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|Constraints on Thermal X-Ray Radiation from SAX J1808.4-3658 and Implications for Neutron Star Neutrino Emission|
Thermal X-ray radiation from neutron star soft X-ray transients inquiescence provides the strongest constraints on the cooling rates ofneutron stars and thus on the interior composition and properties ofmatter in the cores of neutron stars. We analyze new (2006) and archival(2001) XMM-Newton observations of the accreting millisecond pulsar SAXJ1808.4-3658 in quiescence, which provide the most stringent constraintsto date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observationis consistent with a power law of photon index 1.83+/-0.17, withoutrequiring the presence of a blackbody-like component from a neutron staratmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-rayluminosity, at a level of 68+15-13% of thatinferred from the 2001 observation. Simultaneous fitting of allavailable XMM-Newton data allows a constraint on the quiescent neutronstar (0.01-10 keV) luminosity ofLNS<1.1×1031 ergs s-1. Thislimit excludes some current models of neutrino emission mediated by pioncondensates and provides further evidence of additional coolingprocesses, such as neutrino emission via direct Urca processes involvingnucleons and/or hyperons, in the cores of massive neutron stars.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.
|The INTEGRAL Galactic bulge monitoring program: the first 1.5 years|
Aims.The Galactic bulge region is a rich host of variable high-energypoint sources. Since 2005, February 17 we are monitoring the sourceactivity in the Galactic bulge region regularly and frequently, i.e.,about every three days, with the instruments onboard INTEGRAL. Thanks tothe large field of view, the imaging capabilities and the sensitivity athard X-rays, we are able to present for the first time a detailedhomogeneous (hard) X-ray view of a sample of 76 sources in the Galacticbulge region. Methods: We describe the successful monitoring programand show the first results from the start of the monitoring up to 2006,April 21, i.e., for a period of about one and a half year, during threevisibility seasons. We focus on the short (hour), medium (month) andlong-term (year) variability in the hard X-ray bands, i.e., 20-60 keVand 60-150 keV. When available, we discuss the simultaneous observationsin the soft X-ray, 3-10 keV and 10-25 keV, bands. Results: Almost allthe sources in the Galactic bulge region we detect in the 20-60 keV and60-150 keV bands are variable. During the last two and a half weeks ofthe third visibility season most of the known persistent (hard) X-raysources in the Galactic Center region were not detected. Of our sampleof sources, per visibility season we detect 32/33 sources in the 20-60keV band and 8/9 sources in the 60-150 keV band above a signal to noiseof 7. On average, we find per visibility season one active bright(⪆100 mCrab, 20-60 keV) black-hole candidate X-ray transient andthree active weaker (⪉25 mCrab, 20-60 keV) neutron star X-raytransients. Most of the time a clear anti-correlation can be seenbetween the soft and hard X-ray emission in some of the X-ray bursters.Hard X-ray flares or outbursts in X-ray bursters, which have a durationof the order of weeks are accompanied by soft X-ray drops. On the otherhand, hard X-ray drops can be accompanied by soft X-rayflares/outbursts. During the course of our program we found a number ofnew sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGRJ17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here onsome of the high-energy properties of these sources. Conclusions: Thehigh-energy light curves of all the sources in the field of view, andthe high-energy images of the region, are made available through theWWW, as soon as possible after the observations have been performed, athttp://isdc.unige.ch/Science/BULGE/.Appendices are only available in electronic form at http://www.aanda.org
|Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge|
We present near-IR color-magnitude diagrams and physical parameters fora sample of 24 Galactic globular clusters toward the bulge. In thispaper we discuss the properties of 12 new clusters (out of the 24) inaddition to those previously studied and published by our group. Thecompilation includes measurements of the cluster reddening, distance,photometric metallicity, horizontal branch red clump, and red giantbranch morphological (e.g., mean ridgelines) and evolutionary (e.g.,bump and tip) features. The compilation is available in electronic formon the World Wide Web, and it will be updated regularly.Based on data taken at the ESO New Technology Telescope, within theobserving programs 73.D-0313, 75.D-0372, and 77.D-0757.
|Chandra X-Ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)|
We report the detection of six discrete, low-luminosity(LX<1033 ergs s-1) X-ray sources,located within 12" of the center of the collapsed-core globular clusterM30 (NGC 7099), and a total of 13 sources within the half-mass radius,from a 50 ks Chandra ACIS-S exposure. Three sources lie within the verysmall upper limit of 1.9" on the core radius. The brightest of the threecore sources has a luminosity of LX(0.5-6keV)~6×1032 ergs s-1 and a blackbody-likesoft X-ray spectrum, which are both consistent with it being a quiescentlow-mass X-ray binary (qLMXB). We have identified optical counterpartsto four of the six central sources and a number of the outlying sources,using deep Hubble Space Telescope and ground-based imaging. While thetwo proposed counterparts that lie within the core may represent chancesuperpositions, the two identified central sources that lie outside ofthe core have X-ray and optical properties consistent with beingcataclysmic variables (CVs). Two additional sources outside of the corehave possible active binary counterparts. We discuss the X-ray sourcepopulation of M30 in light of its collapsed-core status.
|Integrated-Light Two Micron All Sky Survey Infrared Photometry of Galactic Globular Clusters|
We have mosaicked Two Micron All Sky Survey (2MASS) images to derivesurface brightness profiles in J, H, and Ks for 104 Galacticglobular clusters. We fit these with King profiles and show that thecore radii are identical to within the errors for each of these IRcolors and are identical to the core radii at V in essentially allcases. We derive integrated-light colors V-J, V-H, V-Ks, J-H,and J-Ks for these globular clusters. Each color shows areasonably tight relation between the dereddened colors and metallicity.Fits to these are given for each color. The IR - IR colors have verysmall errors, due largely to the all-sky photometric calibration of the2MASS survey, while the V-IR colors have substantially largeruncertainties. We find fairly good agreement with measurements ofintegrated-light colors for a smaller sample of Galactic globularclusters by M. Aaronson, M. Malkan, and D. Kleinmann from 1977. Ourresults provide a calibration for the integrated light of distantsingle-burst old stellar populations from very low to solarmetallicities. A comparison of our dereddened measured colors withpredictions from several models of the integrated light of single-burstold populations shows good agreement in the low-metallicity domain forV-Ks colors but also shows an offset at a fixed [Fe/H] of~0.1 mag in J-Ks, which we ascribe to photometric systemtransformation issues. Some of the models fail to reproduce the behaviorof the integrated-light colors of the Galactic globular clusters nearsolar metallicity.
|VLT-UVES abundance analysis of four giants in NGC 6553|
Context: .Metal-rich globular clusters trace the formation of bulges.Abundance ratios in the metal-rich globular clusters such as NGC 6553can constrain the formation timescale of the Galactic bulge. Aims: .The purpose of this study is to determine the metallicity andelemental ratios in individual stars of the metal-rich bulge globularcluster NGC 6553. Methods: .A detailed abundance analysis of fourgiants in NGC 6553 is carried out, based on optical high-resolutionéchelle spectra obtained with UVES at the ESO VLT-UT2 Kueyentelescope. Results: .A metallicity [Fe/H] = -0.20 dex is derived,together with α-element enhancement of Mg and Si ([Mg/Fe] = +0.28,[Si/Fe] = +0.21), solar Ca and Ti ([Ca/Fe] = +0.05, [Ti/Fe] = -0.01),and a mild enhancement of the r-process element Eu with [Eu/Fe] = +0.10.A mean heliocentric radial velocity of -1.86 km s-1 ismeasured. We compare our results with previous investigations of thecluster.
|Cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29|
We present Chandra and XMM-Newton X-ray observations that monitor theneutron star cooling of the quasi-persistent neutron star X-raytransients KS 1731-260 and MXB 1659-29 for approximately 4 yr afterthese sources returned to quiescence from prolonged outbursts. In bothsources the outbursts were long enough to significantly heat the neutronstar crust out of thermal equilibrium with the core. We analyse theX-ray spectra by fitting absorbed neutron star atmosphere models to theobservations. The results of our analysis strengthen the preliminaryfindings of Wijnands et al. that in both sources the neutron star crustcools down very rapidly suggesting it has a high heat conductivity andthat the neutron star core requires enhanced core cooling processes.Importantly, we now detect the flattening of the cooling in both sourcesas the crust returns to thermal equilibrium with the core. We measurethe thermal equilbrium flux and temperature in both sources by fitting acurve that decays exponentially to a constant level. The cooling curvescannot be fit with just a simple exponential decay without the constantoffset. We find the constant bolometric flux and effective temperaturecomponents to be (9.2 +/- 0.9) ×10-14ergcm-2 s-1 and 70.0 +/- 1.6 eV inKS 1731-260 and (1.7 +/- 0.3) × 10-14ergcm-2s-1 and 51.6 +/- 1.4 eV in MXB 1659-29. We note that thesevalues are dependent on the assumed distance to the sources and thecolumn density which was tied between the observations due to the lownumber of photons in the latter observations. However, importantly, theshape of the cooling curves is independent of the distance assumed. Inaddition, we find that the crust of KS 1731-260 cools faster than thatof MXB 1659-29 by a factor of ~2, likely due to different crustalproperties. This is the first time that the cooling of a neutron starcrust into thermal equilibrium with the core has been observed in suchdetail.
|An Empirical Tool to Derive Metallicity, Reddening, and Distance for Old Stellar Populations from Near-Infrared Color-Magnitude Diagrams|
We present an empirical method to derive photometric metallicity,reddening, and distance to old stellar populations by using a few majorfeatures of the red giant branch (RGB) in near-IR color-magnitudediagrams. We combine the observed RGB features with a set of equationslinking the global metallicity [M/H] to suitable RGB parameters (colors,magnitudes, and slope), as calibrated from a homogeneous sample ofGalactic globular clusters with different metallicities. This techniquecan be applied to efficiently derive the main population parameters ofold stellar systems, with the goal of using ground-based adaptive opticsand space facilities to probe the stellar content of remote galaxies.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG), operated on the island La Palma bythe Fundacion Galileo Galilei of INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|Dynamical Formation of Close Binaries in Globular Clusters: Cataclysmic Variables|
We answer the long-standing question of which production mechanism isresponsible for the cataclysmic variables (CVs) in globular clusters.Arguments have been given that range from mostly primordial presence toa significant contribution of later dynamical formation in close stellarencounters. We conclude, based on a thorough analysis of a homogeneousChandra data set, that the majority of CVs in dense globular clustershave a dynamical origin.
|A Chandra X-ray observation of the globular cluster Terzan 1|
We present a ~19-ks Chandra Advanced CCD Imaging Spectrometer (ACIS)-Sobservation of the globular cluster Terzan 1. 14 sources are detectedwithin 1.4arcmin of the cluster centre with two of these sourcespredicted to be not associated with the cluster (background activegalactic nuclei or foreground objects). The neutron star X-raytransient, X1732-304, has previously been observed in outburst withinthis globular cluster with the outburst seen to last for at least 12yr.Here, we find four sources that are consistent with the ROSAT positionfor this transient, but none of the sources are fully consistent withthe position of a radio source detected with the Very Large Array thatis likely associated with the transient. The most likely candidate forthe quiescent counterpart of the transient has a relatively softspectrum and an unabsorbed 0.5-10keV luminosity of 2.6 ×1032 erg s-1, quite typical of other quiescentneutron stars. Assuming standard core cooling, from the quiescent fluxof this source we predict long (>400yr) quiescent episodes to allowthe neutron star to cool. Alternatively, enhanced core cooling processesare needed to cool down the core. However, if we do not detect thequiescent counterpart of the transient this gives an unabsorbed0.5-10keV luminosity upper limit of 8 × 1031 ergs-1. We also discuss other X-ray sources within the globularcluster. From the estimated stellar encounter rate of this cluster wefind that the number of sources we detect is significantly higher thanexpected by the relationship of Pooley et al.
|An Empirical Calibration of the Mixing-Length Parameter α|
We present an empirical calibration of the mixing-length free parameterα based on a homogeneous infrared database of 28 Galactic globularclusters spanning a wide metallicity range (-2.15<[Fe/H]<-0.2).Empirical estimates of the red giant effective temperatures have beenobtained from infrared colors. Suitable relations linking thesetemperatures to the cluster metallicity have been obtained and comparedto theoretical predictions. An appropriate set of models for the Sun andPopulation II giants has been computed by using both the standard solarmetallicity (Z/X)solar=0.0275 and the most recently proposedvalue (Z/X)solar=0.0177. We find that when the standard solarmetallicity is adopted, a unique value of α=2.17 can be used toreproduce both the solar radius and the Population II red gianttemperature. Conversely, when the new solar metallicity is adopted, twodifferent values of α are required: α=1.86 to fit the solarradius and α~2.0 to fit the red giant temperatures. However, itmust be noted that regardless the adopted solar reference, theα-parameter does not show any significant dependence onmetallicity.Based on observations collected at the European Southern Observatory(ESO), La Silla, Chile. Also based on observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundacion Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
|Globular cluster system and Milky Way properties revisited|
Aims.Updated data of the 153 Galactic globular clusters are used toreaddress fundamental parameters of the Milky Way, such as the distanceof the Sun to the Galactic centre, the bulge and halo structuralparameters, and cluster destruction rates. Methods: .We build areduced sample that has been decontaminated of all the clusters youngerthan 10 Gyr and of those with retrograde orbits and/or evidence ofrelation to dwarf galaxies. The reduced sample contains 116 globularclusters that are tested for whether they were formed in the primordialcollapse. Results: .The 33 metal-rich globular clusters([Fe/H]≥-0.75) of the reduced sample basically extend to the Solarcircle and are distributed over a region with the projected axial-ratiostypical of an oblate spheroidal, Δ x:Δ y:Δz≈1.0:0.9:0.4. Those outside this region appear to be related toaccretion. The 81 metal-poor globular clusters span a nearly sphericalregion of axial-ratios ≈1.0:1.0:0.8 extending from the central partsto the outer halo, although several clusters in the external regionstill require detailed studies to unravel their origin as accretion orcollapse. A new estimate of the Sun's distance to the Galactic centre,based on the symmetries of the spatial distribution of 116 globularclusters, is provided with a considerably smaller uncertainty than inprevious determinations using globular clusters, R_O=7.2±0.3 kpc.The metal-rich and metal-poor radial-density distributions flatten forR_GC≤2 kpc and are represented well over the full Galactocentricdistance range both by a power-law with a core-like term andSérsic's law; at large distances they fall off as R-3.9. Conclusions: .Both metallicity components appearto have a common origin that is different from that of the dark matterhalo. Structural similarities between the metal-rich and metal-poorradial distributions and the stellar halo are consistent with a scenariowhere part of the reduced sample was formed in the primordial collapseand part was accreted in an early period of merging. This applies to thebulge as well, suggesting an early merger affecting the central parts ofthe Galaxy. The present decontamination procedure is not sensitive toall accretions (especially prograde) during the first Gyr, since theobserved radial density profiles still preserve traces of the earliestmerger(s). We estimate that the present globular cluster populationcorresponds to ≤23±6% of the original one. The fact that thevolume-density radial distributions of the metal-rich and metal-poorglobular clusters of the reduced sample follow both a core-likepower-law, and Sérsic's law indicates that we are dealing withspheroidal subsystems at all scales.
|The XMM-Newton/Chandra monitoring campaign of the Galactic center region. Description of the program and preliminary results|
We present the first results of our X-ray monitoring campaign on a 1.7square degree region centered on Sgr A* using the satellites XMM-Newtonand Chandra. The purpose of this campaign is to monitor the behavior(below 10 keV) of X-ray sources (both persistent and transient) whichare too faint to be detected by monitoring instruments aboard othersatellites currently in orbit (e.g., Rossi X-ray Timing Explorer;INTEGRAL). Our first monitoring observations (using the HRC-I aboardChandra) were obtained on June 5, 2005. Most of the sources detectedcould be identified with foreground sources, such as X-ray active stars.In addition we detected two persistent X-ray binaries (1E 1743.1-2843;1A 1742-294), two faint X-ray transients (GRS 1741.9-2853; XMMJ174457-2850.3), as well as a possible new transient source at aluminosity of a few times 1034 erg s-1. We reporton the X-ray results on these systems and on the non-detection of thetransients in follow-up radio data using the Very Large Array. Wediscuss how our monitoring campaign can help to improve ourunderstanding of the different types of X-ray transients (i.e., the veryfaint ones).
|Nearby Spiral Globular Cluster Systems. I. Luminosity Functions|
We compare the near-infrared (JHK) globular cluster luminosity functions(GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies.We obtained near-infrared photometry with the Persson's AuxiliaryNasmyth Infrared Camera on the Baade Telescope for 38 objects (mostlyglobular cluster candidates) in the Sculptor Group. We also havenear-infrared photometry from the Two Micron All Sky Survey (2MASS)-6Xdatabase for 360 M31 globular cluster candidates and aperture photometryfor 96 Milky Way globular cluster candidates from the 2MASS All-Sky andSecond Incremental Release databases. The M31 6X GCLFs peak at absolutereddening-corrected magnitudes of MJ0=-9.18,MH0=-9.73, and MK0=-9.98.The mean brightness of the Milky Way objects is consistent with that ofM31 after accounting for incompleteness. The average Sculptor absolutemagnitudes (correcting for relative distance from the literature andforeground reddening) are MJ0=-9.18,MH0=-9.70, and MK0=-9.80.NGC 300 alone has absolute foreground-dereddened magnitudesMJ0=-8.87, MH0=-9.39, andMK0=-9.46 using the newest Gieren et al. distance.This implies either that the NGC 300 GCLF may be intrinsically fainterthan that of the larger galaxy M31 or that NGC 300 may be slightlyfarther away than previously thought. Straightforward application of ourM31 GCLF results as a calibrator gives NGC 300 distance moduli of26.68+/-0.14 using J, 26.71+/-0.14 using H, and 26.89+/-0.14 using K.Data for this project were obtained at the Baade 6.5 m telescope, LasCampanas Observatory, Chile.
|RR Lyrae-based calibration of the Globular Cluster Luminosity Function|
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
|Chandra observations of the globular cluster M 54|
We have carried out a Chandra observation of the globular cluster M 54.We detected 7 sources located within the half-mass radius of M 54, at aflux limit of 1.5 × 10-15 erg s-1cm-2 in the 0.3-8 keV energy band. The spatial distributionand the colour/spectral properties of the 7 sources suggest that theyare likely to be cataclysmic variables or LMXBs in the globular cluster.M 54 shows the largest number of X-ray sources with luminosities greaterthan 1032 erg s-1 compared to other globularclusters observed using Chandra and XMM-Newton. We searched for acorrelation between the number of sources above this luminosity levelwith globular cluster parameters. We found evidence that the number ofsources peaks at a King concentration parameter c ~ 1.7-1.9, withglobular clusters which are core-collapsed or have low-c values having asmaller number of sources. We speculate on possible reasons for this.
|Resolved Massive Star Clusters in the Milky Way and Its Satellites: Brightness Profiles and a Catalog of Fundamental Parameters|
We present a database of structural and dynamical properties for 153spatially resolved star clusters in the Milky Way, the Large and SmallMagellanic Clouds, and the Fornax dwarf spheroidal. This databasecomplements and extends others in the literature, such as those ofHarris and Mackey & Gilmore. Our cluster sample comprises 50 ``youngmassive clusters'' in the LMC and SMC, and 103 old globular clustersbetween the four galaxies. The parameters we list include central andhalf-light-averaged surface brightnesses and mass densities; core andeffective radii; central potentials, concentration parameters, and tidalradii; predicted central velocity dispersions and escape velocities;total luminosities, masses, and binding energies; central phase-spacedensities; half-mass relaxation times; and ``κ-space'' parameters.We use publicly available population-synthesis models to computestellar-population properties (intrinsic B-V colors, reddenings, andV-band mass-to-light ratios) for the same 153 clusters plus another 63globulars in the Milky Way. We also take velocity-dispersionmeasurements from the literature for a subset of 57 (mostly old)clusters to derive dynamical mass-to-light ratios for them, showing thatthese compare very well to the population-synthesis predictions. Thecombined data set is intended to serve as the basis for futureinvestigations of structural correlations and the fundamental plane ofmassive star clusters, including especially comparisons between thesystemic properties of young and old clusters.The structural and dynamical parameters are derived from fitting threedifferent models-the modified isothermal sphere of King; an alternatemodified isothermal sphere based on the ad hoc stellar distributionfunction of Wilson; and asymptotic power-law models withconstant-density cores-to the surface-brightness profile of eachcluster. Surface-brightness data for the LMC, SMC, and Fornax clustersare based in large part on the work of Mackey & Gilmore, but includesignificant supplementary data culled from the literature and importantcorrections to Mackey & Gilmore's V-band magnitude scale. Theprofiles of Galactic globular clusters are taken from Trager et al. Weaddress the question of which model fits each cluster best, finding inthe majority of cases that the Wilson models-which are spatially moreextended than King models but still include a finite, ``tidal'' cutoffin density-fit clusters of any age, in any galaxy, as well as or betterthan King models. Untruncated, asymptotic power laws often fit about aswell as Wilson models but can be significantly worse. We argue that theextended halos known to characterize many Magellanic Cloud clusters maybe examples of the generic envelope structure of self-gravitating starclusters, not just transient features associated strictly with youngage.
|The discovery of X-ray binaries in the Sculptor dwarf spheroidal galaxy|
We report the results of a deep Chandra survey of the Sculptor dwarfspheroidal galaxy. We find five X-ray sources with LX of atleast 6 × 1033 ergs-1 with opticalcounterparts establishing them as members of Sculptor. These X-rayluminosities indicate that these sources are X-ray binaries, as no otherknown class of Galactic point sources can reach 0.5-8 keV luminositiesthis high. Finding these systems proves definitively that such objectscan exist in an old stellar population without stellar collisions. Threeof these objects have highly evolved optical counterparts (giants orhorizontal branch stars), as do three other sources whose X-rayluminosities are in the range which includes both quiescent low-massX-ray binaries and the brightest magnetic cataclysmic variables. Wepredict that large area surveys of the Milky Way should also turn uplarge numbers of quiescent X-ray binaries.
|Chandra observations of the millisecond X-ray pulsar IGR J00291+5934 in quiescence|
In this paper we report on our analysis of three Chandra observations ofthe accretion-powered millisecond X-ray pulsar IGR J00291+5934 obtainedduring the late stages of the 2004 outburst. We also report theserendipitous detection of the source in quiescence by ROSAT during MJD48830-48839 (1992 July 26-August 4). The detected 0.3-10 keV sourcecount rates varied significantly between the Chandra observations from(7.2 +/- 1.2) × 10-3, (6.8 +/- 0.9) ×10-3 and (1.4 +/- 0.1) × 10-2 countss-1 for the first, second and third Chandra observations, onMJD 53371.88 (2005 January 1), 53383.99 (2005 January 13) and 53407.57(2005 February 6), respectively. The count rate for the thirdobservation is 2.0 +/- 0.4 times as high as that of the average of thefirst two observations. The unabsorbed 0.5-10 keV source fluxes for thebest-fitting power-law model to the source spectrum were (7.9 +/- 2.5)× 10-14, (7.3 +/- 2.0) × 10-14, and(1.17 +/- 0.22) × 10-13 erg cm-2s-1 for the first, second and third Chandra observations,respectively. We find that this source flux is consistent with thatfound by ROSAT[~(5.4 +/- 2.4) × 10-14 ergcm-2 s-1]. Under the assumption that theinterstellar extinction, NH, does not vary between theobservations, we find that the blackbody temperature during the secondChandra observation is significantly higher than that during the firstand third observations. Furthermore, the effective temperature of theneutron star derived from fitting an absorbed blackbody or neutron staratmosphere model to the data is rather high in comparison with manyother neutron star soft X-ray transients in quiescence, even during thefirst and third observations. If we assume that the source quiescentluminosity is similar to that measured for two other accretion poweredmillisecond pulsars in quiescence, the distance to IGR J00291+5934 is2.6-3.6 kpc.
|4U 1715-390 and NGC 6440 in Outburst|
RXTE Proportional Counter Array scans of the galactic bulge region havebeen continuing. The burster in the globular cluster NGC 6440 (MXB1746-20) appears to be entering outburst. The previous outburst was inOctober 2001. The 2-10 keV fluxes on May 12.6, 15.6 and 16.1 were 5.4+/- 1.0, 7 +/- 3 and 21 +/- 1 mCrab, respectively. We appreciateinquiries by J. in 't Zand and R. Wijnands about this source.
|VLT-UVES analysis of 5 giants in 47 Tucanae|
High resolution spectra of 5 giants, including a horizontal branch star,of the metal-rich globular cluster 47 Tucanae were obtained with theUVES spectrograph at the 8 m VLT UT2-Kueyen telescope. The atmosphericparameters (T_eff, log g, [Fe/H], v_t) were derived from VIJK photometryand spectroscopic data based on Fe I and Fe II lines. Fe I and Fe IIiron abundances and respective total errors [Fe I/H] = -0.66±0.12 and [Fe II/H] = -0.69 ± 0.24 are found. Abundancesof α (O, Mg, Ca, Si, Ti), odd-Z (Na, Al), s- (Ba, La, Zr), andr-process (Eu) elements were determined by means of spectrum synthesis.The main results are [O/Fe] = +0.35 ± 0.11, [Mg/Fe] ≈ [Si/Fe]≈ [Ti/Fe] = +0.23 ± 0.17, [Ca/Fe] = 0.0 ± 0.15, [Ba/Fe]= +0.31 ± 0.22 and [Eu/Fe] = +0.33 ± 0.10. Overabundancesof the α-elements O, Mg, Si and Ti, and of Eu are similar to thoseseen in halo metal-poor stars, whereas a solar Ca-to-Fe ratio resemblesthe values found in bulge stars. An overall metallicity Z=0.006 or [M/H]= -0.45 is thus obtained, as is a mean heliocentric radial velocityv_r^hel = -22.43 ± 1.99 km s-1.
|Three Additional Quiescent Low-Mass X-Ray Binary Candidates in 47 Tucanae|
We identify through their X-ray spectra one certain (W37) and twoprobable (W17 and X4) quiescent low-mass X-ray binaries (qLMXBs)containing neutron stars in a long Chandra X-ray exposure of theglobular cluster 47 Tucanae, in addition to the two previously knownqLMXBs. W37's spectrum is dominated by a blackbody-like componentconsistent with radiation from the hydrogen atmosphere of a 10 kmneutron star. W37's light curve shows strong X-ray variability, which weattribute to variations in its absorbing column depth, and eclipses witha probable 3.087 hr period. For most of our exposures, W37'sblackbody-like emission (assumed to be from the neutron star surface) isalmost completely obscured, yet some soft X-rays (of uncertain origin)remain. Two additional candidates, W17 and X4, present X-ray spectradominated by a harder component, fitted by a power law of photon index~1.6-3. An additional soft component is required for both W17 and X4,which can be fitted with a 10 km hydrogen atmosphere neutron star model.X4 shows significant variability, which may arise from either itspower-law or hydrogen atmosphere spectral component. Both W17 and X4show rather low X-ray luminosities,LX(0.5-10keV)~5×1031 ergs s-1.All three candidate qLMXBs would be difficult to identify in otherglobular clusters, suggesting an additional reservoir of fainter qLMXBsin globular clusters that may be of similar numbers as the group ofpreviously identified objects. The number of millisecond pulsarsinferred to exist in 47 Tuc is less than 10 times larger than the numberof qLMXBs in 47 Tuc, indicating that for typical inferred lifetimes of10 and 1 Gyr, respectively, their birthrates are comparable.
|The X-ray source population of the globular cluster M15: Chandra high-resolution imaging|
The globular cluster M15 was observed on three occasions with the HighResolution Camera on-board Chandra in 2001 in order to investigate theX-ray source population in the cluster centre. After subtraction of thetwo bright central sources, four faint sources were identified within 50arcsec of the core. One of these sources is probably the planetarynebula K648, making this the first positive detection of X-rays from aplanetary nebula inside a globular cluster. Another two are identifiedwith UV variables (one previously known), which we suggest arecataclysmic variables (CVs). The nature of the fourth source is moredifficult to ascertain, and we discuss whether it is possibly aquiescent soft X-ray transient or also a CV.
|X-Ray Variability during the Quiescent State of the Neutron Star X-Ray Transient in the Globular Cluster NGC 6440|
The globular cluster NGC 6440 is known to harbor a bright neutron starX-ray transient. We observed the globular cluster with Chandra on twooccasions when the bright transient was in its quiescent state, in 2000July and 2003 June (both observations were made nearly 2 yr after theend of their preceding outbursts). The quiescent spectrum during thefirst observation is well represented by a two-component model (aneutron star atmosphere model plus a power-law component that dominatesat energies above 2 keV). During the second observation (which wasroughly of equal duration to the first observation) we found that thepower-law component could no longer be detected. Our spectral fitsindicate that the effective temperature of the neutron star surface wasconsistent between the two observations. We conclude that the effect ofthe change in the power-law component caused the 0.5-10 keV flux to be afactor of ~2 lower during the second observation compared to the firstobservation. We discuss plausible explanations for the variations,including variable residual accretion onto the neutron starmagnetosphere or some variation in the interaction of the pulsar windwith the matter still outflowing from the companion star.
|Infrared Photometry of NGC 6791|
We present deep JHK photometry of the old and metal-rich open clusterNGC 6791. The photometry reaches below the main-sequence turnoff toK~16.5 mag. We combine our photometry with that from Stetson et al. toprovide color-magnitude diagrams showing K versus J-K, K versus V-K, andV versus V-K. We study the slope of the red giant branch in the infraredbut find that it is not a useful metallicity indicator for the cluster,nor any metal-rich cluster that lacks a well-populated red giant branch,because it is not linear, as has often been assumed, in K versus J-K.The mean color of the red horizontal-branch/red clump stars provide anestimate of the cluster reddening, E(B-V)=0.14+/-0.04 mag for[Fe/H]=+0.4+/-0.1. The mean magnitudes of these stars also provide agood distance estimate, (m-M)0=13.07+/-0.04. Finally, we findthat the isochrones of Yi et al. provide optimal fits in V versus B-Vand V-K and K versus J-K and V-K for such values if [Fe/H] lies between+0.3 and +0.5 (with a slight preference for +0.5) and ages between 9 Gyr([Fe/H]=+0.3) and 7.5 Gyr ([Fe/H]=+0.5).Based on observations made with the Mayall 4 m Telescope of the NationalOptical Astronomy Observatory.
|The Hard Quiescent Spectrum of the Neutron Star X-Ray Transient EXO 1745-248 in the Globular Cluster Terzan 5|
We present a Chandra observation of the globular cluster Terzan 5 duringtimes when the neutron star X-ray transient EXO 1745-248 located in thiscluster was in its quiescent state. We detected the quiescent systemwith a (0.5-10 keV) luminosity of ~2×1033 ergss-1. This is similar to several other neutron-star transientsobserved in their quiescent states. However, the quiescent X-rayspectrum of EXO 1745-248 was dominated by a hard power-law componentinstead of the soft component that usually dominates the quiescentemission of other neutron star X-ray transients. This soft componentcould not conclusively be detected in EXO 1745-248, and we conclude thatit contributed at most 10% of the quiescent flux in the energy range0.5-10 keV. EXO 1745-248 is only the second known neutron-star transientwhose quiescent spectrum is dominated by the hard component (SAXJ1808.4-3658 is the other one). We discuss possible explanations forthis unusual behavior of EXO 1745-248, its relationship to otherquiescent neutron-star systems, and the impact of our results onunderstanding quiescent X-ray binaries. We also discuss the implicationsof our results on the way that the low-luminosity X-ray sources inglobular clusters are classified.
|Deep ALTAIR+NIRI Imaging of the Disk and Bulge of M31|
Deep J, H, and K' images, recorded with the ALTAIR adaptive opticssystem and NIRI imager on Gemini North, are used to probe the stellarcontent of the disk and bulge of the Local Group galaxy M31. With anFWHM near 0.08" in K, these are the highest angular resolutionnear-infrared images yet obtained of this galaxy. One field samples theouter disk of M31 at a galactocentric distance of roughly 62' along themajor axis. The mean metallicity in this field is close to that of themetal-rich globular cluster NGC 6528, and no stars with [Fe/H]<-0.7are detected. Another field, located on the major axis 9' from thegalaxy center, contains a roughly equal mix of disk and bulge stars. Thered giant branch (RGB) in this field is redder than that of NGC 6528,although it is argued that reddening internal to M31 may be significantin this region of the galaxy. The remaining two fields, located atprojected galactocentric distances of 2' and 4', are dominated by bulgestars. The RGB tip occurs between K=17.0 and 17.2, and the color of theRGB in the field closest to the center of M31 is consistent with that ofNGC 6528. After accounting for random photometric errors, the upper RGBin each field has a width on the (K, J-K) color-magnitude diagrams thatis consistent with a +/-0.5 dex dispersion in [Fe/H], in rough agreementwith what is seen in other disk and spheroid fields in M31. The numberof bright asymptotic giant branch (AGB) and RGB stars also scales withthe r-band surface brightness in all four fields. Thus, we conclude thatthe stellar content does not change markedly from field to field andthat the photometric properties of the brightest AGB stars in the twoinnermost fields are not affected significantly by crowding. Thebrightest star has MK=-8.6 and Mbol=-5.2, althoughthis may not be a reliable measure of the AGB-tip brightness because ofphotometric variability. A population of very bright red stars, which weidentify as C stars, are seen in the three fields that are closest tothe center of M31. The spatial distribution of these objects suggestthat they are well mixed throughout this part of M31, and thus theylikely did not form in a compact region near the galactic nucleus butmore probably in the inner disk. We speculate that these C stars may bethe most luminous members of the intermediate-age population that hasbeen detected previously in studies of the integrated spectrum of thecentral regions of M31.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the National Science Foundation(NSF) on behalf of the Gemini partnership: the NSF (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council of Canada (Canada), CONICYT (Chile), theAustralian Research Council (Australia), CNPq (Brazil), and CONICET(Argentina).
|A robust method for the analysis of integrated spectra from globular clusters using Lick indices|
We define a method for the analysis of the integrated spectra ofextragalactic globular clusters that provides more reliable measures ofthe age, metallicity and α-element abundance ratio than have sofar been achieved. The method involves the simultaneous fitting of up to25 Lick indices in a χ2 fitting technique that maximizesthe use of the available data. Here we compare three sets of singlestellar population (SSP) models of Lick indices to the highsignal-to-noise, integrated spectra of 20 Galactic globular clusters.The ages, [Fe/H] and α-element abundance ratios derived from theSSP models are compared with the results of resolved stellar populationstudies from the literature. We find good consistency with the publishedvalues, with an agreement of better than 0.1 dex in all three derivedparameters. The technique allows the identification of abundance ratioanomalies, such as the known nitrogen overabundance in Galactic globularclusters, and the presence of anomalous horizontal branch morphologies.It also minimizes the impact on the derived parameters of imperfectcalibration to the Lick system, and reduction errors in general. Themethod defined in this paper is therefore robust with respect to many ofthe difficulties that plague the application of SSP models in general.Consequently, it is well suited to the study of extragalactic globularcluster systems.
|Ages and metallicities of star clusters: New calibrations and diagnostic diagrams from visible integrated spectra|
We present homogeneous scales of ages and metallicities for starclusters from very young objects, through intermediate-age ones up tothe oldest known clusters. All the selected clusters have integratedspectra in the visible range, as well as reliable determinations oftheir ages and metallicities. From these spectra equivalent widths (EWs)of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ andHβ Balmer lines have been measured homogeneously. The analysis ofthese EWs shows that the EW sums of the metallic and Balmer H lines,separately, are good indicators of cluster age for objects younger than10 Gyr, and that the former is also sensitive to cluster metallicity forages greater than 10 Gyr. We propose an iterative procedure forestimating cluster ages by employing two new diagnostic diagrams and agecalibrations based on the above EW sums. For clusters older than 10 Gyr,we also provide a calibration to derive their overall metal contents.
|Red giant branch in near-infrared colour-magnitude diagrams - II. The luminosity of the bump and the tip|
We present new empirical calibrations of the red giant branch (RGB) bumpand tip based on a homogeneous near-infrared data base of 24 Galacticglobular clusters. The luminosities of the RGB bump and tip in the J, Hand K bands and their dependence on the cluster metallicity have beenstudied, yielding empirical relationships. By using recenttransformations between the observational and theoretical planes, wealso derived similar calibrations in terms of bolometric luminosity.Direct comparisons between updated theoretical models and observationsshow an excellent agreement. The empirical calibration of the RGB tipluminosity in the near-infrared passbands presented here is afundamental tool to derive distances to galaxies far beyond the LocalGroup, in view of using the new ground-based adaptive optics facilitiesand, in the near future, the James Webb Space Telescope.
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