Haldan N. Cohn

A Deep Chandra Survey of the Globular Cluster 47 Tucanae: Catalog of Point Sources

We have detected 300 X-ray sources within the half-mass radius (279) of the globular cluster 47 Tucanae in a deep (281 ks) Chandra exposure. We perform photometry and simple spectral fitting for our detected sources and construct luminosity functions, X-ray color-magnitude, and color-color diagrams. Eighty-seven X-ray sources show variability on timescales from hours to years. Thirty-one of the new X-ray sources are identified with chromospherically active binaries from the catalogs of Albrow and coworkers. The radial distributions of detected sources imply that roughly 70 are background sources of some kind. The radial distribution of the known millisecond pulsar (MSP) systems is consistent with that expected from mass segregation, if the average neutron star mass is 1.39 ? 0.19 M?. Most source spectra are well fitted by thermal plasma models, except for quiescent low-mass X-ray binaries (qLMXBs; containing accreting neutron stars) and MSPs. We identify three new candidate qLMXBs with relatively low X-ray luminosities. One of the brightest cataclysmic variables (CVs; X10) shows evidence (a 4.7 hr period pulsation and strong soft X-ray emission) for a magnetically dominated accretion flow as in AM Her systems. Most of the bright CVs require intrinsic NH columns of order 1021 cm-2, suggesting a possible DQ Her nature. A group of X-ray sources associated with bright (sub)giant stars also requires intrinsic absorption. By comparing the X-ray colors, luminosities, variability, and quality of spectral fits of the detected MSPs to those of unidentified sources, we estimate that a total of ~25 MSPs exist in 47 Tuc (<60 at 95% confidence), regardless of their radio beaming fraction. We estimate that the total number of neutron stars in 47 Tuc is of order 300, reducing the discrepancy between theoretical neutron star retention rates and observed neutron star populations in globular clusters. Comprehensive tables of source properties and simple spectral fits are provided electronically.

Analysis of the Quiescent Low-Mass X-Ray Binary Population in Galactic Globular Clusters

Quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars have been identified in several globular clusters using Chandra or XMM X-ray observations, via their distinctive soft thermal spectra. We report a complete census of the qLMXB population in these clusters, identifying three additional probable qLMXBs in NGC 6440. We conduct several analyses of the qLMXB population and compare it with the harder, primarily cataclysmic variable (CV), population of low-luminosity X-ray sources with 1031 ergs s-1 < LX < 1032.5 ergs s-1. The radial distribution of our qLMXB sample suggests an average system mass of 1.5 M☉, consistent with a neutron star and low-mass companion. Spectral analysis reveals that no globular cluster qLMXBs, other than the transient in NGC 6440, require an additional hard power-law component, as often observed in field qLMXBs. We identify an empirical lower luminosity limit of ~1032 ergs s-1 among globular cluster qLMXBs. The bolometric luminosity range of qLMXBs implies (in the deep crustal heating model of Brown and collaborators) low time-averaged mass-transfer rates, below the disk stability criterion. The X-ray luminosity functions of the CV populations alone in NGC 6397 and 47 Tuc are shown to differ. The distribution of qLMXBs among globular clusters is consistent with their dynamical formation by either tidal capture or exchange encounters, allowing us to estimate that 7 times more qLMXBs than bright LMXBs reside in globular clusters. The distribution of harder sources (primarily CVs) has a weaker dependence on density than that of the qLMXBs. Finally, we discuss possible effects of core collapse and globular cluster destruction on X-ray source populations.

Dynamical and luminosity evolution of active galactic nuclei : models with a mass spectrum

A multimass energy-space Fokker-Planck code is used to follow the dynamical and luminosity evolution of an AGN model that consists of a dense stellar system surrounding a massive black hole. It is found that stellar evolution and tidal disruption are the predominant mass-loss mechanisms for low-density nuclei, whereas physical collisions dominate in high-density nuclei. For initial central densities greater than 10 million solar masses/cu pc the core of the stellar system contacts due to the removal of kinetic energy by collisions, whereas for densities less than this the core of the stellar system expands due to heating that results from the settling of a small population of stars into orbits tightly bound to the black hole. These mechanisms produce differing power-law slopes in the resulting stellar density cusp surrounding the black hole, -7/4 and -1/2 for low- and high-density nuclei, respectively. 60 refs.

Faint X-Ray Sources in the Globular Cluster Terzan 5

We report our analysis of a Chandra X-ray observation of the rich globular cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6 keV X-ray luminosity of 3 × 1031 ergs s-1 within the half-mass radius of the cluster. Thirty-three of these have LX > 1032 ergs s-1, the largest number yet seen in any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB; identified by Wijnands et al.), another 12 relatively soft sources may be quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5 to the Galactic center sources studied by Muno and collaborators and find the Galactic center sources to have harder X-ray colors, indicating a possible difference in the populations. We cannot clearly identify a metallicity dependence in the production of low-luminosity X-ray binaries in Galactic globular clusters, but a metallicity dependence of the form suggested by Jordan et al. for extragalactic LMXBs is consistent with our data.

Fokker-Planck calculations of star clusters with primordial binaries

The multimass Fokker-Planck code developed by Murphy B.W. & Cohn H. (1988, MNRAS, 232) has been modified to allow for a distribution of internal binary binding energies, as well as the usual distribution of orbital energies in the cluster potential. Interactions between binaries and single stars and between pairs of binaries are simulated; these exchange energy between internal and external degrees of freedom and strongly influence the evolution both of the binary population and of the cluster as a whole

Improved mass and radius constraints for quiescent neutron stars in ω Cen and NGC 6397

We use Chandra and XMM-Newton observations of the globular clusters omega Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favoured by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS.

Discovery of candidate cataclysmic variables in the post-core-collapse globular cluster NGC 6397

A photometric search for objects with H alpha emission in the globular cluster NGC 6397 has been carried out with the Hubble Space Telescope (HST) Planetary Camera. Images were obtained through the F675W (similar to Johnson R) and F656N (H alpha) filters, and photometry carried out on approximately 900 stars detected in the central approximately 38 x 35 arcseconds of the cluster. Limiting magnitudes of R approximately 21 and 19.5 were reached in the F675W and F656N images, respectively. Three H alpha-bright stars are found which have apparent magnitudes in the range R = 17.8-19.5. The corresponding absolute magnitudes (M(sub R) = 5.6-7.3) and inferred H alpha emission-line strengths (EW(H alpha) greater than or approximately equal to 15-26 A) make these objects good candidates to be cataclysmic variables (CVs) in this nearby, post-collapse cluster. All three H alpha-bright objects are found to be UV-bright in a photometric comparison of the F675W images with archival HST Faint Object Camera (FOC) images of the cluster center through the F220W and F346M filters. Such UV excesses further support the identification of these objects as candidates to be CVs in the cluster. The H alpha-bright stars are within the error circles of three X-ray sources previously found with the ROSAT High Resoltion Imager (HRI) and are likley to be the dominant source of the X-ray emission. The implied X-Ray to optical flux ratios (f(sub x)/f(sub v) approximately 0.8-4.2) are consistent with CVs known elsewhere in the Galaxy. Two additional stars with possible H alpha emission are also discussed, along the stars identified on the basis of UV emisssion. Preliminary completeness estimates suggest that these observations should be sensitive to approximately one-third to two-thirds of most varieties of CVs in the cluster, and that therefore greater than or approximately 5-10 CVs are likely to be present in the observed section of NGC 6397. Scaling these numbers to the cluster as a whole would increase these numbers by up to a factor of 2. A total CV population on the order of 5-20 is in reasonable with previous estimates made on the basis of X-ray observations, and with theoretical predictions scaled to this cluster.

Chandra Multiwavelength Plane (ChaMPlane) Survey: An Introduction

We introduce the Chandra Multiwavelength Plane (ChaMPlane) survey, designed to measure or constrain the populations of low-luminosity (LX 1031 ergs s-1) accreting white dwarfs, neutron stars, and stellar mass black holes in the Galactic plane and bulge. ChaMPlane incorporates two surveys, X-ray (Chandra) and optical (NOAO 4 m Mosaic imaging), and a follow-up spectroscopy and IR identification program. The survey has now extended through the first 6 yr of Chandra data using serendipitous sources detected in 105 distinct ACIS-I and ACIS-S fields observed in 154 pointings and covered by 65 deep Mosaic images in V, R, I, and Hα. ChaMPlane incorporates fields with Galactic latitude 12° and selected to be devoid of bright point or diffuse sources, with exposure time 20 ks and (where possible) minimum NH. We describe the scientific goals and introduce the X-ray and optical/IR processing and databases. We derive preliminary constraints on the space density or luminosity function of cataclysmic variables (CVs) from the X-ray/optical data for 14 fields in the Galactic anticenter. The lack of ChaMPlane CVs in these anticenter fields suggests that their space density is ~3 times below the value (3 × 10-5 pc-3) found for the solar neighborhood by previous X-ray surveys. Companion papers describe the X-ray and optical processing in detail, optical spectroscopy of ChaMPlane sources in selected anticenter fields, and IR imaging results for the Galactic center field. An appendix introduces the ChaMPlane Virtual Observatory (VO) for online access to the X-ray and optical images and source catalogs for ready display and further analysis.

Cataclysmic Variables and a Candidate Helium White Dwarf in the Globular Cluster NGC 6397

We have used the Hubble Space Telescope (HST ) and the Faint Object Spectrograph (FOS) to study faint UV stars in the core of the nearby globular cluster NGC 6397. We con—rm the presence of a fourth cataclysmic variable (CV) in NGC 6397 (hereafter CV 4), and we use the photometry of Cool et al. to present evidence that CVs 1¨4 all have faint disks and probably low accretion rates. By combining these results with new UV spectra of CV 1 and the published spectra of Grindlay et al., we present new evi- dence that CVs 1¨3 may be DQ Her systems, as originally suggested by Grindlay et al., and we show that CV 4 may either be a dwarf nova or another magnetic system. Another possibility is that the CVs could be old novae in hibernation between nova eruptions. We also present the —rst spectrum of a member of a new class of UV bright stars in NGC 6397. These faint, hot stars do not vary, unlike the CVs, and are thus denoted as ii non—ickerers ˇˇ (NFs). Like the CVs, their spatial concentration is strongly concentrated toward the cluster center. Using detailed comparisons with stellar atmosphere models, we have determined log g \ 6.25 ^ 1.0 and K for this NF. Using these line parameters T eff \ 17,500 ^ 5,000 and the luminosity of the NF, we show that the NF spectrum is consistent with a helium WD having a mass of D0.25 and an age between 0.1 and 0.5 Gyr (depending on the models used). The NF spec- M _ trum appears to be signi—cantly Doppler shifted from the expected wavelength, suggesting the presence of a dark, massive companion, probably a carbon-oxygen (CO) WD. Subject headings: globular clusters: individual (NGC 6397) ¨ novae, cataclysmic variables

A Chandra X-Ray Study of the Dense Globular Cluster Terzan 5

We report a Chandra ACIS-I observation of the dense globular cluster Terzan 5. The previously known transient low-mass X-ray binary (LMXB) EXO 1745-248 in the cluster entered a rare high state during our 2000 August observation, complicating the analysis. Nevertheless, nine additional sources clearly associated with the cluster are also detected, ranging from LX(0.5-2.5 keV) = 5.6 × 1032 down to 8.6 × 1031 ergs s-1. Their X-ray colors, luminosities, and spectral fitting indicate that five of them are probably cataclysmic variables and four are likely quiescent LMXBs containing neutron stars. We estimate the total number of sources between LX(0.5-2.5 keV) = 1032 and 1033 ergs s-1 as 11.4 by the use of artificial point source tests and note that the numbers of X-ray sources are similar to those detected in NGC 6440. The improved X-ray position allowed us to identify a plausible infrared counterpart to EXO 1745-248 on our 1998 Hubble Space Telescope NICMOS images. This blue star (F110W = 18.48, F187W = 17.30) lies within 02 of the boresighted LMXB position. Simultaneous Rossi X-Ray Timing Explorer spectra, combined with the Chandra spectrum, indicate that EXO 1745-248 is an ultracompact binary system and show a strong broad 6.55 keV iron line and an 8 keV smeared reflection edge.