X-ray Sources In Globular Clusters Research Articles

Dataset of Classified Chandra Sources in Globular Clusters

We present a collection of classified X-ray sources in Globular Clusters (GCs) observed by the Chandra X-ray Observatory, including active binaries, cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries. We cross-match the most accurate published positions from multiwavelength observations of these sources to the Chandra Source Catalog Release 2.1, and the HST UV Globular Cluster Survey to extract their multiwavelength properties. The data set can be accessed via an interactive website (https://home.gwu.edu/~kargaltsev/XCLASS_GC) and used as a training data set for machine-learning classification of unidentified X-ray sources in GCs.

Globular cluster ultraluminous X-ray sources in the furthest early-type galaxies

ABSTRACT Ultraluminous X-ray sources (ULXs) in globular clusters (GCs) are low-mass X-ray binaries that achieve high X-ray luminosities through a currently uncertain accretion mechanism. Using archival Chandra and Hubble Space Telescope observations, we perform a volume-limited search (≲70 Mpc) of 21 of the most massive ($\gt 10^{11.5} \, \mathrm{M}_\odot$) early-type galaxies to identify ULXs hosted by GC candidates. We find a total of 34 ULX candidates above the expected background within five times the effective radius of each galaxy, with 10 of these ($\sim 29.4{{\ \rm per\ cent}}$) potentially hosted by a GC. A comparison of the spatial and luminosity distributions of these new candidate GC ULXs with previously identified GC ULXs shows that they are similar: both samples peak at LX ∼ a few × 1039 erg s−1 and are typically located within a few effective radii of their host galaxies.

Probing M87 Globular Clusters for Flaring Ultraluminous X-Ray Sources

Abstract We present X-ray analysis of three short Chandra observations of M87. The basis of this analysis is the search for possible new ultraluminous X-ray sources (ULXs) in M87's globular clusters (GCs) and attempt to quantify possible variability within the observations. We searched Chandra ObsIDs 1808 (2000 July 30, 14 ks) Wilson & Yang, 3975 (2002 November 17, 5 ks) and 3977 (2003 February 4, 5 ks) Harris et al. and identified one previously discovered GC ULX, and two new GC ULX candidates. Analysis of the light-curves revealed no new evidence of variability in these sources for the duration of the observations.

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

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.

X-ray sources in Galactic globular clusters and old open clusters

AbstractThe features and make up of the population of X-ray sources in Galactic star clusters reflect the properties of the underlying stellar environment. Cluster age, mass, stellar encounter rate, binary frequency, metallicity, and maybe other properties as well, determine to what extent we can expect a contribution to the cluster X-ray emission from low-mass X-ray binaries, millisecond pulsars, cataclysmic variables, and magnetically active binaries. Sensitive X-ray observations withXMM-Newton and certainlyChandra have yielded new insights into the nature of individual sources and the effects of dynamical encounters. They have also provided a new perspective on the collective X-ray properties of clusters, in which the X-ray emissivities of globular clusters and old open clusters can be compared to each other and to those of other environments. I will review our current understanding of cluster X-ray sources, focusing on star clusters older than about 1 Gyr, illustrated with recent results.

X-ray spectral variability of ultraluminous X-ray sources in extragalactic globular clusters

A number of ultraluminous X-ray sources (ULXs) are physically associated with extragalactic globular clusters (GCs). We undertake a systematic X-ray analysis of eight of the brightest of these sources. We fit the spectra of the GC ULXs to single power law and single disk models. We find that the data never require that any of the sources change between a disk and a power law across successive observations. The GC ULXs best fit by a single disk show a bimodal distribution: they either have temperatures well below 0.5 keV, or variable temperatures ranging above 0.5 keV up to 2~keV. The GC ULXs with low kT have significant changes in luminosity but show little or no change in kT. By contrast, the sources with higher kT either change in both kT and $L_X$ together, or show no significant change in either parameter. Notably, the X-ray characteristics may be related to the optical properties of these ULXs, with the two lowest kT sources showing optical emission lines.

Dynamical formation of cataclysmic variables in globular clusters

The formation and evolution of X-ray sources in globular clusters is likely to be affected by the cluster internal dynamics and the stellar interactions in the cluster dense environment.Several observational studies have revealed a correlation between the number of X-ray sources and the stellar encounter rate and provided evidence of the role of dynamics in the formation of X-ray binaries. We have performed a survey of Monte-Carlo simulations aimed at exploring the connection between the dynamics and formation of cataclysmic variables (CVs) and the origin of the observed correlation between the number of these objects, $N_{\rm cv}$, and the stellar encounter rate, $\Gamma$.The results of our simulations show a correlation between $N_{\rm cv}$ and $\Gamma$ as found in observational data, illustrate the essential role played by dynamics, and shed light on the dynamical history behind this correlation. CVs in our simulations are more centrally concentrated than single stars with masses close to those of turn-off stars, although this trend is stronger for CVs formed from primordial binaries undergoing exchange encounters, which include a population of more massive CVs absent in the group of CVs formed from binaries not suffering any component exchange.

Demonstrating the likely neutron star nature of five M31 globular cluster sources withSwift-NuSTAR spectroscopy

We present the results of a joint Swift-NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6-8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM-Newton. We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disk winds in older work that predict which systems will be persistent and which will be transient.

Testing the deep-crustal heating model using quiescent neutron-star very-faint X-ray transients and the possibility of partially accreted crusts in accreting neutron stars

It is assumed that accreting neutron stars (NSs) in LMXBs are heated due to the compression of the existing crust by the accreted matter which gives rise to nuclear reactions in the crust. It has been shown that most of the energy is released deep in the crust by pycnonuclear reactions involving low-Z elements. We discuss if NSs in very-faint X-ray transients (VFXTs; those which have peak X-ray luminosities < 1E36 erg/s) can be used to test this model. Unfortunately we cannot conclusively answer this because of the large uncertainties in our estimates of the accretion rate history of those VFXTs, both the short-term (less than a few tens of thousands of years) and the one throughout their lifetime. The latter is important because it can be so low that the NSs might not have accreted enough matter to become massive enough that enhanced cooling processes become active. Therefore, they could be relatively warm compared to other systems for which such enhanced cooling processed have been inferred. However, the amount of matter can also not be too low because then the crust might not have been replaced significantly by accreted matter and thus a hybrid crust of partly accreted and partly original, albeit further compressed matter, might be present. This would inhibit the full range of pycnonuclear reactions to occur and thus very likely decreasing the amount of heat deposited in the crust. Furthermore, better understanding is needed how a hybrid crust affects other properties such as the thermal conductivity. We also show that some individual NS LMXBs might have hybrid crusts as well as the NSs in HMXBs. This has to be taken into account when studying the cooling properties of those systems when they are in quiescence. We show that the VFXTs are likely not the dominate transients that are associated with the brightest low-luminosity X-ray sources in globular clusters as was hypothesized.

Strong [O iii] and [N ii] emission lines in globular clusters from photoionized R Corona Borealis star winds

Abstract The globular cluster X-ray source CXO J033831.8–352604 in NGC 1399 has recently been found to show strong emission lines of [O iii] and [N ii] in its optical spectrum in addition to ultraluminous X-ray emission with a soft X-ray spectrum. It was further suggested that this system contained an intermediate-mass black hole which had tidally disrupted a white dwarf, producing the strong emission lines without detectable hydrogen emission. We show that an alternative exists which can explain the data more naturally in which the oxygen- and nitrogen-rich material is ejected from an R Corona Borealis (RCB) star. The scenario we propose here does not require an intermediate-mass black hole as the accretor, but also does not exclude the possibility.

Globular cluster x-ray sources

Globular clusters and x-ray astronomy have a long and fruitful history. Uhuru and OSO-7 revealed highly luminous (> 10(36) ergs(-1)) x-ray sources in globular clusters, and Einstein and ROSAT revealed a larger population of low-luminosity (< 10(33) ergs(-1)) x-ray sources. It was realized early on that the high-luminosity sources were low-mass x-ray binaries in outburst and that they were orders of magnitude more abundant per unit mass in globular clusters than in the rest of the galaxy. However, the low-luminosity sources proved difficult to classify. Many ideas were put forth--low-mass x-ray binaries in quiescence (qLMXBs), cataclysmic variables (CVs), active main-sequence binaries (ABs), and millisecond pulsars (MSPs)--but secure identifications were scarce. In ROSAT observations of 55 clusters, about 25 low-luminosity sources were found. Chandra has now observed over 80 Galactic globular clusters, and these observations have revealed over 1,500 x-ray sources. The superb angular resolution has allowed for many counterpart identifications, providing clues to the nature of this population. It is a heterogeneous mix of qLMXBs, CVs, ABs, and MSPs, and it has been shown that the qLMXBs and CVs are both, in part, overabundant like the luminous LMXBs. The number of x-ray sources in a cluster correlates very well with its encounter frequency. This points to dynamical formation scenarios for the x-ray sources and shows them to be excellent tracers of the complicated internal dynamics. The relation between the encounter frequency and the number of x-ray sources has been used to suggest that we have misunderstood the dynamical states of globular clusters.

X-RAY SOURCES AND THEIR OPTICAL COUNTERPARTS IN THE GALACTIC GLOBULAR CLUSTER M12 (NGC 6218)

We study a Chandra X-ray Observatory ACIS-S observation of the Galactic globular clusterM12. With a 26 ks exposure time, we detect 6 X-ray sources inside the half-mass radius (2.16 arcminutes) of which two are inside the core radius (0.72 arcminutes) of the cluster. If we assume these sources are all associated with globular cluster M12, the luminosity L_{X} among these sources between 0.3-7.0 keV varies roughly from 10^{30} to 10^{32} ergs s^{-1}. For identification, we also analyzed the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and Wide Field and Planetary Camera 2 (WFPC2) data and identified the optical counterparts to five X-ray sources inside the HST ACS field of view. According to the X-ray and optical features, we found 2-5 candidate active binaries (ABs) or cataclysmic variables (CVs) and 0-3 background galaxies within the HST ACS field of view. Based on the assumption that the number of X-ray sources scales with the encounter rate and the mass of the globular cluster, we expect 2 X-ray source inside M12, and the expectation is consistent with our observational results. Therefore, the existence of identified X-ray sources (possible CVs or ABs) in M12 suggests the primordial origin of X-ray sources in globular clusters which is in agreement with previous studies.

The first two transient supersoft X-ray sources in M 31 globular clusters and the connection to classical novae

Classical novae (CNe) have been found to represent the major class of supersoft X-ray sources (SSS) in our neighbour galaxy M 31. We determine properties and evolution of the two first SSSs ever discovered in the M 31 globular cluster (GC) system. We have used XMM-Newton, Chandra and Swift observations of the centre region of M 31 to discover both SSS and to determine their X-ray light curves and spectra. We performed detailed analysis of XMM-Newton EPIC PN spectra of the source in Bol 111 (SS1) using blackbody and NLTE white dwarf (WD) atmosphere models. For the SSS in Bol 194 (SS2) we used optical monitoring data to search for an optical counterpart. Both GC X-ray sources were classified as SSS. We identify SS1 with the CN M31N 2007-06b recently discovered in the M 31 GC Bol 111. For SS2 we did not find evidence for a recent nova outburst and can only provide useful constraints on the time of the outburst of a hypothetical nova. The only known CN in a M 31 GC can be identified with the first SSS found in a M31 GC. We discuss the impact of our observations on the nova rate for the M 31 GC system.

NewXMM‐NewtonAnalysis of Three Bright X‐Ray Sources in M31 Globular Clusters, Including a New Black Hole Candidate

We present detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3--10 keV lightcurve exhibited a r.m.s variability of ~10%, and its 0.3--7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44$\pm$0.12. Its variability and emission is characteristic of low mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behaviour is typically observed at luminosities $\la$10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of 2.5$\pm0.2\times 10^{38}$ erg s$^{-1}$, or{~140%} Eddington for a 1.4 $M_{\odot}$ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 appears to have been consistently bright for ~30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007, January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or neutron star.

Observational Evidence for the Origin of X-ray Sources in Globular Clusters

AbstractLow-mass X-ray binaries, recycled pulsars, cataclysmic variables and magnetically active binaries are observed as X-ray sources in globular clusters. We discuss the classification of these systems, and find that some presumed active binaries are brighter than expected. We discuss a new statistical method to determine from observations how the formation of X-ray sources depends on the number of stellar encounters and/or on the cluster mass. We show that cluster mass is not a proxy for the encounter number, and that optical identifications are essential in proving the presence of primordial binaries among the low-luminosity X-ray sources.

XMM-Newton observations of faint X-ray sources in globular clusters

Using the new generation of X-ray observatories, we are now beginning to identify populations of close binaries in globular clusters, previously elusive in the optical domain because of the high stellar density. These binaries are thought to be, at least in part, responsible for delaying the inevitable core collapse of globular clusters and their identification is therefore essential in understanding the evolution of globular clusters, as well as being valuable in the study of the binaries themselves. Here, we present observations made with XMM-Newton of six globular clusters, in which we have identified neutron star low mass X-ray binaries and their descendants (millisecond pulsars), cataclysmic variables and other types of binaries. We discuss not only the characteristics of these binaries, but also their formation and evolution in globular clusters and their use in tracing the dynamical history of these clusters.

An explanation for long flares from extragalactic globular cluster X-ray sources

Repeatedly flaring X-ray binaries have recently been discovered in NGC 4697 by Sivakoff and collaborators. We show that these flares can be explained as the result of eccentric binaries in globular clusters which accrete more rapidly at periastron than during the rest of the binary orbit. We show that theoretical timescales for producing eccentricities and circularising the binaries are consistent with what is needed to produce the observed population of flaring sources, although the circularisation timescales are highly uncertain on both observational and theoretical grounds. This model makes two clear theoretical predictions (1) the flares should be seen to be strictly periodic if adequate sampling is provided, and that periodicity should be of approximately 15 hours (2) this class of flaring behaviour should be seen only in globular cluster sources, and predominantly in the densest globular clusters. We also test the model for producing eccentricities through fly-by's of a third star near the binary in a globular cluster against a much larger database of millisecond pulsar observations than has been used in past work, and find that the theoretical cross sections for producing eccentricity in binaries are in reasonable agreement with most of the data, provided that the pulsar ages are about $4\times10^9$ years.

Creating ultra-compact binaries in globular clusters through stable mass transfer

A binary in which a slightly evolved star starts mass transfer to a neutron star can evolve towards ultra-short orbital periods under the influence of magnetic braking. This is called magnetic capture. We investigate in detail for which initial orbital periods and initial donor masses binaries evolve to periods less than 30–40 min within the Hubble time. We show that only small ranges of initial periods and masses lead to ultra-short periods, and that for those only a small time interval is spent at ultra-short periods. Consequently, only a very small fraction of any population of X-ray binaries is expected to be observed at ultra-short period at any time. If 2 to 6 of the 13 bright X-ray sources in globular clusters have an ultra-short period, as suggested by recent observations, their formation cannot be explained by the magnetic capture model.

M31 Globular Cluster X‐Ray Sources:XMM‐NewtonandChandraResults

We present the results of an M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 arcmin2 of M31. We detected 43 X-ray sources coincident with GC candidates from various optical surveys. The inferred isotropic X-ray luminosities of GC sources lie between ~1035 and ~1039 ergs s-1 in the 0.3-10 keV energy band. The spectral properties of the 31 brightest sources in our sample were found to be similar to those of the low-mass X-ray binaries (LMXBs) located in the bulge and the GCs of the Milky Way. The spectral distribution of the M31 GC X-ray sources is consistent with that derived for the bulge of M31 and other nearby galaxies of different morphological type. Several sources demonstrate a correlation between the level of X-ray flux and the hardness of their energy spectrum reminiscent of the Galactic Z and atoll sources. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on timescales from days to years. The X-ray source RX J0043.2+4127, coincident with GC Bo 163, has been found to show recurrent transient outbursts with peak luminosities of ~1038 ergs s-1. Several sources in our sample show significant variability on a timescale of individual observations, ranging from aperiodic fluctuations to regular dipping. The X-ray luminosity function of GC sources is found to be significantly different from that of the point sources in the bulge and disk of M31. The luminosity distribution of M31 GC sources has ~10 times higher peak luminosity and a much higher fraction of bright sources than the Milky Way GC distribution. Six persistent sources in our sample (or ~14% of the total number) have luminosities exceeding 1038 ergs s-1 during all observations, and three other sources occasionally exceed that luminosity level. Our observations indicate that GC sources make the dominant contribution to the bright source counts in the areas of M31 covered by the survey: ~40% of the total number of sources with luminosities above 1037 ergs s-1 reside in GCs, with the fraction of GC sources rising to 67%-90% for luminosities above 1038 ergs s-1. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way but surprisingly close to that of early-type galaxies. We found that the brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that GCs hosting bright X-ray sources are optically brighter and more metal-rich than the rest of the M31 GCs, in agreement with previous studies. The brightest sources, with luminosities above ~1038 ergs s-1, show a tendency to reside in more metal-poor clusters. The remarkable similarities between the properties of the M31 GC X-ray sources and those of Galactic neutron star LMXBs lead us to expect most of the persistent M31 GC X-ray sources to be LMXB systems with neutron star primaries. However, the current X-ray spectral and timing data cannot rule out the possibility of finding active accreting black holes in our GC source sample.

The Hard Quiescent Spectrum of the Neutron Star X‐Ray Transient EXO 1745−248 in the Globular Cluster Terzan 5

We present a Chandra observation of the globular cluster Terzan 5 during times when the neutron star X-ray transient EXO 1745-248 located in this cluster was in its quiescent state. We detected the quiescent system with a (0.5-10 keV) luminosity of ~2 × 1033 ergs s-1. This is similar to several other neutron-star transients observed in their quiescent states. However, the quiescent X-ray spectrum of EXO 1745-248 was dominated by a hard power-law component instead of the soft component that usually dominates the quiescent emission of other neutron star X-ray transients. This soft component could not conclusively be detected in EXO 1745-248, and we conclude that it contributed at most 10% of the quiescent flux in the energy range 0.5-10 keV. EXO 1745-248 is only the second known neutron-star transient whose quiescent spectrum is dominated by the hard component (SAX J1808.4-3658 is the other one). We discuss possible explanations for this unusual behavior of EXO 1745-248, its relationship to other quiescent neutron-star systems, and the impact of our results on understanding quiescent X-ray binaries. We also discuss the implications of our results on the way that the low-luminosity X-ray sources in globular clusters are classified.