Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

M 28



Upload your image

DSS Images   Other Images

Related articles

Completing the census of (bright) variable stars in galactic globular clusters .
We present a long-term project aimed at completing the census of(bright) variable stars in Galactic globular clusters. While our mainaim is to obtain a reliable assessment of the populations of RR Lyraeand type II Cepheid stars in the Galactic globular cluster system, dueattention is also being paid to other types of variables, including SXPhoenicis stars, long-period variables, and eclipsing binaries.

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.

Chandra X-Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae
We present spectral and long-timescale variability analyses of ChandraX-Ray Observatory ACIS-S observations of the 19 millisecond pulsars(MSPs) with precisely known positions in the globular cluster 47Tucanae. The X-ray emission of the majority of these MSPs is welldescribed by a thermal (blackbody or neutron star hydrogen atmosphere)spectrum with a temperature Teff~(1-3)×106K, emission radius Reff~0.1-3 km, and luminosityLX~1030-1031 ergs s-1. Forseveral MSPs, there are indications that a second thermal component isrequired, similar to what is seen in some nearby field MSPs. Theobserved radiation most likely originates from the heated magnetic polarcaps of the MSPs. The small apparent scatter in LX isconsistent with thermal emission from the polar caps of a global dipolefield, although the small emission areas may imply either a more complexsmall-scale magnetic field configuration near the neutron star surfaceor nonuniform polar cap heating. The radio eclipsing binary MSPs 47 TucJ, O, and W show a significant nonthermal (power-law) component, withspectral photon index Γ~1-1.5, which most likely originates in anintrabinary shock formed due to interaction between the relativisticpulsar wind and matter from the stellar companion. We reexamine theX-ray-spin-down luminosity relation (LX-E˙ relation) andfind that for the MSPs with thermal spectraLX~E˙β, where β~0.2+/-1.1. Due tothe large uncertainties in both parameters, the result is consistentwith both the linear LX-E˙ relation and the flatterLX~E˙0.5 predicted by polar cap heatingmodels. In terms of X-ray properties, we find no clear systematicdifferences between MSPs in globular clusters and in the field of theGalaxy. We discuss the implications of these results on the presentunderstanding of the X-ray emission properties of MSPs.

Pulsar Wind Nebulae and the Nonthermal X-Ray Emission of Millisecond Pulsars
The nonthermal, nonpulsed X-ray emission of MSPs is investigated. As inyoung pulsars, MSPs emit a relativistic wind, which in interacting withthe ISM and/or a binary companion can significantly contribute to thenonpulsed emission of these pulsars. An application and extension of asimple model developed for young pulsars is applied to the old recycledMSP B1957+20. It is found that the pulsar wind can indeed contribute toboth the resolved and unresolved X-ray emission. For other MSPs in theGalactic field for which the spectral index of the nonpulsed componenthas been measured (i.e., PSR B1937+21, PSR J0218+4232) the contributionof the pulsar wind to the nonpulsed X-ray luminosity is estimated. Forthe MSPs in the core regions of globular clusters, the pulsar windnebula is likely affected by its interaction with the dense stellarenvironment, possibly leading to a diminished contribution to the totalX-ray emission. In this case, the existence of nonthermal nonpulsedX-ray emission is more likely for binary than for isolated MSPs, withthe emission arising from the interaction of the relativistic pulsarwind and a binary companion. Our study suggests that the magnetizationparameter in the pulsar wind nebulae of MSPs is significantly largerthan that of the Crab Nebula, by about a factor of 10. The nebulaepowered by rapidly rotating neutron stars either in isolation or in thequiescent state of soft X-ray transients can contribute to the faintX-ray source population associated with nonaccreting neutron stars. Theemission from MSPs moving at high velocities (>~100 kms-1) through regions of the ISM characterized by magneticfield strengths <~0.1 mG may appear spatially extended with atail-like morphology. Thus, MSPs may also contribute to the faintfilamentary X-ray source subpopulation in the Galaxy.

Surface Brightness Profiles of Galactic Globular Clusters from Hubble Space Telescope Images
The Hubble Space Telescope (HST) allows us to study the central surfacebrightness profiles of globular clusters at unprecedented detail. Wehave mined the HST archives to obtain 38 WFPC2 images of Galacticglobular clusters with adequate exposure times and filters, which we useto measure their central structure. We outline a reliable method toobtain surface brightness profiles from integrated light that we test onan extensive set of simulated images. Most clusters have central surfacebrightness about 0.5 mag brighter than previous measurements made fromground-based data, with the largest differences around 2 mag. Includingthe uncertainties in the slope estimates, the surface brightness slopedistribution is consistent with half of the sample having flat cores andthe remaining half showing a gradual decline from 0 to -0.8[dlogΣ/dlogr)]. We deproject the surface brightness profiles in anonparametric way to obtain luminosity density profiles. Thedistribution of luminosity density logarithmic slopes shows similarfeatures, with half of the sample between -0.4 and -1.8. These resultsare in contrast to our theoretical bias that the central regions ofglobular clusters are either isothermal (i.e., flat central profiles) orvery steep (i.e., luminosity density slope approximately -1.6) forcore-collapse clusters. With only 50% of our sample having centralprofiles consistent with isothermal cores, King models appear torepresent most globular clusters in their cores poorly.

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.

Hot Populations in M87 Globular Clusters
To explore the production of UV-bright stars in old, metal-richpopulations like those in elliptical galaxies, we have obtained HubbleSpace Telescope (HST) Space Telescope Imaging Spectrograph far- andnear-UV photometry of globular clusters (GCs) in four fields in thegiant elliptical (gE) galaxy M87. To a limit of mFUV~25 wedetect a total of 66 GCs in common with the deep HST optical-band studyof Kundu et al. Despite strong overlap in V- and I-band properties, theM87 GCs have UV-optical properties that are distinct from clusters inthe Milky Way and in M31. M87 clusters, especially metal-poor ones,produce larger hot horizontal-branch populations than do Milky Wayanalogs. In color plots including the near-UV band, the M87 clustersappear to represent an extension of the Milky Way sequence. Cluster massis probably not a factor in these distinctions. The most metal-rich M87GCs in our sample are near solar metallicity and overlap the local Egalaxy sample in estimated Mg2 line indices. Nonetheless, theclusters produce much more UV light at a given Mg2, being upto 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do notappear to represent a transition between Milky Way-type clusters and Egalaxies. The differences are in the correct sense if the clusters aresignificantly older than the E galaxies.Comparisons with Galactic open clusters indicate that the hot stars lieon the extreme horizontal branch, rather than being blue stragglers, andthat the extreme horizontal branch becomes well populated for ages>~5 Gyr. Existing model grids for clusters do not match theobservations well, due to poorly understood giant branch mass loss orperhaps high helium abundances. We find that 41 of our UV detectionshave no optical-band counterparts. Most appear to be UV-brightbackground galaxies seen through M87. Eleven near-UV variable sourcesdetected at only one epoch in the central field are probably classicalnovae. Two recurrent variable sources have no obvious explanation butcould be related to activity in the relativistic jet.

Surface-brightness fluctuations in stellar populations. IAC-star models for the optical and near-IR wavelengths
Aims.A new theoretical calibration of surface-brightness fluctuations(SBF) for single age, single metallicity stellar populations ispresented for the optical and near-IR broad-band filters, as well as forthe HST WFPC2 and ACS filters. Methods: .The IAC-star code isused. Two Padua and the Teramo stellar evolution libraries have beenconsidered. A set of single-burst stellar populations (SSP) with a widerange of ages (3 Gy-15 Gy) and metallicities (Z = 0.0001-0.03) have beencomputed using each one of the three considered stellar evolutionlibraries. For each SSP, color indexes and SBF magnitudes are given forthe filters U, B, V, R, I, J, H, K, {F218W}, {F336W}, {F439W}, {F450W},{F555W} and {F814W}, and for the first time, an uncertainty has beenestimated for the SBF theoretical calibration. Results: .Althoughsome differences might be addressed, the Padua and Teramo stellarevolution libraries provide comparable SBF results. A detailedcomparison of the present SBF calibrations with both previouscalibrations and observational data is also presented. Comparing thedifferent models with observational data, Padua based models reproducefairly well the optical data for globular clusters, while Teramo basedmodels fits both optical galaxies and globular clusters data, as well.In the near-IR wavelengths, the Teramo based models provide the only SBFtheoretical calibration to date able to properly reproduce theobservational data for superclusters, with intermediate-to-lowmetallicity. As a conclusion, Teramo based models work better than anyother calibration reproducing observational data for the near-IRwavelengths. Furthermore, the age-metallicity degeneracy is broken forlow metallicity (Z≤0.0037) stellar populations. Finally, a clearrelation between the B SBF absolute magnitude of a stellar populationand its metallicity is found for intermediate to old populations, so theB-band fluctuation magnitude is proposed as a metallicity tracer. Thepresent theoretical calibration shows that the analysis of SBF providesa very powerful tool in the study and characterization of unresolvedstellar populations.

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.

Deepsky delights.
Not Available

On the origin of the radial mass density profile of the Galactic halo globular cluster system
We investigate what may be the origin of the presently observed spatialdistribution of the mass of the Galactic Old Halo globular clustersystem. We propose its radial mass density profile to be a relic of thedistribution of the cold baryonic material in the protogalaxy. Assumingthat this one arises from the profile of the whole protogalaxy minus thecontribution of the dark matter (and a small contribution of the hot gasby which the protoglobular clouds were bound), we show that the massdistributions around the Galactic centre of this cold gas and of the OldHalo agree satisfactorily. In order to demonstrate our hypothesis evenmore conclusively, we simulate the evolution with time, up to an age of15Gyr, of a putative globular cluster system whose initial massdistribution in the Galactic halo follows the profile of the coldprotogalactic gas. We show that beyond a galactocentric distance oforder 2-3kpc, the initial shape of such a mass density profile ispreserved despite the complete destruction of some globular clusters andthe partial evaporation of some others. This result is almostindependent of the choice of the initial mass function for the globularclusters, which is still ill determined. The shape of these evolvedcluster system mass density profiles also agrees with the presentlyobserved profile of the Old Halo globular cluster system, thusstrengthening our hypothesis. Our result might suggest that theflattening shown by the Old Halo mass density profile at short distancesfrom the Galactic centre is, at least partly, of primordial origin.

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.

A Library of Integrated Spectra of Galactic Globular Clusters
We present a new library of integrated spectra of 40 Galactic globularclusters, obtained with the Blanco 4 m telescope and the R-Cspectrograph at the Cerro Tololo Inter-American Observatory. The spectracover the range ~3350-6430 Å with ~3.1 Å (FWHM) resolution.The spectroscopic observations and data reduction were designed tointegrate the full projected area within the cluster core radii in orderto properly sample the light from stars in all relevant evolutionarystages. The S/N values of the flux-calibrated spectra range from 50 to240 Å-1 at 4000 Å and from 125 to 500Å-1 at 5000 Å. The selected targets span a widerange of cluster parameters, including metallicity, horizontal-branchmorphology, Galactic coordinates, Galactocentric distance, andconcentration. The total sample is thus fairly representative of theentire Galactic globular cluster population and should be valuable forcomparison with similar integrated spectra of unresolved stellarpopulations in remote systems. For most of the library clusters, ourspectra can be coupled with deep color-magnitude diagrams and reliablemetal abundances from the literature to enable the calibration ofstellar population synthesis models. In this paper we present a detailedaccount of the observations and data reduction. The spectral library ispublicly available in electronic format from the National OpticalAstronomical Observatory Web site.

Pulsar Timing and the Detection of Black Hole Binary Systems in Globular Clusters
The possible existence of intermediate-mass binary black holes (IMBBHs)in globular clusters (GCs) offers us a unique geometry in which todetect spacetime oscillations. For certain pulsar-IMBBH configurationspossible within a GC, the usual far-field plane wave approximation forthe IMBBH metric perturbation severely underestimates the induced pulsetime-of-arrival (TOA) fluctuations. In this Letter, the expected TOAfluctuations induced by an IMBBH lying close to the line of sightbetween a pulsar and the Earth are calculated for the first time. For anIMBBH consisting of 10 and 103 Msolar components,a 10 yr orbital period, and located 0.1 lt-yr from the Earth-pulsar lineof sight, the induced TOA fluctuations will be of order 5-500 ns.

High-Energy Emission from Millisecond Pulsars
The X-ray and γ-ray spectrum of rotation-powered millisecondpulsars is investigated in a model for acceleration and pair cascades onopen field lines above the polar caps. Although these pulsars have lowsurface magnetic fields, their short periods allow them to have largemagnetospheric potential drops, but the majority do not producesufficient pairs to completely screen the accelerating electric field.In these sources, the primary and secondary electrons continue toaccelerate to high altitude, and their Lorentz factors are limited bycurvature and synchrotron radiation reaction. The accelerating particlesmaintain high Lorentz factors and undergo cyclotron resonant absorptionof radio emission that produces and maintains a large pitch angle,resulting in a strong synchrotron component. The resulting spectraconsist of several distinct components: curvature radiation from primaryelectrons dominating from 1 to 100 GeV, synchrotron radiation fromprimary and secondary electrons dominating up to about 100 MeV, and muchweaker inverse Compton radiation from primary electrons at 0.1-1 TeV. Wefind that the relative size of these components depends on pulsarperiod, period derivative, and neutron star mass and radius, with thelevel of the synchrotron component also depending sensitively on theradio emission properties. This model is successful in describing theobserved X-ray and γ-ray spectrum of PSR J0218+4232 as synchrotronradiation, peaking around 100 MeV and extending up to a turnover aroundseveral GeV. The predicted curvature radiation components from a numberof millisecond pulsars, as well as the collective emission from themillisecond pulsars in globular clusters, should be detectable withAGILE and GLAST. We also discuss a hidden population of X-ray-quiet andradio-quiet millisecond pulsars that have evolved below the pair deathline, some of which may be detectable by telescopes sensitive above 1GeV.

The Australia Telescope National Facility Pulsar Catalogue
We have compiled a new and complete catalog of the main properties ofthe 1509 pulsars for which published information currently exists. Thecatalog includes all spin-powered pulsars, as well as anomalous X-raypulsars and soft gamma-ray repeaters showing coherent pulsed emission,but excludes accretion-powered systems. References are given for alldata listed. We have also developed a new World Wide Web interface foraccessing and displaying either tabular or plotted data with the optionof selecting pulsars to be displayed via logical conditions on parameterexpressions. The Web interface has an ``expert'' mode giving access to awider range of parameters and allowing the use of custom databases. Forusers with locally installed software and database on Unix or Linuxsystems, the catalog may be accessed from a command-line interface.C-language functions to access specified parameters are also available.The catalog is updated from time to time to include new information.

The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.

Using X-rays to Probe the Compact Binary Content of Globular Clusters
Globular clusters (GCs) harbour a large number of close binaries whichare hard to identify optically due to high stellar densities. Observingthese GCs in X-rays, in which the compact binaries are bright,diminishes the over-crowding problem. Using the new generation of X-rayobservatories, it is possible to identify populations of neutron starlow mass X-ray binaries, cataclysmic variables and millisecond pulsarsas well as other types of binaries. We present the spectra of a varietyof binaries that we have identified in four GCs observed by XMM-Newton.We show that through population studies we can begin to understand theformation of individual classes of binaries in GCs and hence start tounfold the complex evolutionary paths of these systems.

The pulsar observation and research with 50m radio telescope
The 50m radio telescope of the National Astronomical Observatories willwork for pulsar observations. This paper first reviews main results ofthe pulsar surveys and research since the first pulsar was discoveredfor 35 years. Recently observation and research of pulsars have become aremarkable task in astrophysics. Therefore the future work on pulsarobservations by using the 50m radio telescope is discussed in thispaper.

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.

Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo
We investigate the hypothesis that some fraction of the globularclusters presently observed in the Galactic halo formed in externaldwarf galaxies. This is done by means of a detailed comparison betweenthe `old halo', `young halo' and `bulge/disc' subsystems defined by Zinnand the globular clusters in the Large Magellanic Cloud, SmallMagellanic Cloud, and Fornax and Sagittarius dwarf spheroidal galaxies.We first use high-quality photometry from Hubble Space Telescope imagesto derive a complete set of uniform measurements of horizontal branch(HB) morphology in the external clusters. We also compile structural andmetallicity measurements for these objects and update the data base ofsuch measurements for the Galactic globular clusters, including newcalculations of HB morphology for 11 objects. Using these data togetherwith recent measurements of globular cluster kinematics and ages weexamine the characteristics of the three Galactic cluster subsystems.Each is quite distinct in terms of their spatial and age distributions,age-metallicity relationships, and typical orbital parameters, althoughwe observe some old halo clusters with ages and orbits more similar tothose of young halo objects. In addition, almost all of the Galacticglobular clusters with large core radii fall into the young halosubsystem, while the old halo and bulge/disc ensembles are characterizedby compact clusters. We demonstrate that the majority of the externalglobular clusters are essentially indistinguishable from the Galacticyoung halo objects in terms of HB morphology, but ~20-30 per cent ofexternal clusters have HB morphologies most similar to the Galactic oldhalo clusters. We further show that the external clusters have adistribution of core radii which very closely matches that for the younghalo objects. The old halo distribution of core radii can be very wellrepresented by a composite distribution formed from ~83-85 per cent ofobjects with structures typical of bulge/disc clusters, and ~15-17 percent of objects with structures typical of external clusters. Takentogether our results fully support the accretion hypothesis. We concludethat all 30 young halo clusters and 15-17 per cent of the old haloclusters (10-12 objects) are of external origin. Based on cluster numbercounts, we estimate that the Galaxy may have experienced approximatelyseven merger events with cluster-bearing dwarf-spheroidal-type galaxiesduring its lifetime, building up ~45-50 per cent of the mass of theGalactic stellar halo. Finally, we identify a number of old halo objectswhich have properties characteristic of accreted clusters. Several ofthe clusters associated with the recently proposed dwarf galaxy in CanisMajor fall into this category.

Radio emission as a test of the existence of intermediate-mass black holes in globular clusters and dwarf spheroidal galaxies
We take the established relation between black hole mass, X-rayluminosity and radio luminosity and show that intermediate-mass blackholes (IMBHs), such as those predicted to exist at the centres ofglobular clusters (GCs), will be easily identifiable objects in deepradio observations. We show that the radio observations will be far moresensitive than any possible X-ray observations. We also discuss thelikely optical photometric and spectroscopic appearance of such systemsin the event that radio detections are made.

On the origin of red giant depletion through low-velocity collisions
We investigate a means of explaining the apparent paucity of red giantstars within post-core-collapse globular clusters. We propose thatcollisions between the red giants and binary systems can lead to thedestruction of some proportion of the red giant population, by eitherknocking out the core of the red giant or by forming a common envelopesystem which will lead to the dissipation of the red giant envelope.Treating the red giant as two point masses, one for the core and anotherfor the envelope (with an appropriate force law to take account of thedistribution of mass), and the components of the binary system alsotreated as point masses, we utilize a four-body code to calculate thetime-scales on which the collisions will occur. We then perform a seriesof smooth particle hydrodynamics runs to examine the details of masstransfer within the system. In addition, we show that collisions betweensingle stars and red giants lead to the formation of a common envelopesystem which will destroy the red giant star. We find that low-velocitycollision between binary systems and red giants can lead to thedestruction of up to 13 per cent of the red giant population. This couldhelp to explain the colour gradients observed in PCC globular clusters.We also find that there is the possibility that binary systems formedthrough both sorts of collision could eventually come into contactperhaps producing a population of cataclysmic variables.

Investigating the Faint X-ray Sources in Globular Clusters with XMM-Newton
Globular clusters (GCs) harbour a large number of faint X-ray sourceswhose nature, until recently, was largely unknown. Using the new X-rayobservatories, it is possible to identify populations of low mass X-raybinaries, cataclysmic variables, millisecond pulsars, as well as othertypes of binaries belonging to the GCs, along with fore- and backgroundobjects. We present a variety of binaries, identified in four GCsobserved by XMM-Newton. We show that through population studies we canbegin to understand the formation of individual classes of binaries andhence start to unfold the complex evolutionary paths of such systems.

Chandra X-ray observations of the globular cluster M28 and optical HST identification of the X-ray sources
We present here the results of Chandra X-Ray Observatory observations ofNGC 6626 (M 28), and a first search for optical counterparts by usingarchival HST data. 46 sources have been detected within thefield-of-view of Chandra, 12 of which lie within 1 core radius from thecluster center. We measured for the first time the unconfused X-Rayspectrum of the ms pulsar B1821-24 and present spectral identificationfor other 5 bright unidentified sources. The brightest one showsspectral parameters suggesting that it is a transiently accretingneutron star in a low-mass X-ray binary in quiescent phase. Theapparently extended core emission seen by ROSAT is due to superpositionof multiple source for which the X-ray luminosity function isdetermined. Using Chandra derived position, we also present apreliminary report on the search for the optical counterparts of thesesources by using archival HST data.

Microsecond Timing of PSR B1821-24 with Chandra High Resolution Camera-S
We perform absolute timing of PSR B1821-24 in M28, using a 50 ksobservation with Chandra/HRC-S. We have obtained the highestsignal-to-noise ratio X-ray pulsed light curve of this source to date,detecting two X-ray pulses, as well as significant nonpulsed emission: apersistent X-ray flux that comprises 15%+/-3% of the total X-ray flux ofthe pulsar. The Gaussian width of the sharp X-ray peak is 34+/-3 μsin time, implying a size of the X-ray beam as it crosses the line ofsight of 4.0d+/-0.4d. We find evidence for a significant trailingcomponent in both X-ray peaks of the pulse profile. Including threeRXTE/PCA observations in our analysis and tying the phases togetherusing a radio ephemeris obtained at Nançay, we find that theabsolute phases in the X-ray wander with respect to this radio ephemerisby up to 60 μs, likely as a result of the variable dispersionmeasure, which changes the pulse arrival time in the radio band but notthe X-ray band. The present analysis makes clear that pulsar timingnoise properties in millisecond pulsars such as PSR B1821-24-hithertoonly studied at radio wavelengths, where variable dispersion measurerequires a significant correction-can be studied at X-ray wavelengths,where the effect of variable dispersion measure is negligible. We alsoexamine the known uncertainties in the absolute Chandra/HRC-S timingaccuracy, which amount to +/-12 μs. We limit the amount of lineardrift in the relative timing accuracy of HRC-S to less than3×10-10 s s-1.

A Microglitch in the Millisecond Pulsar PSR B1821-24 in M28
We report on the observation of a very small glitch observed for thefirst time in a millisecond pulsar, PSR B1821-24, located in theglobular cluster M28. Timing observations were mainly conducted with theNançay radio telescope (France), and confirmation comes from the140 ft radio telescope at Green Bank and the new Green Bank Telescopedata. This event is characterized by a rotation frequency step of 3 nHz,or 10-11 in fractional frequency change, along with a shortduration limited to a few days or a week. A marginally significantfrequency derivative step was also found. This glitch follows the maincharacteristics of those in the slow-period pulsars but is 2 orders ofmagnitude smaller than the smallest ever recorded. Such an event must bevery rare for millisecond pulsars since no other glitches have beendetected when the cumulated number of years of millisecond pulsar timingobservations up to 2001 is around 500 for all these objects. However,pulsar PSR B1821-24 is one of the youngest among the old recycled ones,and there is likely a correlation between age, or a related parameter,and timing noise. While this event happens on a much smaller scale, therequired adjustment of the star to a new equilibrium figure as it spinsdown is a likely common cause for all glitches.

M31's Undisturbed Thin Disk of Globular Clusters
We show that there is a subsystem of the M31 globular clusters withthin-disk kinematics. These clusters span the entire metallicity rangeof the M31 globular cluster system, in contrast to the (thick) diskglobular clusters in the Milky Way, which are predominantly metal-rich.Disk globular clusters are found across the entire disk of M31 and form~40% of the clusters projected on its disk. The existence of such a disksystem suggests that there was a relatively large thin disk in placevery early in M31's history. Accurate measures of the ages of theseclusters will constrain the epoch of disk formation in M31. There iscurrently no strong evidence for differences in age between Milky Wayand M31 globular clusters. While age differences are subtle for oldpopulations, it is unlikely that disk clusters with [Fe/H] around -2.0were formed after significant star formation began in the galaxy, as theprotocluster gas would be enriched by supernova ejecta. Thus it islikely that M31 had a rather large disk in place at early epochs. Thevery existence of such a cold disk means that M31 has suffered nomergers with an object of 10% or more of the disk mass since theclusters were formed. This makes the suggestion of Brown et al. that M31could have suffered an equal-mass merger 6-8 Gyr ago less viable.

Infrared Echelle Spectroscopy of Palomar 6 and M71
We present high-resolution infrared echelle spectroscopy for theglobular clusters Palomar 6 and M71. Our mean heliocentric radialvelocity of Pal 6 is +180.6+/-3.2 km s-1 and is 20 kms-1 lower than that found by Minniti in 1995. Contrary to theprevious metallicity estimates using low-resolution spectroscopy, ourresults show that Pal 6 has an intermediate metallicity, with[Fe/H]=-1.0+/-0.1, and is slightly more metal poor than M71. Reasonablechanges in the surface temperature or the microturbulent velocity of themodel atmospheres do not affect [Fe/H] at more than +/-0.2 dex. In spiteof its high metallicity, on the basis of the spectrum of a singlecluster member the [Si/Fe] and [Ti/Fe] ratios of Pal 6 appear to beenhanced by 0.4 and 0.5 dex, respectively, suggesting that the Galacticinner halo may have experienced a very rapid chemical enrichmenthistory.Based on observations made with the Infrared Telescope Facility, whichis operated by the University of Hawaii under contract to the NationalAeronautics and Space Administration.

Spectral and timing properties of the X-ray emission from the millisecond pulsar PSR B1821-24
We report results on the timing and spectral analysis of observations ofthe millisecond pulsar PSR B1821-24 with RXTE, BeppoSAX and Chandra.The X-ray light curve is characterized by two narrow peaks at a phasedistance of 0.452±0.002. The average pulsed emission, over therange 1.6-20 keV, is well represented by a single power law with aphoton index α=1.30+0.05-0.02 and unabsorbed(2-10 keV) pulsed X-ray flux of 3.9×10-13 ergcm-2 s-1. We searched for a possible bunching ofX-ray photons to verify if the X ray emission has a time structuresimilar to that of giant pulses and found a negative result.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:18h24m30.00s
Apparent magnitude:6.9

Catalogs and designations:
Proper Names   (Edit)
MessierM 28
NGC 2000.0NGC 6626

→ Request more catalogs and designations from VizieR