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

Abstract

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.

Exploring the Mass Segregation Effect of X-Ray Sources in Globular Clusters. IV. Evidence of Black Hole Burning in ω Centauri

“…From Figure-1, it can be seen that the X-ray sources are uniformly dispersed in the Chandra FoV of ω Cen, and few bright sources (F p 1.0 × 10 −6 photon cm −2 s −1 ) are located within the cluster core. These features are different from other GCs studied in this series of work (i.e., 47 Tuc, Terzan 5 and M 28; Cheng et al 2019aCheng et al ,b, 2020, where the bright X-ray sources are mainly concentrated in the cluster center and significant radial distribution dips, a signature of mass segregation, can be observed in the F p − R diagram. To explore the mass segregation of X-ray sources in ω Cen, we study the radial surface density profile of X-ray sources in .…”

“…The AE binomial no-source probability (P B ) parameter was adopted to evaluate the significance of X-ray sources. As demonstrated in the case of 47 Tucanae, we set a more stringent threshold value of P B 1 × 10 −3 for validated X-ray sources, which was proved to be helpful in optimizing the source completeness and reliability (Cheng et al 2019a). On the other hand, the threshold value of P B also offers us a criterion to calculate the limiting detection sensitivity across the FoV of ω Cen.…”

“…Following the procedures presented in Cheng et al (2019a), we checked the source validation and calculated the detection sensitivity maps within the Chandra FoV of ω Cen. Our final source catalog contains 300 X-ray sources, with P B 1 × 10 −3 in either the full, soft or hard energy band.…”

“…As a result, it is the heaviest objects (m ∼ m eq ) that determine the stellar equipartition and dominate the mass segregation (dσ/σ ∝ dm/m eq ) of GCs (Bianchini et al 2016). Assuming that the CXB sources are uniformly distributed in the Chandra FoV, we corrected the CXB contamination and calculated the value of RX using the method presented in Cheng et al (2019a). In all panels, the blue horizontal dashed lines (with RX,BSS = 1) represent a uniform distribution of X-ray sources (or BSS) relative to the integrated light of the cluster.…”

Properties of Cataclysmic Variables in Globular Clusters

“…Interestingly, spectra of such CVs could be relatively easy to obtain, since crowding should not be a big problem in the outer parts. With respect to observations, Cheng et al (2019a [164]; 2019b [165]; 2020 [166]) recently investigated the phenomenon of mass segregation in the GCs 47 Tuc, Terzan 5 and M28. These authors confirmed that there are some X-ray sources located outside the half-light radii and showed that the radial distribution of X-ray sources in these clusters is bimodal, with features similar to those found in the radial distribution of blue stragglers, they being modulated by mass segregation of primordial binaries.…”