41 research outputs found
Exotica in the Globular Cluster M4, Studied with Chandra, HST, and the VLA
No full textUsing the Hubble Ultraviolet Globular Cluster Survey (HUGS) and additional
HST archival data, we have carried out a search for optical counterparts to the
low-luminosity Chandra X-ray sources in the globular cluster M4 (NGC 6121). We
have also searched for optical or X-ray counterparts to radio sources detected
by the VLA. We find 24 new confident optical counterparts to Chandra sources
for a total of 40, including the 16 previously identified. Of the 24 new
identifications, 18 are stellar coronal X-ray sources (active binaries, ABs),
the majority located along the binary sequence in a V-I colour-magnitude
diagram and generally showing an H-alpha excess. In addition to confirming the
previously detected cataclysmic variable (CV, CX4), we identify one confident
new CV (CX76), and two candidates (CX81 and CX101). One MSP is known in M4
(CX12), and another strong candidate has been suggested (CX1); we identify some
possible MSP candidates among optical and radio sources, such as VLA20, which
appears to have a white dwarf counterpart. One X-ray source with a sub-subgiant
optical counterpart and a flat radio spectrum (CX8, VLA31) is particularly
mysterious. The radial distribution of X-ray sources suggests a relaxed
population of average mass ~ 1.2 - 1.5 Msun. Comparing the numbers of ABs,
MSPs, and CVs in M4 with other clusters indicates that AB numbers are
proportional to cluster mass (primordial population), MSPs to stellar encounter
rate (dynamically formed population), while CVs seem to be produced both
primordially and dynamically.Comment: 30 pages, 12 figures, 2 pages of supplementary material containing
finding chart
Searching for exotic object companions in the dense core of NGC 362
No full textThe dense cores of globular clusters (GCs) are efficient environments for the production of exotic stellar populations, including millisecond pulsars (MSPs), low-mass X-ray binaries (LMXBs), and cataclysmic variables (CVs). Most of these objects likely form through two- and three-body interactions and are useful tracers of the cluster’s dynamical evolution. In this work, we explore the exotic object population in the galactic GC NGC 362, searching for the optical counterpart of 33 X-ray sources identified within 1′ from the cluster centre. To this end, we exploited a large Hubble Space Telescope dataset obtained in eight different epochs and covering a wavelength range from the near UV to the optical I band. To identify the most promising counterparts, we followed a multi-step analysis based on four main ingredients, namely, positional coincidence, position in the colour–magnitude diagrams, Hα excess, and photometric variability. In addition, we complemented the photometric analysis with spectroscopic information coming from the analysis of MUSE radial velocity curves. Thanks to this multi-diagnostic approach, we were able to identify 28 high-confidence optical counterparts, including several candidate MSPs, active binaries, and CVs. The most intriguing counterparts include a candidate black widow system, an eclipsing binary blue straggler, and a system in outburst, potentially representing either an LMXB or a nova eruption from a CV. The candidate MSPs proposed in this work will contribute to ongoing radio analyses with MeerKAT for the identification and detailed study of MSPs in NGC 362
An Investigation of the Radial Dependence of the Galaxy Luminosity Function in Abell Clusters
No full textImproved mass and radius constraints for quiescent neutron stars in omega Cen and NGC 6397
No full textWe use Chandra and XMM-Newton observations of the globular clusters omega Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favoured by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS
The Dynamics of the Merging Galaxy Cluster System A2256: Evidence for a New Subcluster
No full textWe present 236 new radial velocities of galaxies in the cluster A2256 measured with the WIYN Hydra Multi-Object Spectrograph. Combined with the previous work of Fabricant, Kent, & Kurtz, we have velocities for a total of 319 galaxies, of which 277 are cluster members. In addition to the new radial velocities, we present a 3 × 3 image mosaic in the R band of the central 19′ × 19′ region of A2256, from which we obtained photometry for 861 galaxies. These data provide strong evidence for a merger event between two groups. In addition, we present evidence for the presence of a third group, on the outer reaches of the system, that is just now beginning to merge with the system
FOKKER-PLANCK MODELS FOR M15 WITHOUT A CENTRAL BLACK HOLE: THE ROLE OF THE MASS FUNCTION
No full text
Identification of Faint<i>Chandra</i>X-Ray Sources in the Core-collapsed Globular Cluster NGC 6752
No full textWe have searched for optical identifications for 39 Chandra X-ray sources that lie within the 1farcm9 half-mass radius of the nearby (), core-collapsed globular cluster, NGC 6752, using deep Hubble Space Telescope ACS/WFC imaging in , , and Hα. Photometry of these images allows us to classify candidate counterparts based primarily on color–magnitude and color–color diagram location. The color–color diagram is particularly useful for quantifying the Hα-line equivalent width. In addition to recovering 11 previously detected optical counterparts, we propose 20 new optical IDs. In total, there are 16 likely or less certain cataclysmic variables (CVs), 9 likely or less certain chromospherically active binaries, 3 galaxies, and 3 active galactic nuclei. The latter three sources, which had been identified as likely CVs by previous investigations, now appear to be extragalactic objects based on their proper motions. As we previously found for NGC 6397, the CV candidates in NGC 6752 fall into a bright group that is centrally concentrated relative to the turnoff-mass stars and a faint group that has a spatial distribution that is more similar to that of the turnoff-mass stars. This is consistent with an evolutionary scenario in which CVs are produced by dynamical interactions near the cluster center and diffuse to larger radius orbits as they age
A Comprehensive Analysis of X-Ray Sources in Terzan 5 Using Chandra Observations
No full textWe analyze photometry, spectra, and variability of over 100 faint X-ray sources in the globular cluster Terzan 5, using 737 ks of Chandra data. X-ray colors and spectral fitting allow for clear separation of foreground sources (with less extinction than the cluster), quiescent low-mass X-ray binaries (qLMXBs), and sources with harder spectra. We identify 22 candidate qLMXBs, over twice that found in any other cluster. This is consistent with Terzan 5’s stellar interaction rate, the highest among Galactic globular clusters. We do not see qLMXBs dominated by thermal emission below L _X ∼ 10 ^32 erg s ^−1 , though qLMXBs with stronger nonthermal emission could be missed. We find that more than 50% of the qLMXB sources have a neutron star thermal component contributing over 80% of the total luminosity. We report an unusual spectral feature around 1.75 keV in the combined spectrum of Ter 5 X-3. The concentration of the qLMXBs within the cluster is consistent with that of a population of mass 1.46 ± 0.14 M _⊙ . We identify secure X-ray counterparts to millisecond pulsars Terzan 5 ar and Terzan 5 at, using positional coincidence and orbital X-ray light curves matching those expected for spider pulsars
