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Register a new userExploring the Mass Segregation Effect of X-ray Sources in Globular Clusters: The Case of 47 Tucanae
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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 $Rlesssim7.5arcmin$) 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 $Rsim 100arcsec$ and $Rsim 170arcsec$ for the faint ($L_{X}lesssim 5.0times 10^{30} {rm erg,s^{-1}}$) and bright ($L_{X}gtrsim 5.0times 10^{30} {rm erg,s^{-1}}$) 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 $Rsim 200arcsec$ citep{ferraro2004}. By fitting the radial distribution of each group of sources with a generalized King model, we estimated an average mass of $1.51pm0.17 M_{odot}$, $1.44pm0.15 M_{odot}$ and $1.16pm0.06 M_{odot}$ 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.
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Using archival {it Chandra} observations with a total effective exposure of 734 ks, we derive an updated catalog of point sources in the massive globular cluster Terzan 5. Our catalog covers an area of $58.1, rm arcmin^{2}$ ($Rleq 4.3 , rm arcmin$) with 489 X-ray sources, and more than $75%$ of these sources are first detected in this cluster. We find significant dips in the radial distribution profiles of X-ray sources in Terzan 5, with the projected distance and width of the distribution dips for bright ($L_{X} gtrsim 9.5times 10^{30} {rm erg ,s^{-1}}$) X-ray sources are larger than that of the faint ($L_{X} lesssim 9.5times 10^{30} {rm erg ,s^{-1}}$) sources. By fitting the radial distribution of the X-ray sources with ageneralized King model, we estimated an average mass of $1.48pm0.11,M_{odot}$ and $1.27pm0.13,M_{odot}$ for the bright and faint X-ray sources, respectively. These results are in agreement with that observed in 47 Tuc, which may suggest a universal mass segregation effect for X-ray sources in GCs. Compared with 47 Tuc, we show that the two-body relaxation timescale of Terzan 5 is much smaller, but its dynamical age is significantly younger than 47 Tuc. These features suggest that the evolution of Terzan 5 is not purely driven by two-body relaxation, and tidal stripping effect also plays an important role in accelerating the dynamical evolution of this cluster.
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 ($L_{X} gtrsim 4.5times 10^{30} {rm erg ,s^{-1}}$) X-ray sources are larger than the faint ($L_{X} lesssim 4.5times 10^{30} {rm erg ,s^{-1}}$) sources. The generalized King model fitting give a slightly larger average mass for the bright sources ($1.30pm0.15,M_{odot}$) than the faint sources ($1.09pm0.14,M_{odot}$), 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.
Using X-ray sources as sensitive probes of stellar dynamical interactions in globular clusters (GCs), we study the mass segregation and binary burning processes in $omega$ Cen. We show that the mass segregation of X-ray sources is quenched in $omega$ Cen, while the X-ray source abundance of $omega$ Cen is much smaller than other GCs, and the binary hardness ratio (defined as $L_{rm X}/(L_{rm K}f_{b})$, with $f_{b}$ the binary fraction, $L_{rm X}$ and $L_{rm K}$ the cumulative X-ray and K band luminosity of GCs, respectively) of $omega$ Cen is located far below the $L_{rm X}/(L_{rm K}f_{b})-sigma_{c}$ correlation line of the dynamically normal GCs. These evidences suggest that the binary burning processes are highly suppressed in $omega$ Cen, and other heating mechanisms, very likely a black hole subsystem (BHS), are essential in the dynamical evolution of $omega$ Cen. Through the black hole burning processes (i.e., dynamical hardening of the BH binaries), the BHS is dominating the energy production of $omega$ Cen, which also makes $omega$ Cen a promising factory of gravitational-wave sources in the Galaxy.
Based on over 5400 BV images of 47 Tuc collected between 1998 and 2010 we obtained light curves of 65 variables, 21 of which are newly detected objects. New variables are located mostly just outside of the core in a region poorly studied by earlier surveys of the cluster. Among them there are four detached eclipsing binaries and five likely optical counterparts of X-ray sources. Two detached systems are promising targets for follow-up observations. We briefly discuss properties of the most interesting new variables.
Multi-decade observing campaigns of the globular clusters 47 Tucanae and M15 have led to an outstanding number of discoveries. Here, we report on the latest results of the long-term observations of the pulsars in these two clusters. For most of the pulsars in 47 Tucanae we have measured, among other things, their higher-order spin period derivatives, which have in turn provided stringent constraints on the physical parameters of the cluster, such as its distance and gravitational potential. For M15, we have studied the relativistic spin precession effect in PSR B2127+11C. We have used full-Stokes observations to model the precession effect, and to constrain the system geometry. We find that the visible beam of the pulsar is swiftly moving away from our line of sight and may very soon become undetectable. On the other hand, we expect to see the opposite emission beam sometime between 2041 and 2053.
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