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

Abstract

Abstract 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 X-ray sources within an area of ∼475 arcmin2, 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, and the projected distance and width of the distribution dip for bright (L X ≳ 4.5 × 1030 erg s−1) X-ray sources are larger than for faint (L X ≲ 4.5 × 1030 erg s−1) sources. Fitting with the “generalized King model” gives a slightly larger average mass for the bright sources (1.30 ± 0.15 M ⊙) than for the faint ones (1.09 ± 0.14 M ⊙), which supports a universal delay in mass segregation between heavy objects in globular clusters. We show that the dynamical age of M28 is comparable to that of Terzan 5 and much smaller than that of 47 Tuc, but M28 is evolving faster (i.e., with a shorter 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 compared with its outskirts, which indicates that M28 may have suffered an early phase of primordial binary disruption within its central region, and the mass segregation effect will erase such a phenomenon as clusters evolve to an older dynamical age.