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Register a new userExploring the Mass Segregation Effect of X-ray Sources in Globular Clusters. II. The Case of Terzan 5
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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.
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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.
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.
We report and study the outburst of a new transient X-ray binary (XRB) in Terzan 5, the third detected in this globular cluster, Swift J174805.3-244637 or Terzan 5 X-3. We find clear spectral hardening in Swift/XRT data during the outburst rise to the hard state, thanks to our early coverage (starting at L_X ~ 4x10^{34} ergs/s) of the outburst. This hardening appears to be due to the decline in relative strength of a soft thermal component from the surface of the neutron star (NS) during the rise. We identify a {Type I X-ray burst} in Swift/XRT data with a long (16 s) decay time, indicative of {hydrogen burning on the surface of the} NS. We use Swift/BAT, Maxi/GSC, Chandra/ACIS, and Swift/XRT data to study the spectral changes during the outburst, identifying a clear hard-to-soft state transition. We use a Chandra/ACIS observation during outburst to identify the transients position. Seven archival Chandra/ACIS observations show evidence for variations in Terzan 5 X-3s non-thermal component, but not the thermal component, during quiescence. The inferred long-term time-averaged mass accretion rate, from the quiescent thermal luminosity, suggests that if this outburst is typical and only slow cooling processes are active in the neutron star core, such outbursts should recur every ~10 years.
Globular cluster are believed to boost the rate of compact binary mergers which may launch a certain type of cosmological gamma-ray bursts (GRBs). Therefore globular clusters appear to be potential sites to search for remnants of such GRBs. The very-high-energy (VHE) gamma-ray source HESS J1747-248 recently discovered in the direction of the Galactic globular cluster Terzan 5 is investigated for being a GRB remnant. Signatures created by the ultra-relativistic outflow, the sub-relativistic ejecta and the ionizing radiation of a short GRB are estimated for an expected age of such a remnant of t > 10^4 years. The kinetic energy of a short GRB could roughly be adequate to power the VHE source in a hadronic scenario. The age of the proposed remnant estimated from its extension possibly agrees with the occurrence of such events in the Galaxy. Sub-relativistic merger ejecta could shock-heat the ambient medium. Further VHE observations can probe the presence of a break towards lower energies expected for particle acceleration in ultra-relativistic shocks. Deep X-ray observations would have the potential to examine the presence of thermal plasma heated by the sub-relativistic ejecta. The identification of a GRB remnant in our own Galaxy may also help to explore the effect of such a highly energetic event on the Earth.
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