- ID:
- ivo://CDS.VizieR/J/MNRAS/479/2834
- Title:
- 233 X-ray sources in omega Centauri
- Short Name:
- J/MNRAS/479/2834
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We identify 233 X-ray sources, of which 95 are new, in a 222ks exposure of omega Centauri with the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer detector. The limiting unabsorbed flux in the core is fX(0.5-6.0keV)~=3x10^-16^erg/s/cm^2^ (Lx~=1x10^30^erg/s at 5.2kpc). We estimate that ~60+/-20 of these are cluster members, of which ~30 lie within the core (r_c_=155-arcsec), and another ~30 between 1-2 core radii. We identify four new optical counterparts, for a total of 45 likely identifications. Probable cluster members include 18 cataclysmic variables (CVs) and CV candidates, one quiescent low-mass X-ray binary, four variable stars, and five stars that are either associated with omega Cen's anomalous red giant branch or are sub-subgiants. We estimate that the cluster contains 40+/-10 CVs with Lx>10^31^erg/s, confirming that CVs are underabundant in omega Cen relative to the field. Intrinsic absorption is required to fit X-ray spectra of six of the nine brightest CVs, suggesting magnetic CVs, or high-inclination systems. Though no radio millisecond pulsars (MSPs) are currently known in omega Cen, more than 30 unidentified sources have luminosities and X-ray colours like those of MSPs found in other globular clusters; these could be responsible for the Fermi-detected gamma-ray emission from the cluster. Finally, we identify a CH star as the counterpart to the second brightest X-ray source in the cluster and argue that it is a symbiotic star. This is the first such giant/white dwarf binary to be identified in a globular cluster.
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- ID:
- ivo://CDS.VizieR/J/ApJ/625/796
- Title:
- X-ray sources in the globular cluster 47 Tuc
- Short Name:
- J/ApJ/625/796
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have detected 300 X-ray sources within the half-mass radius (2.79') of the globular cluster 47 Tucanae in a deep (281ks) Chandra exposure. We perform photometry and simple spectral fitting for our detected sources and construct luminosity functions, X-ray color-magnitude, and color-color diagrams. Eighty-seven X-ray sources show variability on timescales from hours to years. Thirty-one of the new X-ray sources are identified with chromospherically active binaries from the catalogs of Albrow and coworkers (2001, Cat. <J/ApJ/559/1060>). We estimate that the total number of neutron stars in 47 Tuc is of order 300, reducing the discrepancy between theoretical neutron star retention rates and observed neutron star populations in globular clusters. Comprehensive tables of source properties and simple spectral fits are provided electronically.
- ID:
- ivo://CDS.VizieR/J/A+A/490/641
- Title:
- X-ray sources toward NGC 2808
- Short Name:
- J/A+A/490/641
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using new Chandra X-ray observations and existing XMM-Newton X-ray and Hubble far ultraviolet observations, we aim to detect and identify the faint X-ray sources belonging to the Galactic globular cluster NGC 2808 in order to understand their role in the evolution of globular clusters. We present a Chandra X-ray observation of the Galactic globular cluster NGC 2808. We classify the X-ray sources associated with the cluster by analysing their colours and variability. Previous observations with XMM-Newton and far ultraviolet observations with the Hubble Space Telescope are re-investigated to help identify the Chandra sources associated with the cluster. We compare our results to population synthesis models and observations of other Galactic globular clusters. We detect 113 sources, of which 16 fall inside the half-mass radius of NGC 2808 and are concentrated towards the cluster core.
- ID:
- ivo://CDS.VizieR/J/ApJ/741/86
- Title:
- XRBs and star clusters in NGC 4449
- Short Name:
- J/ApJ/741/86
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present 23 candidate X-ray binaries with luminosities down to 1.8x10^36^erg/s, in the nearby starburst galaxy NGC 4449, from observations totaling 105ks taken with the ACIS-S instrument on the Chandra Space Telescope. We determine count rates, luminosities, and colors for each source, and perform spectral fits for sources with sufficient counts. We also compile a new catalog of 129 compact star clusters in NGC 4449 from high-resolution, multi-band optical images taken with the Hubble Space Telescope, doubling the number of clusters known in this galaxy. The UBVI, H{alpha} luminosities of each cluster are compared with predictions from stellar evolution models to estimate their ages and masses. We find strong evidence for a population of very young massive, black hole binaries, which comprise nearly 50% of the detected X-ray binaries in NGC 4449. Approximately a third of these remain within their parent star clusters, which formed {tau}<=6-8Myr ago, while others have likely been ejected from their parent clusters. We also find evidence for a population of somewhat older X-ray binaries, including both supergiant and Be-binaries, which appear to be associated with somewhat older {tau}~100-400Myr star clusters, and one X-ray binary in an ancient ({tau}~10Gyr) globular cluster. Our results suggest that detailed information on star clusters can significantly improve constraints on X-ray binary populations in star-forming galaxies.
- ID:
- ivo://CDS.VizieR/J/AJ/162/63
- Title:
- ZTF light curve of 51 stars in 12 globular clusters
- Short Name:
- J/AJ/162/63
- Date:
- 21 Mar 2022 11:55:52
- Publisher:
- CDS
- Description:
- In this work, we aimed to derive the gri-band period-luminosity (PL) and period-luminosity-color (PLC) relations for late-type contact binaries, for the first time, located in globular clusters, using the homogeneous light curves collected by the Zwicky Transient Factory (ZTF). We started with 79 contact binaries in 15 globular clusters, and retained 30 contact binaries in 10 globular clusters that have adequate numbers of data points in the ZTF light curves and are unaffected by blending. Magnitudes at mean and maximum light of these contact binaries were determined using a fourth-order Fourier expansion, while extinction corrections were done using the Bayerstar2019 3D reddening map together with adopting the homogeneous distances to their host globular clusters. After removing early-type and "anomaly" contact binaries, our derived gri-band PL and period-Wesenheit (PW) relations exhibited a much larger dispersion with large errors on the fitted coefficients. Nevertheless, the gr-band PL and PW relations based on this small sample of contact binaries in globular clusters were consistent with those based on a larger sample of nearby contact binaries. Good agreements of the PL and PW relations suggested both samples of contact binaries in the local Solar neighborhood and in the distant globular clusters can be combined and used to derive and calibrate the PL, PW, and PLC relations. The final derived gr-band PL, PW, and PLC relations were much improved over those based on the limited sample of contact binaries in the globular clusters.
