X rays from old open clusters: M 67 and NGC 188

We have observed the old open clusters M 67 and NGC 188 with the ROSAT PSPC. In M 67 we detect a variety of X-ray sources. The X-ray emission by a cataclysmic variable, a single hot white dwarf, two contact binaries, and some RS CVn systems is as expected. The X-ray emission by two binaries located below the subgiant branch in the Hertzsprung Russell diagram of the cluster, by a circular binary with a cool white dwarf, and by two eccentric binaries with orbital period > 700 d is puzzling. Two members of NGC 188 are detected, including the FK Com type star D719. Another possible FK Com type star, probably not a member of NGC 188, is also detected.Comment: 10 pages, 5 figures. Accepted for publication on Astronomy & Astrophysic

Gravitational waves from double white dwarfs

Double white dwarfs could be important sources for space based gravitational wave detectors like OMEGA and LISA. We use population synthesis to predict the current population of double white dwarfs in the Galaxy and the gravitational waves produced by this population. We simulate a detailed power spectrum for an observation with an integration time of 10^6 s. At frequencies below ~3 mHz confusion limited noise dominates. At higher frequencies a few thousand double white dwarfs are resolved individually. Including compact binaries containing neutron stars and black holes in our calculations yields a further few hundred resolved binaries and some tens which can be detected above the double white dwarf noise at low frequencies. We find that binaries in which one white dwarf transfers matter to another white dwarf are rare, and thus unimportant for gravitational wave detectors. We discuss the uncertainties and compare our results with other authors.Comment: 6 pages, to appear in the proceedings of the XXXIVth Rencontres de Moriond on "Gravitational Waves and Experimental Gravity", January 23-30, 199

The distance and luminosity probability distributions derived from parallax and flux with their measurement errors with application to the millisecond pulsar PSR J0218+4232

We use a Bayesian approach to derive the distance probability distribution for one object from its parallax with measurement uncertainty for two spatial distribution priors, viz. a homogeneous spherical distribution and a galactocentric distribution - applicable for radio pulsars - observed from Earth. We investigate the dependence on measurement uncertainty, and show that a parallax measurement can underestimate or overestimate the actual distance, depending on the spatial distribution prior. We derive the probability distributions for distance and luminosity combined, and for each separately, when a flux with measurement error for the object is also available, and demonstrate the necessity of and dependence on the luminosity function prior. We apply this to estimate the distance and the radio and gamma-ray luminosities of PSR J0218+4232. The use of realistic priors improves the quality of the estimates for distance and luminosity, compared to those based on measurement only. Use of a wrong prior, for example a homogeneous spatial distribution without upper bound, may lead to very wrong results.Comment: 10 pages, 9 figures, accepted 27-04-2016 to Astronomy and Astrophysic

A census with ROSAT of low-luminosity X-ray sources in globular clusters

I analyze 101 observations from the ROSAT archive to search for X-ray sources in or near 55 globular clusters. New sources are found in the cores of NGC362 (a double source), NGC6121 (marginally significant), NGC6139, and NGC6266; and outside the cores of NGC6205, NGC6352 and NGC6388. More accurate positions are determined for the X-ray sources in some ten clusters. The improved position for the source in NGC6341 excludes the suggested ultraviolet counterpart. It is shown that one of the two sources reported near the core NGC6626 is spurious, as is the detection of a pulsar period in the PSPC data of this cluster; the central source is resolved in three sources. One source reported previously in NGC6304 is demoted to an upper limit. For 20 cluster cores better upper limits to the X-ray luminosity are obtained. From a statistical analysis I argue that several sources outside the cluster cores may well belong to the clusters. All spectral energy distributions observed so far are relatively soft, with bremsstrahlung temperatures =~0.9keV; there is evidence however that bremsstrahlung spectra do not correctly describe the spectra. The X-ray luminosity per unit mass for the cluster as a whole does not depend on the concentration; the luminosity per unit mass for the core may increase with the cluster concentration.Comment: 24 pages, 8 figures, 5 tables, accepted for publication in Astronomy and Astrophysic

Chandra Observation of the Globular Cluster NGC 6440 and the Nature of Cluster X-ray Luminosity Functions

As part of our campaign to determine the nature of the various source populations of the low-luminosity globular cluster X-ray sources, we have obtained a Chandra X-ray Observatory ACIS-S3 image of the globular cluster NGC 6440. We detect 24 sources to a limiting luminosity of ~2 times 10^31 erg/s (0.5-2.5keV) inside the cluster's half-mass radius, all of which lie within ~2 core radii of the cluster center. We also find excess emission in and around the core which could be due to unresolved point sources. Based upon X-ray luminosities and colors, we conclude that there are 4-5 likely quiescent low-mass X-ray binaries and that most of the other sources are cataclysmic variables. We compare these results to Chandra results from other globular clusters and find the X-ray luminosity functions differ among the clusters.Comment: 9 pages, 4 figures, accepted by ApJ, minor changes, added table of clusters' physical parameter

X-ray and Optical Study of Low Core Density Globular Clusters NGC6144 and E3

We report on the Chandra X-ray Observatory and Hubble Space Telescope observation of two low core density globular clusters, NGC6144 and E3. By comparing the number of X-ray sources inside the half-mass radius to those outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among which 4 are expected to be background sources; 3 X-ray sources are also found within the half-mass radius of E3, of which 3 is expected to be background source. Therefore, we cannot exclude that all our sources are background sources. However, combining the results from X-ray and optical observations, we found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be cataclysmic variables and that 1 source in NGC6144 is an active binary, based on the X-ray and optical properties. The number of faint X-ray sources in NGC6144 and E3 found with Chandra and HST is higher than a prediction based on collision frequency, but is closer to that based on mass. Our observations strongly suggest that the compact binary systems in NGC6144 and E3 are primordial in origin.Comment: 28 pages, 9 figures, 6 tables, Accepted for publication in Ap