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ASCA Observations of the Two Nearest Globular Clusters, M 4 and NGC 6397
We report on ASCA observations of the two nearby globular clusters, M 4and NGC 6397. Although 14 and 8 sources were detected at more than the 5σ confidence level from each cluster field, respectively, themajority of them, except for the cores of both clusters, were foundunlikely to be associated with the clusters. The core of NGC 6397 showsoptically thin thermal plasma emission with a temperature of kTi gtrsim12 keV. This result supports the results of HST, ROSAT, and Chandra,indicating that the main constituent of the NGC 6397 core is cataclysmicvariables (CVs). The high temperature suggests it to be magnetic CVs,which supports the result of the HST spectroscopy. The core of M 4 wasunambiguously detected at a gtrsim 5 σ confidence level for thefirst time. Although we cannot carry out a detailed spectral analysis onthe M 4 core due to a statistical limitation, the hardness ratio of thedetected photons is also consistent with CVs. We compared the ASCAfluxes of the core sources to those from other X-ray observations. Theabsorption-corrected 0.5-10keV flux of the NGC 6397 core with ASCA wasobtained to be 2.5 × 10-12 erg cm-2s-1, and had been constant within a factor of ˜ 2during 1993-2001. The M 4 core flux obtained under the same condition is2.6 × 10-13 erg cm-2 s-1, whichseems to be ˜ 4 times as high as the ROSAT HRI flux, although thepossibility of a constant flux between the ASCA and HRI observationscannot be excluded due to a statistical limitation. Spectral andtemporal properties of other out-of-core sources, including the radiogalaxy PKS 1620 ‑ 260 and the G0V star V972 Sco, are alsodescribed.
| 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-raysources in or near 55 globular clusters. New sources are found in thecores of NGC 362 (a double source), NGC 6121 (marginally significant),NGC 6139, and NGC 6266; and outside the cores of NGC 6205, NGC 6352 andNGC 6388. More accurate positions are determined for the X-ray sourcesin some ten clusters. The improved position for the source in NGC 6341excludes the suggested ultraviolet counterpart. It is shown that one ofthe two sources reported near the core of NGC 6626 is spurious, as isthe detection of a pulsar period in the PSPC data of this cluster; thecentral source is resolved in three sources. One source reportedpreviously in NGC 6304 is demoted to an upper limit. For 20 clustercores better upper limits to the X-ray luminosity are obtained. From astatistical analysis I argue that several sources outside the clustercores may well belong to the clusters. All spectral energy distributionsobserved so far are relatively soft, with bremsstrahlung temperatures =~0.9 keV; there is evidence however that bremsstrahlung spectra do notcorrectly describe the spectra. The X-ray luminosity per unit mass forthe cluster as a whole does not depend on the concentration; theluminosity per unit mass for the core may increase with the clusterconcentration.
| ROSAT observations of ten globular clusters with large core radii.
We present X-ray observations of ten globular clusters observed with theRosat PSPC. The clusters were chosen to have large core radii and to benearby. Three clusters contain X-ray sources which are probablyassociated with the cluster. One is the previously discovered X-raytransient H1825-331 in NGC 6652. The other two, in NGC 6366 and NGC6809, are new members of the class of low-luminosity sources, withluminosities in the region of 10^32^erg/s. Upper limits can be placed onthe source temperatures of both sources, making them similar to sourcesfound in other globular clusters by Rosat .
| uvby beta Photometry of Stars of "Astrophysical Interest"
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| HD147491 is Variable, but it is not a delta Scuti Star
Not Available
| A catalogue of variable stars in the lower instability strip.
Identifications, positions, photometry, spectra, some pulsationalfeatures, other astrophysical parameters and literature for 302pulsating variable stars in the lower instability strip, near the ZAMS,are given. About 185 stars have near homogeneous photometric informationin the Stroemgren's uvby-β photometric system. Thiscatalogue/database covers information published until November 1993.
| The 70th Name-List of Variable Stars
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| HD 147491 - a New delta Scuti Variable Star
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| A photoelectric BVRI sequence in the field of the globular cluster NGC 6121 (M4)
A photoelectric BVRI sequence of eleven stars homogeneously covering thefield of the Galactic globular cluster NGC 6121 (M4) is presented usingdata acquired in the Cousins BVRI photometric system. Three of the starsin the sequence are also on the list of Alcaino and Liller (1984), and acomparison is made with their data. This photometric sequence can beused in the calibration of future observations of NGC 6121 with otherdetectors.
| On the Distance of the Scorpio / Centaurus Dark Cloud
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| The c-m diagram of the globular cluster M4.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977A&AS...27..367L&db_key=AST
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