Exploring the Mass Segregation Effect of X-ray Sources in Globular Clusters. III. Signs of Binary Disruption in M28

Zhongqun Cheng, Huijun Mu, Zhiyuan Li, Xiaojie Xu, Wei Wang, Xiangdong Li

Published 2020-02-27Version 1

Using archival {\it 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 X-ray sources within an area of $\sim475\, \rm arcmin^{2}$, 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 ($L_{X} \gtrsim 4.5\times 10^{30} {\rm\ erg\ \,s^{-1}}$) X-ray sources are larger than the faint ($L_{X} \lesssim 4.5\times 10^{30} {\rm\ erg\ \,s^{-1}}$) sources. The "generalized King model" fitting give a slightly larger average mass for the bright sources ($1.30\pm0.15\,M_{\odot}$) than the faint sources ($1.09\pm0.14\,M_{\odot}$), 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 ($R \lesssim 1.5 \rm~arcmin$) 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.