X-ray Sources and their Optical Counterparts in the Globular Cluster M4

We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is Lx~10e29 erg/sec for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify 6 X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.Comment: 11 pages, 6 figures, accepted for publication in ApJ. Figure 1 and 5 are of reduced qualit

The contribution of Utrecht to the study of Compact Stars in Globular Clusters

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Spectroscopy in Utrecht: a brief history

In the first half of the 20th century, Leonard Ornstein turned the Physics Laboratory of Utrecht University into a center for testing the nascent quantum mechanics through accurate measurements of line intensities in flame spectra. The microphotometer of instrument builderWillem Moll was the key to this success. Marcel Minneart used this instrument to study stellar spectra at the Utrecht Astronomical Institute, and in particular for the Photometric Atlas of the Solar Spectrum. When the opportunity came in the 1960s to build instruments for research from space, it was natural that Kees de Jager opted for spectroscopy, in X-ray observations of the Sun. The expertise developed in this program allowed the Space Research Organisation of the Netherlands to be a principal player in the development of X-ray spectrographs on the Einstein, EXOSAT, Chandra and XMM-Newton satellites

The astrophysics of ultra-compact binaries : a white paper for the astro2010 decadal review

Ultra-compact binaries are objects which have orbital periods shorter than one hour. Both stars must be compact and are typically degenerate and hydrogen deficient. The class includes interacting AM CVn stars, ultra-compact X-ray binaries, detached double white dwarfs, double neutron stars, white dwarf/neutron star binaries and as yet unobserved binaries such as black holes with neutron star or white dwarf companions

The star catalogue of Hevelius : machine-readable version and comparison with the modern Hipparcos Catalogue

The catalogue by Johannes Hevelius with the positions and magnitudes of 1564 entries was published by his wife Elisabeth Koopman in 1690. We provide a machine-readable version of the catalogue, and briefly discuss its accuracy on the basis of comparison with data from the modern Hipparcos Catalogue. We compare our results with an earlier analysis by Rybka (1984), finding good overall agreement. The magnitudes given by Hevelius correlate well with modern values. The accuracy of his position measurements is similar to that of Brahe, with σ = 2´ for longitudes and latitudes, but with more errors >5´ than expected for a Gaussian distribution. The position accuracy decreases slowly with magnitude. The fraction of stars with position errors larger than a degree is 1.5%, rather smaller than the fraction of 5% in the star catalogue of Brahe

Three editions of the star catalogue of Tycho Brahe : machine-readable versions and comparison with the modern Hipparcos Catalogue

Tycho Brahe completed his catalogue with the positions and magnitudes of 1004 fixed stars in 1598. This catalogue circulated in manuscript form. Brahe edited a shorter version with 777 stars, printed in 1602, and Kepler edited the full catalogue of 1004 stars, printed in 1627. We provide machine-readable versions of the three versions of the catalogue, describe the differences between them and briefly discuss their accuracy on the basis of comparison with modern data from the Hipparcos Catalogue. We also compare our results with earlier analyses by Dreyer (1916, Tychonis Brahe Dani Scripta Astronomica, Vol. II) and Rawlins (1993, DIO, 3, 1), finding good overall agreement. The magnitudes given by Brahe correlate well with modern values, his longitudes and latitudes have error distributions with widths of 2´, with excess numbers of stars with larger errors (as compared to Gaussian distributions), in particular for the faintest stars. Errors in positions larger than 10´, which comprise about 15% of the entries, are likely due to computing or copying errors

High-resolution X-ray spectroscopy of the interstellar medium : XMM-Newton observation of the LMXB GS 1826−238

Aims. The interstellar medium (ISM) has a multiphase structure characterized by gas, dust, and molecules. The gas can be found in different charge states: neutral, weakly ionized (warm) and highly ionized (hot). It is possible to probe the multiphase ISM through the observation of its absorption lines and edges in the X-ray spectra of background sources. Methods. We present a high-quality RGS spectrum of the low-mass X-ray binary GS 1826−238 with an unprecedentedly detailed treatment of the absorption features caused by the dust and both the neutral and ionized gas of the ISM. We constrain the column density ratios within the different phases of the ISM and measure the abundances of elements such as O, Ne, Fe, and Mg. Results. We found significant deviations from the protosolar abundances: oxygen is over-abundant by a factor 1.23 ± 0.05, neon by 1.75±0.11, iron by 1.37±0.17, and magnesium by 2.45±0.35. The abundances are consistent with the measured metallicity gradient in our Galaxy: the ISM appears to be metal-rich in the inner regions. The spectrum also shows the presence of warm and hot ionized gas. The gas column has a total ionization degree of less than 10%. We also show that dust plays an important role as expected from the position of GS 1826−238: most iron appears to be bound in dust grains, while 10−40% of oxygen consist of a mixture of dust and molecules

The mass of the black hole in 1A 0620–00, revisiting the ellipsoidal light curve modelling

The mass distribution of stellar mass black holes can provide important clues to supernova modeling, but observationally it is still ill constrained. Therefore it is of importance to make black hole mass measurements as accurate as possible. The X-ray transient 1A 0620-00 is well studied, with a published black hole mass of $6.61\pm0.25\,$M$_{\odot}$, based on an orbital inclination $i$ of $51.0\pm0.9$ degrees. This was obtained by Cantrell et al. (2010), as an average of independent fits to $V$-, $I$- and $H$-band light curves. In this work we perform an independent check on the value of $i$ by re-analyzing existing YALO/SMARTS $V$-, $I$- and $H$-band photometry, using different modeling software and fitting strategy. Performing a fit to the three light curves simultaneously, we obtain a value for $i$ of $54.1\pm1.1$ degrees, resulting in a black hole mass of $5.86\pm0.24\,$M$_{\odot}$. Applying the same model to the light curves individually, we obtain $58.2\pm1.9$, $53.6\pm1.6$ and $50.5\pm2.2$ degrees for $V$-, $I$- and $H$-band, respectively, where the differences in best-fitting $i$ are caused by the contribution of the residual accretion disc light in the three different bands. We conclude that the mass determination of this black hole may still be subject to systematic effects exceeding the statistical uncertainty. Obtaining more accurate masses would be greatly helped by continuous phase-resolved spectroscopic observations simultaneous with photometry.Comment: 9 pages, 5 figures, 2 tables, accepted for publication in MNRA

The masses of PSR J1911-5958A and its white dwarf compnion.

We present spectroscopic and photometric observations of the optical counterpart to PSR J1911−5958A, a millisecond pulsar located towards the globular cluster NGC6752. We measure radial velocities from the spectra and determine the systemic radial velocity of the binary and the radial-velocity amplitude of the white-dwarf orbit. Combined with the pulsar orbit obtained from radio timing, we infer a mass ratio of MPSR/MWD = 7.36 ± 0.25. The spectrum of the counterpart is that of a hydrogen atmosphere, showing Balmer absorption lines up to H12, and we identify the counterpart as a helium-core white dwarf of spectral type DA5. Comparison of the spectra with hydrogen atmosphere models yield a temperature Teff = 10 090±150 K and a surface gravity log g = 6.44±0.20 cgs. Using mass-radius relations appropriate for low-mass helium-core white dwarfs, we infer the white-dwarf mass MWD = 0.18 ± 0.02 M and radius RWD = 0.043 ± 0.009 R . Combined with the mass ratio, this constrains the pulsar mass to MPSR = 1.40+0.16 −0.10 M . If we instead use the white-dwarf spectrum and the distance of NGC6752 to determine the white-dwarf radius, we find RWD = 0.058±0.004 R . For the observed temperature, the mass-radius relations predict a white-dwarf mass of MWD = 0.175 ± 0.010 M , constraining the pulsar mass to MPSR = 1.34 ± 0.08 M . We find that the white-dwarf radius determined from the spectrum and the systemic radial velocity of the binary are only marginally consistent with the values that are expected if PSR J1911−5958A is associated with NGC6752. We discuss possible causes to explain this inconsistency, but conclude that our observations do not conclusively confirm nor disprove the assocation of the pulsar binary with the globular cluster