation timescale) than 47 Tuc. These features may suggest an acceleration effect of cluster dynamical evolution by tidal shock in M28. Besides, we find an abnormal deficiency of X-ray sources in the central region (R 1.5 ′ ) of M28 than its outskirts, which indicate that M28 may have suffered an early phase of primordial binary disruption within its central region, and mass segregation effect will erase such a phenomenon as cluster evolve to older dynamical age.