Search

We conduct one of the largest systematic investigations of bright X-ray binaries (XRBs) in both young star clusters and ancient globular clusters (GCs) using a sample of six nearby star-forming galaxies. Combining complete CXO X-ray source catalogs with optical Physics at High Angular Resolution in Nearby Galaxies-Hubble Space Telescope cluster catalogs, we identify a population of 33 XRBs within or near their parent clusters. We find that GCs that host XRBs in spiral galaxies appear to be brighter, more compact, denser, and more massive than the general GC population. However, these XRB hosts do not appear to be preferentially redder or more metal-rich, pointing to a possible absence of the metallicity-boosted formation of low-mass X-ray binaries (LMXBs) that is observed in the GCs of older galaxies. We also find that a smaller fraction of LMXBs is found in spiral GC systems when compared with those in early-type galaxies: between 8% and 50%, or an average of 20% across galaxies in our sample. Although there is a non-negligible probability of a chance superposition between an XRB and an unrelated young cluster, we find that among clusters younger than 10 Myr, which most likely host high-mass XRBs, the fraction of clusters associated with an XRB increases at higher cluster masses and densities. The X-ray luminosity of XRBs appears to increase with the mass of the cluster host for clusters younger than ∼400 Myr, while the inverse relation is found for XRBs in GCs. [ABSTRACT FROM AUTHOR]

We report a peculiar variable blue star in the globular cluster (GC) NGC 6397, using Hubble Space Telescope optical imaging. Its position in the colour–magnitude diagrams and its spectrum are consistent with this star being a helium-core white dwarf (He WD) in a binary system. The optical light curve shows a periodicity at 18.5 h. We argue that this periodicity is due to the rotation of the WD and possibly due to magnetic spots on the surface of the WD. This would make this object the first candidate magnetic He WD in any GC, and the first candidate magnetic WD in a detached binary system in any GC and one of the few He WDs with a known rotation period and of magnetic nature. Another possibility is that this system is an He WD in a binary system with another WD or another degenerate object, which would make this object one of the few candidate non-accreting double-degenerate binaries in any GC. [ABSTRACT FROM AUTHOR]

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 L X ∼ a few × 1039 erg s−1 and are typically located within a few effective radii of their host galaxies. [ABSTRACT FROM AUTHOR]

NGC 4472 is home to five ultraluminous X-ray sources hosted by globular clusters. These sources have been suggested as good black hole candidates in extragalactic globular clusters—a highly sought after population that may provide observational information regarding the progenitors of merging black hole binaries. In this body of work, we present X-ray and optical follow-up to one of these sources, CXOUJ1229410+075744 (GCU1). We find no evidence of [OIII] optical emission in GCU1 which indicates a lack of significant evidence for super-Eddington outflows, unlike what is seen in a handful of ULXs in extragalactic GCs. X-ray monitoring from 2019 to 2021 shows no detected X-ray emission above a few × 1038 erg/s. Comparisons of the multiwavelength properties to disc-dominated, near Eddington Galactic black hole low-mass X-ray binaries (GRS 1915+105 and XTEJ1817-330) suggests that GCU1 may show similar behaviour to GRS 1915+105 in terms of X-ray variability and similar relationships between LX and kT , with GCU1 showing maximum X-ray luminosities on order of higher magnitude. [ABSTRACT FROM AUTHOR]

Radio continuum observations offer a new window on compact objects in globular clusters compared to typical X-ray or optical studies. As part of the MAVERIC survey, we have used the Australia Telescope Compact Array to carry out a deep (median central noise level ≈4  |$\mu$| Jy beam-1) radio continuum survey of 26 southern globular clusters at central frequencies of 5.5 and 9.0 GHz. This paper presents a catalogue of 1285 radio continuum sources in the fields of these 26 clusters. Considering the surface density of background sources, we find significant evidence for a population of radio sources in seven of the 26 clusters, and also identify at least 11 previously known compact objects (six pulsars and five X-ray binaries). While the overall density of radio continuum sources with 7.25-GHz flux densities ≳ 20  |$\mu$| Jy in typical globular clusters is relatively low, the survey has already led to the discovery of several exciting compact binaries, including a candidate ultracompact black hole X-ray binary in 47 Tuc. Many of the unclassified radio sources near the centres of the clusters are likely to be true cluster sources, and multiwavelength follow-up will be necessary to classify these objects and better understand the demographics of accreting compact binaries in globular clusters. [ABSTRACT FROM AUTHOR]

Transitional millisecond pulsars are millisecond pulsars that switch between a rotation-powered millisecond pulsar state and an accretion-powered X-ray binary state, and are thought to be an evolutionary stage between neutron star low-mass X-ray binaries and millisecond pulsars. So far, only three confirmed systems have been identified in addition to a handful of candidates. We present the results of a multiwavelength study of the low-mass X-ray binary NGC 6652B in the globular cluster NGC 6652, including simultaneous radio and X-ray observations taken by the Karl G. Jansky Very Large Array and the Chandra X-ray Observatory, and optical spectroscopy and photometry. This source is the second brightest X-ray source in NGC 6652 (⁠|$L_{\textrm {X}}\sim 1.8 \times 10^{34}{\, \mathrm{erg\, s}^{-1}}$|⁠) and is known to be variable. We observe several X-ray flares over the duration of our X-ray observations, in addition to persistent radio emission and occasional radio flares. Simultaneous radio and X-ray data show no clear evidence of anticorrelated variability. Optical spectra of NGC 6652B indicate variable, broad H α emission that transitions from double-peaked emission to absorption over a time-scale of hours. We consider a variety of possible explanations for the source behaviour, and conclude that based on the radio and X-ray luminosities, short time-scale variability and X-ray flaring, and optical spectra, NGC 6652B is best explained as a transitional millisecond pulsar candidate that displays prolonged X-ray flaring behaviour. However, this could only be confirmed with observations of a change to the rotation-powered millisecond pulsar state. [ABSTRACT FROM AUTHOR]

Recently, Bahramian et al. investigated a large sample of globular clusters (GCs) and found that bright intermediate polars (IPs) are a factor of 10 less frequent in GCs than in the Galactic field. We theoretically investigate here this discrepancy based on GC numerical simulations. We found that, due to disruptive dynamical interaction, there is on average a reduction of only half of bright IP progenitors, which is clearly not enough to explain the observed deficiency. However, if the rotation- and crystallization-driven dynamo scenario recently proposed by Schreiber et al. is incorporated in the simulations, the observed rareness of bright IPs in GCs can be reproduced. This is because bright cataclysmic variables (CVs) in GCs are typically very old systems (≳10 Gyr), with white dwarfs that almost fully crystallized before mass transfer started, which does not allow strong magnetic fields to be generated. The observed mass density of bright IPs in GCs can be recovered if around one-third of the bright CVs dynamically formed through mergers have magnetic field strengths similar to those of IPs. We conclude that the observed paucity of bright IPs in GCs is a natural consequence of the newly proposed rotation- and crystallization-driven dynamo scenario. [ABSTRACT FROM AUTHOR]

We investigate potential correlations between radio source counts (after background corrections) of 22 Galactic globular clusters (GCs) from the MAVERIC survey and the stellar encounter rates (Γ) and masses (M) of the GCs. Applying a radio luminosity limit of , we take a census of radio sources in the core and those within the half-light radius of each cluster. By following a maximum likelihood method and adopting a simplified linear model, we find an unambiguous dependence of core radio source counts on Γ and/or M at 90% confidence, but no clear dependence of source counts within the half-light radius on either Γ or M. Five of the identified radio sources in GC cores above our adopted limit are millisecond pulsars or neutron star X-ray binaries, the dependence of which on Γ is well known, but another is a published black hole (BH) X-ray binary candidate, and 10 others are not identified. Accounting for these verified cluster members increases the significance of the correlation with M and/or Γ (to 99% confidence) for fits to core and half-light region source counts, while excluding a dependence on Γ alone at 90% (core) and 68% (half-light) confidence. This is consistent with published dynamical simulations of GC BH interactions that argue Γ will be a poor predictor of the distribution of accreting BHs in GCs. Future multiwavelength follow-up to verify cluster membership will enable stronger constraints on the dependence of radio source classes on cluster properties, promising a new view on the dynamics of BHs in GCs. [ABSTRACT FROM AUTHOR]

We report optical modulation of the companion to the X-ray source U18 in the globular cluster NGC 6397. U18, with combined evidence from radio and X-ray measurements, is a strong candidate as the second redback in this cluster, initially missed in pulsar searches. This object is a bright variable star with an anomalous red colour and optical variability (∼0.2 mag in amplitude) with a periodicity ∼1.96 d that can be interpreted as the orbital period. This value corresponds to the longest orbital period for known redback candidates and confirmed systems in Galactic globular clusters and one of the few with a period longer than 1 d. [ABSTRACT FROM AUTHOR]

We present a deep (∼330 ks) Chandra survey of the Galactic globular cluster M30 (NGC 7099). Combining the new Cycle 18 with the previous Cycle 3 observations we report a total of 10 new X-ray point sources within the 1 |${_{.}^{\prime}}$| 03 half-light radius, compiling an extended X-ray catalogue of a total of 23 sources. We incorporate imaging observations by the Hubble Space Telescope and the Karl G. Jansky Very Large Array from the MAVERIC survey to search for optical and radio counterparts to the new and old sources. Two X-ray sources are found to have a radio counterpart, including the known millisecond pulsar PSR J2140−2310A, the radio position of which also matches a previously reported faint optical counterpart which is slightly redder than the main sequence. We found optical counterparts to 18 of the 23 X-ray sources, identifying two new cataclysmic variables (CVs), five new CV candidates, two new candidates of RS CVn type of active binary (AB), and two new candidates of BY Dra type of AB. The remaining unclassified X-ray sources are likely background active galactic nuclei (AGNs), as their number is consistent with the expected number of AGN at our X-ray sensitivity. Finally, our analysis of radial profiles of different source classes suggests that bright CVs are more centrally distributed than faint CVs in M30, consistent with other core-collapsed globular clusters. [ABSTRACT FROM AUTHOR]

Owing to its massive, dense core, Terzan 5 has the richest population of millisecond pulsars (MSPs) known among Galactic globular clusters. Here we report new deep 2–8 GHz radio continuum observations of Terzan 5 obtained with the Karl G. Jansky Very Large Array. We have identified a total of 24 sources within the cluster half-light radius, including 17 within the core radius. Nineteen are associated with previously studied MSPs and X-ray binaries. Three of the new radio sources have steep radio spectra and are located within the cluster core, as expected for MSPs. These three sources have hard X-ray photon indices (Γ = 1.3–1.5) and highly variable X-ray emission, suggesting they are binary MSPs belonging to the spider class. For the most X-ray luminous of these sources, the redback spider classification is confirmed by its X-ray light curve, which shows an orbital period of 12.32 hr and double-peaked structure around X-ray maximum. The likely discovery of bright binary MSPs in a well-studied cluster like Terzan 5 highlights how deep radio continuum imaging can complement pulsar search and timing observations in finding probable eclipsing systems. The other new radio source in the core has a flat radio spectrum and is X-ray faint (erg s−1) with a photon index Γ = 2.1 ± 0.5, consistent with the properties expected for a quiescent stellar-mass black hole X-ray binary. [ABSTRACT FROM AUTHOR]

The MAVERIC survey is the first deep radio continuum imaging survey of Milky Way globular clusters, with a central goal of finding and classifying accreting compact binaries, including stellar-mass black holes. Here we present radio source catalogs for 25 clusters with ultra-deep Karl G. Jansky Very Large Array observations. The median observing time was 10 hr per cluster, resulting in typical rms sensitivities of 2.3 and 2.1 μJy per beam at central frequencies of 5.0 and 7.2 GHz, respectively. We detect nearly 1300 sources in our survey at 5σ, and while many of these are likely to be background sources, we also find strong evidence for an excess of radio sources in some clusters. The radio spectral index distribution of sources in the cluster cores differs from the background, and shows a bimodal distribution. We tentatively classify the steep-spectrum sources (those much brighter at 5.0 GHz) as millisecond pulsars and the flat-spectrum sources as compact or other kinds of binaries. These provisional classifications will be solidified with the future addition of X-ray and optical data. The outer regions of our images represent a deep, relatively wide-field (∼0.4 deg2) and high-resolution C band background survey, and we present source counts calculated for this area. We also release radio continuum images for these 25 clusters to the community. [ABSTRACT FROM AUTHOR]

Globular clusters host a variety of lower-luminosity (LX < 1035 erg s−1) X-ray sources, including accreting neutron stars (NSs) and black holes (BHs), millisecond pulsars (MSPs), cataclysmic variables, and chromospherically active binaries. In this paper, we provide a comprehensive catalog of more than 1100 X-ray sources in 38 Galactic globular clusters (GCs) observed by the Chandra X-ray Observatory's Chandra/ACIS detector. The targets are selected to complement the MAVERIC survey's deep radio continuum maps of Galactic GCs. We perform photometry and spectral analysis for each source, determine a best-fit model, and assess the possibility of it being a foreground or background source based on its spectral properties and location in the cluster. We also provide basic assessments of variability. We discuss the distribution of X-ray binaries in GCs and their X-ray luminosity function, and we carefully analyze systems with LX > 1033 erg s−1. Among these moderately bright systems, we discover a new source in NGC 6539 that may be a candidate accreting stellar-mass BH or a transitional MSP. We show that quiescent NS low-mass X-ray binaries in GCs may spend ∼2% of their lifetimes as transitional MSPs in their active (LX > 1033 erg s−1) state. Finally, we identify a substantial underabundance of bright (LX > 1033 erg s−1) intermediate polars in GCs compared to the Galactic field, in contrast with the literature of the past two decades. [ABSTRACT FROM AUTHOR]

We have identified seven ultraluminous X-ray sources (ULXs) that are coincident with globular cluster candidates (GC) associated with M87. ULXs in the old GC environment represent a new population of ULXs, and ones likely to be black holes. In this study, we perform detailed X-ray spectroscopic follow-up to seven GC ULXs across a wealth of archival Chandra observations and long time baseline of 16 yr. This study brings the total known sample of GC ULXs to 17. Two of these sources show variability in their X-ray luminosity of an order of magnitude over many years, and one of these sources shows intra-observational variability on the scale of hours. While the X-ray spectra of the majority of GC ULXs are best fit by single-component models, one of the sources studied in this paper is the second GC ULX to be best fit by a two-component model. We compare this new sample of GC ULXs to the previously studied sample, and compare the X-ray and optical properties counterparts across the samples. We find that the clusters that host ULXs in M87 have metallicities from g − z  = 1.01 to g − z  = 1.70. The best-fitting power-law indices of the X-ray spectra range from Γ = 1.37 to 2.21, and the best-fitting inner blackbody disc temperatures range from kT = 0.56 to 1.90 keV. [ABSTRACT FROM AUTHOR]

Using a 16.2-h radio observation by the Australia Telescope Compact Array and archival Chandra data, we found >5σ radio counterparts to four known and three new X-ray sources within the half-light radius (r h) of the Galactic globular cluster NGC 6397. The previously suggested millisecond pulsar (MSP) candidate, U18, is a steep-spectrum (S ν ∝ να; |$\alpha =-2.0^{+0.4}_{-0.5}$|⁠) radio source with a 5.5-GHz flux density of 54.7 ± 4.3  |$\mu \mathrm{ Jy}$|⁠. We argue that U18 is most likely a 'hidden' MSP that is continuously hidden by plasma shocked at the collision between the winds from the pulsar and companion star. The non-detection of radio pulsations so far is probably the result of enhanced scattering in this shocked wind. On the other hand, we observed the 5.5-GHz flux of the known MSP PSR J1740−5340 (U12) to decrease by a factor of >2.8 during epochs of 1.4-GHz eclipse, indicating that the radio flux is absorbed in its shocked wind. If U18 is indeed a pulsar whose pulsations are scattered, we note the contrast with U12's flux decreases in eclipse, which argues for two different eclipse mechanisms at the same radio frequency. In addition to U12 and U18, we also found radio associations for five other Chandra X-ray sources, four of which are likely background galaxies. The last, U97, which shows strong H α variability, is mysterious; it may be either a quiescent black hole low-mass X-ray binary or something more unusual. [ABSTRACT FROM AUTHOR]

RZ2109 is the first of several extragalactic globular clusters shown to host an ultraluminous X-ray source. RZ2109 is particularly notable because optical spectroscopy shows it has broad, luminous [O  iii ] λλ4959,5007 emission, while also having no detectable hydrogen emission. The X-ray and optical characteristics of the source in RZ2109 make it a good candidate for being a stellar mass black hole accreting from a white dwarf donor (i.e. an ultracompact black hole X-ray binary). In this paper we present optical spectroscopic monitoring of the [O  iii ]5007 emission line from 2007 to 2018. We find that the flux of the emission line is significantly lower in recent observations from 2016 to 2018 than it was in earlier observations in 2007–2011. We also explore the behaviour of the emission line shape over time. Both the core and the wings of the emission line decline over time, with some evidence that the core declines more rapidly than the wings. However, the most recent observations (in 2019) unexpectedly show the emission line core rebrightening [ABSTRACT FROM AUTHOR]

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 disc models. We find that the data never require that any of the sources change between a disc and a power law across successive observations. The GC ULXs best fit by a single disc 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 LX 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. [ABSTRACT FROM AUTHOR]

We present high spatial resolution, far ultraviolet (FUV) F140LP observations of 12 massive globular clusters in M 31 obtained using the ACS/SBC on the Hubble Space Telescope (HST). These observations resolve the cluster profiles to scales similar to their core radii and enable the study of the spatial distribution of blue and extreme horizontal branch (HB) stars, which dominate the emission in the F140LP images. We confirm that some of these clusters have excess FUV emission, suggesting additional hot populations beyond those expected by canonical single stellar population models. We find no evidence that the hot populations are spatially distinct from the majority populations in these clusters, as would be expected if the excess FUV emission is a result of a dynamically enhanced population of extreme-HB stars. We conclude that a second population of stars with significantly enhanced helium abundance is a viable explanation for the observed FUV emission that is both bright and distributed similarly to the rest of the cluster light. Our results support the use of FUV observations as a path to characterizing helium-enhanced subpopulations in extragalactic clusters. These M 31 clusters also show a correlation such that more massive and denser clusters are relatively FUV bright. Similar to extant Milky Way results, this may indicate the degree of helium enrichment, or second population fraction increases with cluster mass. [ABSTRACT FROM AUTHOR]

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 disc winds in older work that predict which systems will be persistent and which will be transient. [ABSTRACT FROM AUTHOR]

In this paper, we address the question: what is the probability of stellar-mass black hole (BH) binaries co-existing in a globular cluster with an intermediate-mass black hole (IMBH)? Our results suggest that the detection of one or more BH binaries can strongly constrain the presence of an IMBH in most Galactic globular clusters. More specifically, the detection of one or more BH binaries could strongly indicate against the presence of an IMBH more massive than ≳103 M⊙ in roughly 80 per cent of the clusters in our sample. To illustrate this, we use a combination of N-body simulations and analytic methods to weigh the rate of formation of BH binaries against their ejection and/or disruption rate via strong gravitational interactions with the central (most) massive BH. The eventual fate of a sub-population of stellar-mass BHs (with or without binary companions) is for all BHs to be ejected from the cluster by the central IMBH, leaving only the most massive stellar-mass BH behind to form a close binary with the IMBH. During each phase of evolution, we discuss the rate of inspiral of the central BH–BH pair as a function of both the properties of the binary and its host cluster. [ABSTRACT FROM PUBLISHER]

We present a comprehensive high spatial resolution imaging study of globular clusters (GCs) in NGC 1399, the central giant elliptical cD galaxy in the Fornax galaxy cluster, conducted with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). Using a novel technique to construct drizzled point-spread function libraries for HST/ACS data, we accurately determine the fidelity of GC structural parameter measurements from detailed artificial star cluster experiments and show the superior robustness of the GC half-light radius, rh, compared with other GC structural parameters, such as King core and tidal radius. The measurement of rh for the major fraction of the NGC 1399 GC system reveals a trend of increasing rh versus galactocentric distance, Rgal, out to about 10 kpc and a flat relation beyond. This trend is very similar for blue and red GCs, which are found to have a mean size ratio of rh, red/rh, blue = 0.82 ± 0.11 at all galactocentric radii from the core regions of the galaxy out to ∼40 kpc. This suggests that the size difference between blue and red GCs is due to internal mechanisms related to the evolution of their constituent stellar populations. Modeling the mass density profile of NGC 1399 shows that additional external dynamical mechanisms are required to limit the GC size in the galaxy halo regions to rh ≈ 2 pc. We suggest that this may be realized by an exotic GC orbit distribution function, an extended dark matter halo, and/or tidal stress induced by the increased stochasticity in the dwarf halo substructure at larger galactocentric distances. We compare our results with the GC rh distribution functions in various galaxies and find that the fraction of extended GCs with rh ⩾ 5 pc is systematically larger in late-type galaxies compared with GC systems in early-type galaxies. This is likely due to the dynamically more violent evolution of early-type galaxies. We match our GC rh measurements with radial velocity data from the literature and split the resulting sample at the median rh value into compact and extended GCs. We find that compact GCs show a significantly smaller line-of-sight velocity dispersion, 〈 σcmp〉 = 225 ± 25 km s–1, than their extended counterparts, 〈 σext〉 = 317 ± 21 km s–1. Considering the weaker statistical correlation in the GC rh color and the GC rh-Rgal relations, the more significant GC size-dynamics relation appears to be astrophysically more relevant and hints at the dominant influence of the GC orbit distribution function on the evolution of GC structural parameters. [ABSTRACT FROM AUTHOR]

We present an analysis of optical spectra from the globular cluster RZ2109 in NGC 4472, which hosts the first unambiguous globular cluster black hole. We use these spectra to determine the elemental composition of the emission line system associated with this source, and to constrain the age and metallicity of the host globular cluster. For the emission line system of RZ2109, our analysis indicates the [O III] λ5007 equivalent width is 33.82 ± 0.39 Å and the Hβ equivalent width is 0.32 ± 0.32 Å, producing a formal [O III] λ5007/Hβ emission line ratio of 106 for a 3200 km s–1 measurement aperture covering the full velocity width of the [O III] λ5007 line. Within a narrower 600 km s–1 aperture covering the highest luminosity velocity structure in the line complex, we find [O III] λ5007/Hβ = 62. The measured [O III] λ5007/Hβ ratios are significantly higher than can be produced in radiative models of the emission line region with solar composition, and the confidence interval limits exclude all but models which have gas masses much larger than those for a single star. Therefore, we conclude that the region from which the [O III] λ5007 emission originates is hydrogen-depleted relative to solar composition gas. This finding is consistent with emission from an accretion-powered outflow driven by a hydrogen-depleted donor star, such as a white dwarf, being accreted onto a black hole. [ABSTRACT FROM AUTHOR]

We report the discovery of a candidate stellar-mass black hole in the Milky Way globular cluster M62. We detected the black hole candidate, which we call M62-VLA1, in the core of the cluster using deep radio continuum imaging from the Karl G. Jansky Very Large Array. M62-VLA1 is a faint source with a flux density of 18.7 ± 1.9 μJy at 6.2 GHz and a flat radio spectrum (α = –0.24 ± 0.42, for Sν = να). M62 is the second Milky Way cluster with a candidate stellar-mass black hole; unlike the two candidate black holes previously found in the cluster M22, M62-VLA1 is associated with a Chandra X-ray source, supporting its identification as a black hole X-ray binary. Measurements of its radio and X-ray luminosity, while not simultaneous, place M62-VLA1 squarely on the well-established radio-X-ray correlation for stellar-mass black holes. In archival Hubble Space Telescope imaging, M62-VLA1 is coincident with a star near the lower red giant branch. This possible optical counterpart shows a blue excess, Hα emission, and optical variability. The radio, X-ray, and optical properties of M62-VLA1 are very similar to those for V404 Cyg, one of the best-studied quiescent stellar-mass black holes. We cannot yet rule out alternative scenarios for the radio source, such as a flaring neutron star or background galaxy; future observations are necessary to determine whether M62-VLA1 is indeed an accreting stellar-mass black hole. [ABSTRACT FROM AUTHOR]

We consider the effect of compact stellar remnants on the interstellar medium of a massive star cluster following the initial burst of star formation. We argue that accretion on to stellar-mass black holes is an effective mechanism for rapid gas depletion in clusters of all masses, as long as they contain progenitor stars more massive than ≳50 M⊙. This scenario appears especially attractive for the progenitor systems of present-day massive globular clusters which likely had masses above M ≳ 107 M⊙. In such clusters, alternative mechanisms such as supernovae and stellar winds cannot provide a plausible explanation for the sudden removal of the primordial gas reservoir that is required to explain their complex chemical enrichment history.In order to consider different regimes in the rate of gas accretion on to stellar-mass black holes, we consider both the Bondi–Hoyle approximation as well as Eddington-limited accretion. For either model, our results show that the cluster gas can be significantly depleted within only a few tens of Myr. In addition, this process will affect the distribution of black hole masses and, by extension, may accelerate the dynamical decoupling of the black hole population and, ultimately, their dynamical ejection. Moreover, the time-scales for gas depletion are sufficiently short that the accreting black holes could significantly affect the chemistry of subsequent star formation episodes.The gas depletion times and final mass in black holes are not only sensitive to the assumed model for the accretion rate, but also to the initial mass of the most massive black hole which, in turn, is determined by the upper mass cut-off of the stellar initial mass function. Given that the mass function of ‘dark’ remnants is a crucial parameter for their dynamical ejection, our results imply that their accretion history can have an important bearing on the observed present-day cluster mass-to-light ratio. In particular, we show that the expected increase of the upper mass cut-off with decreasing metallicity could contribute to the observed anticorrelation between the mass-to-light ratio and the metallicity of globular clusters. [ABSTRACT FROM AUTHOR]

ABSTRACT We explore the possible contribution of classical and recurrent novae from isolated white dwarfs accreting from the intracluster medium to the abundances of 'second-generation' globular cluster stellar populations. We show that under reasonable assumptions the helium abundances of clusters can be enhanced substantially by these novae and argue that novae should be considered as an important, and perhaps even dominant, channel in the evolution of the intracluster medium. We also discuss a possible test for whether helium enhancement really is the cause of the multiple main sequences in globular clusters that is independent of the positions of stars in the colour-magnitude diagram. [ABSTRACT FROM AUTHOR]

We present a multi-facility study of the optical spectrum of the extragalactic globular cluster RZ2109, which hosts a bright black hole X-ray source. The optical spectrum of RZ2109 shows strong and very broad [O III] λλ4959, 5007 emission in addition to the stellar absorption lines typical of a globular cluster. We use observations over an extended period of time to constrain the variability of these [O III] emission lines. We find that the equivalent width of the lines is similar in all of the data sets; the change in L[O III] λ5007 is ≤ 10% between the first and last observations, which were separated by 467 days. The velocity profile of the line also shows no significant variability over this interval. Using a simple geometric model, we demonstrate that the observed [O III] λ5007 line velocity structure can be described by a two-component model with most of the flux contributed by a bipolar conical outflow of about 1600 km s-1, and the remainder from a Gaussian component with an FWHM of several hundred km s-1. [ABSTRACT FROM AUTHOR]

Accurate stellar population synthesis models are vital in understanding the properties and formation histories of galaxies. In order to calibrate and test the reliability of these models, they are often compared with observations of star clusters. However, relatively little work has compared these models in the ugriz filters, despite the recent widespread use of this filter set. In this paper, we compare the integrated colors of globular clusters in the Sloan Digital Sky Survey (SDSS) with those predicted from commonly used simple stellar population (SSP) models. The colors are based on SDSS observations of M31's clusters and provide the largest population of star clusters with accurate photometry available from the survey. As such, it is a unique sample with which to compare SSP models with SDSS observations. From this work, we identify a significant offset between the SSP models and the clusters' g -- r colors, with the models predicting colors which are too red by g -- r ~ 0.1. This finding is consistent with previous observations of luminous red galaxies in the SDSS, which show a similar discrepancy. The identification of this offset in globular clusters suggests that it is very unlikely to be due to a minority population of young stars. The recently updated SSP model of Maraston & Strömbäck better represents the observed g -- r colors. This model is based on the empirical MILES stellar library, rather than theoretical libraries, suggesting an explanation for the g -- r discrepancy. [ABSTRACT FROM AUTHOR]

BSTRACT A variety of different proxies for the stellar collision rates in globular clusters is used in the literature, depending on the quality of data available. We present comparisons between these proxies and the full integrated collision rate for different King models. The most commonly used proxy, Γ, defined to be , where ρ0 is the central cluster density, and rc is the core radius based on the 1966 King model, is an accurate representation of the collision rate from the King model to within about 25 per cent for all but the least concentrated globular clusters. By integrating over King models with a range of parameters, we show that , defined to be , where is the average density within the half-light radius, and rh is the half-light radius, is only marginally better correlated with the full King model collision rate than is the cluster luminosity. The two galaxies where results of King model fitting have been reported in detail show a dearth of core-collapsed clusters relative to that seen in the Milky Way, indicating that the core radii of the most concentrated clusters are probably slightly overestimated, even with excellent data. Recent work has suggested that shallower than linear relations exist between proxies for Γ and the probability that a cluster will contain an X-ray source; we show that there is a similarly shallower than linear relationship between Γ and that can explain the relationship where is used; we also show that reasonable measurement errors are likely to produce a shallower than linear relationship even when Γ itself is used. We thus conclude that the existing evidence is all consistent with the idea that X-ray binary formation rates are linearly proportional to cluster collision rates. We also find, through comparison with Gunn-Griffin models (sometimes referred to as multimass King models) suggestive evidence that the retention fractions of neutron stars in globular clusters may be related to the present day concentration parameters, which would imply that the most concentrated clusters today were the most concentrated clusters at the time of their supernovae. [ABSTRACT FROM AUTHOR]

We present a wide-field study of the globular cluster (GC)/low-mass X-ray binary (LMXB) connection in the giant elliptical NGC 1399. The large field of view of the Advanced Camera for Surveys/WFC, combined with Hubble Space Telescope and Chandra high resolution, allow us to constrain the LMXB formation scenarios in elliptical galaxies. We confirm that NGC 1399 has the highest LMXB fraction in GCs of all nearby elliptical galaxies studied so far, even though the exact value depends on galactocentric distance due to the interplay of a differential GC versus galaxy light distribution and the GC color dependence. In fact, LMXBs are preferentially hosted by bright, red GCs out to >5 Reff of the galaxy light. The finding that GCs hosting LMXBs follow the radial distribution of their parent GC population argues against the hypothesis that the external dynamical influence of the galaxy affects the LMXB formation in GCs. On the other hand, field-LMXBs closely match the host galaxy light, thus indicating that they are originally formed in situ and not inside GCs. We measure GC structural parameters, finding that the LMXB formation likelihood is influenced independently by mass, metallicity, and GC structural parameters. In particular, the GC central density plays a major role in predicting which GCs host accreting binaries. Finally, our analysis shows that LMXBs in GCs are marginally brighter than those in the field, and in particular the only color-confirmed GC with Lx > 1039 erg s-1 shows no variability, which may indicate a superposition of multiple LMXBs in these systems. [ABSTRACT FROM AUTHOR]

We investigate the effect of stellar density on the ultraviolet (UV) emission from M31's globular clusters (GCs). Published far-UV (FUV) and near-UV (NUV) colours from Galaxy Evolution and Explorer ( GALEX) observations are used as a probe into the temperature of the horizontal branch (HB) stars in these clusters. From these data, we demonstrate a significant relationship between the core density of a cluster and its FUV-NUV colour, with dense clusters having bluer UV colours. These results are consistent with a population of (FUV bright) extreme-HB (EHB) stars, the production of which is related to the stellar density in the clusters. Such a relationship may be expected if the formation of EHB stars is enhanced in dense clusters due to dynamical interactions. We also consider the contribution of low-mass X-ray binaries (LMXBs) to the integrated FUV luminosity of a cluster. We note that two of the three metal-rich clusters, (recently) identified by Rey et al. as having a FUV excess, are known to host LMXBs in outburst. Considering the FUV luminosity of Galactic LMXBs, we suggest that a single LMXB is unlikely to produce more than 10 per cent of the observed FUV luminosity of clusters that contain a significant population of blue-HB stars. [ABSTRACT FROM AUTHOR]

ABSTRACT We discuss CXOU 1229410+075744, a new black hole candidate in a globular cluster in the elliptical galaxy NGC 4472. By comparing two Chandra observations of the galaxy, we find a source that varies by at least a factor of 4, and has a peak luminosity of at least 2 × 1039 erg s−1. As such, the source varies by significantly more than the Eddington luminosity for a single neutron star, and is a strong candidate for being a globular cluster black hole. The source's X-ray spectrum also evolves in a manner consistent with what would be expected from a single accreting stellar mass black hole. We consider the properties of the host cluster of this source and the six other strong black hole X-ray binary candidates, and find that there is suggestive evidence that black hole X-ray binary formation is favoured in bright and metal-rich clusters, just as is the case for bright X-ray sources in general. [ABSTRACT FROM AUTHOR]

The globular cluster X-ray source CXO J352604 in NGC 1399 has recently been found to show strong emission lines of [O ] and [N ] 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. [ABSTRACT FROM AUTHOR]

We investigate low-mass X-ray binaries (LMXBs) in the M31 globular cluster (GC) system using data from the 2XMMi catalogue. These X-ray data are based on all publicly available XMM–Newton observations of the galaxy. This new survey provides the most complete and homogeneous X-ray survey of M31's GCs to date, covering >80 per cent of the confirmed old clusters in the galaxy. We associate 41 X-ray sources with confirmed old clusters in the recent M31 cluster catalogue of Peacock et al. Comparing these data with previous surveys of M31, it is found that three of these clusters are newly identified, including a bright transient source in the cluster B128. Four additional clusters, that are not detected in these 2XMMi data, have previously been associated with X-ray sources from Chandra or ROSAT observations. Including these clusters, we identify 45 clusters in M31 which are associated with X-ray emission. By considering the latest optical GC catalogues, we identify that three of the previously proposed X-ray clusters are likely to be background galaxies and two have stellar profiles. We consider the properties of LMXB hosting clusters and confirm significant trends between the presence of an LMXB and the metallicity, luminosity and stellar collision rate of a cluster. We consider the relationship between the luminosity and stellar collision rate of a cluster and note that LMXB hosting clusters have higher than average stellar collision rates for their mass. Our findings strongly suggest that the stellar collision rate is the dominant parameter related to the presence of LMXBs. This is consistent with the formation of LMXBs in GCs through dynamical interactions. [ABSTRACT FROM AUTHOR]

We present an updated catalogue of M31 globular clusters (GCs) based on images from the Wide Field Camera (WFCAM) on the United Kingdom Infrared Telescope and from the Sloan Digital Sky Survey (SDSS). Our catalogue includes new, self-consistent ugriz and K-band photometry of these clusters. We discuss the difficulty of obtaining accurate photometry of clusters projected against M31 due to small-scale background structure in the galaxy. We consider the effect of this on the accuracy of our photometry and provide realistic photometric error estimates. We investigate possible contamination in the current M31 GC catalogues using the excellent spatial resolution of these WFCAM images combined with the SDSS multicolour photometry. We identify a large population of clusters with very blue colours. Most of these have recently been proposed by other works as young clusters. We distinguish between these, and old clusters, in the final classifications. Our final catalogue includes 416 old clusters, 156 young clusters and 373 candidate clusters. We also investigate the structure of M31's old GCs using previously published King model fits to these WFCAM images. We demonstrate that the structure and colours of M31's old GC system are similar to those of the Milky Way. One GC (B383) is found to be significantly brighter in previous observations than observed here. We investigate all of the previous photometry of this GC and suggest that this variability appears to be genuine and short lived. We propose that the large increase in its luminosity may have been due to a classical nova in the GC at the time of the previous observations in 1989. [ABSTRACT FROM AUTHOR]

We present the results of a deep radio observation of the globular cluster NGC 2808. We show that there are no sources detected within the core of the cluster, placing constraints on both the pulsar population of the cluster and the mass of a possible intermediate-mass black hole in NGC 2808. We compare the results for this cluster with other constraints on intermediate-mass black holes derived from accretion measures. With the exception of G1 in M 31 which has previously shown radio emission, even with considerably more conservative assumptions, only the clusters with the poorest of observational constraints are consistent with falling on the relation. This result is interpreted in terms of the fundamental differences between galaxies and globular clusters. [ABSTRACT FROM AUTHOR]

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 the periastron than during the rest of the binary orbit. We show that theoretical time-scales for producing eccentricities and circularizing the binaries are consistent with what is needed to produce the observed population of flaring sources, although the circularization time-scales are highly uncertain on both observational and theoretical grounds. This model makes two clear theoretical predictions: (i) the flares should be seen to be strictly periodic if adequate sampling is provided and that periodicity should be of approximately 15 h; and (ii) 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-bys of a third star near the binary in a globular cluster against a much larger data base 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 × 109 yr. [ABSTRACT FROM AUTHOR]

Intermediate-mass black holes (IMBHs) have been alternatively predicted to be quite common in the centres of globular clusters or nearly impossible to form and retain in the centres of globular clusters. As it has been recently shown that radio observations are currently the most sensitive observational technique for detecting such objects, we have obtained new deep radio observations of Omega Cen, and have reanalyzed older observations of M 15 in the hope of constraining the masses of possible black holes in their centres. In both cases, upper limits of about 100 μJy are found at GHz frequencies. We find that if the Bondi–Hoyle accretion rate truly represents the spherical accretion rate onto a black hole, then the masses of the black holes in the centres of these two galaxies are severely constrained – with mass limits of less than about 100 solar masses in both cases. If more realistic assumptions are made based on recent work showing the Bondi rate to be a severe overestimate, then the data for Omega Cen are marginally consistent with a black hole of about 1/1000 of the mass of the cluster (i.e. about ). The data for M 15 are then only marginally consistent with previous reports of a ∼2000 solar mass black hole, and we note that there is considerable hope for either detecting the black hole or improving this upper limit with current instrumentation. Finally, we discuss the possibility that the radio source near the core of the Ursa Minor dwarf spheroidal galaxy is a black hole. [ABSTRACT FROM AUTHOR]

We take the established relation between black hole mass, X-ray luminosity and radio luminosity and show that intermediate-mass black holes (IMBHs), such as those predicted to exist at the centres of globular clusters (GCs), will be easily identifiable objects in deep radio observations. We show that the radio observations will be far more sensitive than any possible X-ray observations. We also discuss the likely optical photometric and spectroscopic appearance of such systems in the event that radio detections are made. [ABSTRACT FROM AUTHOR]

Hundreds of stellar-mass black holes probably form in a typical globular star cluster, with all but one predicted to be ejected through dynamical interactions. Some observational support for this idea is provided by the lack of X-ray-emitting binary stars comprising one black hole and one other star ('black-hole/X-ray binaries') in Milky Way globular clusters, even though many neutron-star/X-ray binaries are known. Although a few black holes have been seen in globular clusters around other galaxies, the masses of these cannot be determined, and some may be intermediate-mass black holes that form through exotic mechanisms. Here we report the presence of two flat-spectrum radio sources in the Milky Way globular cluster M22, and we argue that these objects are black holes of stellar mass (each ?10-20 times more massive than the Sun) that are accreting matter. We find a high ratio of radio-to-X-ray flux for these black holes, consistent with the larger predicted masses of black holes in globular clusters compared to those outside. The identification of two black holes in one cluster shows that ejection of black holes is not as efficient as predicted by most models, and we argue that M22 may contain a total population of ?5-100 black holes. The large core radius of M22 could arise from heating produced by the black holes. [ABSTRACT FROM AUTHOR]

A correction to the article "Radio observations of NGC 2808 and other globular clusters: constraints on intermediate-mass black holes," that was published in a previous issue is presented.

We present optical Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) spectroscopy of RZ 2109, a globular cluster (GC) in the elliptical galaxy NGC 4472. This GC is notable for hosting an ultraluminous X-ray source as well as associated strong and broad [O III] λλ4959, 5007 emission. We show that the HST/STIS spectroscopy spatially resolves the [O III] emission in RZ 2109. While we are unable to make a precise determination of the morphology of the emission-line nebula, the best-fitting models all require that the [O III] λ5007 emission has a half-light radius in the range 3-7 pc. The extended nature of the [O III] λ5007 emission is inconsistent with published models that invoke an intermediate-mass black hole origin. It is also inconsistent with the ionization of ejecta from a nova in the cluster. The spatial scale of the nebula could be produced via the photoionization of a strong wind driven from a stellar mass black hole accreting at roughly its Eddington rate. [ABSTRACT FROM AUTHOR]

We have searched for [O III] 5007 emission in high-resolution spectroscopic data from FLAMES/GIRAFFE Very Large Telescope observations of 174 massive globular clusters (GCs) in NGC 4472. No planetary nebulae (PNe) are observed in these clusters, constraining the number of PNe per bolometric luminosity, α < 0.8 × 10–7 PN/L☼. This is significantly lower than the rate predicted from stellar evolution, if all stars produce PNe. Comparing our results to populations of PNe in galaxies, we find most galaxies have a higher α than these GCs (more PNe per bolometric luminosity—though some massive early-type galaxies do have similarly low α). The low α required in these GCs suggests that the number of PNe per bolometric luminosity does not increase strongly with decreasing mass or metallicity of the stellar population. We find no evidence for correlations between the presence of known GC PNe and either the presence of low-mass X-ray binaries (LMXBs) or the stellar interaction rates in the GCs. This, and the low α observed, suggests that the formation of PNe may not be enhanced in tight binary systems. These data do identify one [O III] emission feature, this is the (previously published) broad [O III] emission from the cluster RZ 2109. This emission is thought to originate from the LMXB in this cluster, which is accreting at super-Eddington rates. The absence of any similar [O III] emission from the other clusters favors the hypothesis that this source is a black hole LMXB, rather than a neutron star LMXB with significant geometric beaming of its X-ray emission. [ABSTRACT FROM AUTHOR]