-
[show abstract]
[hide abstract]
ABSTRACT: We observed the globular cluster NGC 6652 with Chandra for 47.5 ks, detecting
six known X-ray sources, as well as five previously undetected X-ray sources.
Source A (XB 1832-330) is a well-known bright low-mass X-ray binary (LXMB). The
second brightest source, B, has a spectrum that fits well to either a power-law
model (Gamma ~ 1.3) or an absorbed hot gas emission model (kT ~ 34 keV). Its
unabsorbed 0.5-10 keV luminosity (L_X = 1.6+-0.1*10^34 erg/s) is suggestive of
a neutron star primary; however, Source B exhibits unusual variability for a
LMXB, varying by over an order of magnitude on timescales of ~ 100 s. Source
C's spectrum contains a strong low-energy component below 1 keV. Its spectrum
is well fit to a simplified magnetic cataclysmic variable (CV) model, thus the
soft component may be explained by a hot polar cap of a magnetic CV. Source D
has an average L_X (0.5-10 keV) ~ 9*10^32 erg/s, and its spectrum is well fit
to a neutron star atmosphere model. This is indicative of a quiescent neutron
star LXMB, suggesting Source D may be the third known LMXB in NGC 6652. Source
E has L_X (0.5-10 keV) ~ 3*10^32 erg/s, while Source F has L_X (0.5-10 keV) ~
1*10^32 erg/s. Their relatively hard X-ray spectra are well-fit by power-law or
plasma emission models. Five newly detected fainter sources have luminosities
between 1-5*10^31 erg/s. NGC 6652 has an unusually flat X-ray luminosity
function compared to other globular clusters, which may be connected to its
extremely high central density.
The Astrophysical Journal 05/2012; 751(1). · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Our 5 ks Chandra ACIS-S observation of the globular cluster NGC 6652 detected
7 X-ray sources, 3 of which are previously unidentified. This cluster hosts a
well-known bright low-mass X-ray binary, source A (or XB 1832-330). Source B
shows unusual rapid flaring variability, with an average L_X(0.5-10 keV)
~2*10^{34} ergs/s, but with minutes-long flares up to L_X=9*10^{34} ergs/s. Its
spectrum can be fit by an absorbed power-law of photon index Gamma~1.24, and
hardens as the countrate decreases. This suggests that part or all of the
variation might be due to obscuration by the rim of a highly inclined accretion
disk. Sources C and D, with L_X ~10^{33} ergs/s, have soft and unusual spectra.
Source C requires a very soft component, with a spectrum peaking at 0.5 keV,
which might be the hot polar cap of a magnetically accreting polar cataclysmic
variable.
Source D shows a soft spectrum (fit by a power-law of photon index ~2.3) with
marginal evidence for an emission line around 1 keV; its nature is unclear. The
faint new sources E, F, and G have luminosities of 1-2*10^{32} ergs/s, if
associated with the cluster (which is likely). E and F have relatively hard
spectra (consistent with power-laws with photon index ~1.5). G lacks soft
photons, suggesting absorption with N_H>10^{22} cm^{-2}.
05/2011;
-
C. O. Heinke,
D. Altamirano, H. N. Cohn,
P. M. Lugger,
S. A. Budac,
M. Servillat,
M. Linares,
T. E. Strohmayer,
C. B. Markwardt,
R. Wijnands,
J. H. Swank,
C. Knigge,
C. Bailyn,
J. E. Grindlay
[show abstract]
[hide abstract]
ABSTRACT: We have identified a new transient luminous low-mass X-ray binary, NGC 6440 X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular cluster NGC 6440. The discovery outburst (July 28-31, 2009) peaked at L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Four other outbursts (May 29-June 4, Aug. 29-Sept. 1, Oct. 1-3, and Oct. 28-31 2009) have been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440 X-2), with similar peak luminosities and decay times. Optical and infrared imaging did not detect a clear counterpart, with best limits of V>21, B>22 in quiescence from archival HST imaging, g'>22 during the August outburst from Gemini-South GMOS imaging, and J>~18.5$ and K>~17 during the July outburst from CTIO 4-m ISPI imaging. Archival Chandra X-ray images of the core do not detect the quiescent counterpart, and place a bolometric luminosity limit of L_{NS}< 6*10^31 ergs/s (one of the lowest measured) for a hydrogen atmosphere neutron star. A short Chandra observation 10 days into quiescence found two photons at NGC 6440 X-2's position, suggesting enhanced quiescent emission at L_X~6*10^31 ergs/s . NGC 6440 X-2 currently shows the shortest recurrence time (~31 days) of any known X-ray transient, although regular outbursts were not visible in the bulge scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may be easily missed by current X-ray monitoring. Comment: 13 pages (emulateapj), 8 (color) figures, ApJ in press. Revised version adds 5th outburst (Oct./Nov. 2009), additional discussion of possible causes of short outburst recurrence time.
11/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: We report the most accurate X-ray position of the X-ray source in the giant globular cluster G1 in M31 by using the Chandra X-ray Observatory, Hubble Space Telescope (HST), and Canada-France-Hawaii Telescope (CFHT). G1 is clearly detected with Chandra and by cross-registering with HST and CFHT images, we derive a 1-sigma error radius of 0.15 arcsec, significantly smaller than the previous measurement by XMM-Newton. We conclude that the X-ray emission of G1 is likely to come from within the core radius of the cluster. We have considered a number of possibilities for the origin of the X-ray emission but can rule all but two scenarios out: it could be due to either accretion onto a central intermediate-mass black hole (IMBH), or an ordinary low-mass X-ray binary (LMXB). Based on the X-ray luminosity and the Bondi accretion rate, an IMBH accreting from the cluster gas seems unlikely and we suggest that the X-rays are due to accretion from a companion. Alternatively, the probability that a 1.5 solar masses cluster LMXB lies within the 95 per cent X-ray error circle is about 0.7. Therefore we cannot rule out a single LMXB as the origin of the X-ray emission. While we cannot distinguish between different models with current observations, future high-resolution and high-sensitivity radio imaging observations will reveal whether there is an IMBH at the centre of G1. Comment: 5 pages, 1 figure, accepted for publication in MNRAS
10/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: We present results of a study of the central regions of NGC 6397 using Hubble
Space Telescope's Advanced Camera for Surveys, focusing on a group of 24 faint
blue stars that form a sequence parallel to, but brighter than, the more
populated sequence of carbon-oxygen white dwarfs (CO WDs). Using F625W, F435W,
and F658N filters with the Wide Field Channel we show that these stars, 18 of
which are newly discovered, have magnitudes and colors consistent with those of
helium-core white dwarfs (He WDs) with masses ~ 0.2-0.3 Msun. Their
H-alpha--R625 colors indicate that they have strong H-alpha absorption lines,
which distinguishes them from cataclysmic variables in the cluster. The radial
distribution of the He WDs is significantly more concentrated to the cluster
center than that of either the CO WDs or the turnoff stars and most closely
resembles that of the cluster's blue stragglers. Binary companions are required
to explain the implied dynamical masses. We show that the companions cannot be
main-sequence stars and are most likely heavy CO WDs. The number and
photometric masses of the observed He WDs can be understood if ~1-5% of the
main-sequence stars within the half-mass radius of the cluster have white dwarf
companions with orbital periods in the range ~1-20 days at the time they reach
the turnoff. In contrast to the CO WD sequence, the He WD sequence comes to an
end at R625 ~ 24.5, well above the magnitude limit of the observations. We
explore the significance of this finding in the context of thick vs. thin
hydrogen envelope models and compare our results to existing theoretical
predictions. In addition, we find strong evidence that the vast majority of the
CO WDs in NGC 6397 down to Teff ~ 10,000 K are of the DA class. Finally, we use
the CO WD sequence to measure a distance to the cluster of 2.34 +- 0.13 kpc.
04/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: We present the results of direct Fokker-Plank simulations of the dynamical evolution of a cluster of identical stars that undergoes core collapse followed by a binary-driven reexpansion phase, including a careful treatment of the effects of velocity-space anisotropy. We have substantially refined the two-dimensional Fokker-Planck code first developed by Cohn to improve the energy conservation by a factor of about 100 and to include energy input from binaries produced by three-body interactions. The treatment of anisotropy allows us to construct realistic models for the evolution of global structure, including the development of a halo by ejection of mass from the central regions. We have focused on the evolution of an isolated cluster, with no tidal boundary, in order to provide a benchmark for comparison with simulations that include tidal mass loss. The overall evolution of the central regions of the cluster resembles that of isotropic Fokker-Planck models, with strongly nonlinear core oscillations observed for total star number N>104. However, radial-orbit-dominated anisotropy becomes significant within the central regions during core collapse phases and is present at large amplitude in the halo at all times. The outer regions of the cluster expand much more rapidly than is observed in isotropic simulations, with the expansion of the radius containing 90% of the cluster mass strongly accelerating at the time of the first core collapse. There is close agreement between the expansion observed here and that seen in the shorter duration direct N-body integrations for N~104 reported by Spurzem & Aarseth and Makino. In contrast, the halo expansion that we observe is more rapid than that seen in the somewhat lower resolution anisotropic Fokker-Planck simulations reported by Takahashi. Our simulations indicate a close connection between the collapsed core and the outer halo: frequent stellar encounters in the dense core rapidly boost stars to large orbital apocenter distances on highly elongated, low angular momentum orbits. These orbits provide an efficient energy and mass transport conduit between the inner and outer regions. This process may lead to more rapid globular cluster dissolution in the Galactic tidal field than had previously been inferred in recent studies based on isotropic Fokker-Planck models.
The Astrophysical Journal 01/2009; 518(1):233. · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
The Astrophysical Journal 12/2008; 635(2):920. · 6.02 Impact Factor
-
R. F. Elsner,
C. O. Heinke, H. N. Cohn,
P. M. Lugger,
J. E. Maxwell,
I. H. Stairs,
S. M. Ransom,
J. W. T. Hessels,
W. Becker,
R. H. H. Huang,
P. D. Edmonds,
J. E. Grindlay,
S. Bogdanov,
K. Ghosh,
M. C. Weisskopf
[show abstract]
[hide abstract]
ABSTRACT: We observed the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-ray Observatory to study its faint X-ray populations. Five X-ray sources were found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A. The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2 h. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources were found within the half-mass radius, reaching to a limiting luminosity of 1.5 10^30 erg/s (0.3-8 keV). From a radial distribution analysis, we find that 18+/-6 of these 29 sources are associated with M71, somewhat more than predicted, and that 11+/-6 are background sources, both galactic and extragalactic. M71 appears to have more X-ray sources between L_X=10^30--10^31 erg/s than expected by extrapolating from other studied clusters using either mass or collision frequency. We explore the spectra and variability of these sources, and describe the results of ground-based optical counterpart searches. Comment: 36 pages including 7 figures and 8 tables, accepted by The Astrophysical Journal
The Astrophysical Journal 07/2008; · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: For purposes of designing targeted cataclysmic variable (CV) detection surveys and interpreting results of other projects with many CV detections such as the ChaMPlane Survey, we have created a model of the CV distribution in the Galaxy. It is modeled as a warped, flared exponential disk with a gaussian vertical distribution. Extinction is based on a detailed Galactic dust and gas model. A luminosity function for CVs is also incorporated, based on a smoothed version of published data. We calculate predicted field detection rates as a function of the limiting magnitude expected for the detecting system (i.e. WIYN/Hydra or NOAO 4m/Mosaic). Monte-Carlo techniques are used to assess statistical fluctuations in these rates. We have created maps of the expected CV distribution for the full non-bulge Galactic plane (20<l<340, |b|<15) for use in both the ChaMPlane Survey and future CV surveys. Assuming a CV distribution with a scale height of 160 pc, the ChaMPlane observational result of 5 CVs in 13 northern fields is best fit by a CV local space density of 0.9^{+1.5}_{-0.5}*10^-5 pc^-3, with the range representing the 95% confidence interval. Comment: 11 pages, 5 figures. Accepted for publication in the Astrophysical Journal
11/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: We report the determination of high-accuracy radial velocities for 299 members of the globular cluster M92 using the Hydra multi-object spectrograph on the WIYN telescope. We have concentrated on stars outside of the central region of the cluster, located up to 14'4 from the cluster center. Candidate members were selected for spectroscopy based on a photometric metallicity index determined from 3-band Washington photometry, also obtained with the WIYN telescope. The median error in the velocities is 0.35 km/s. We find the heliocentric radial velocity of the cluster to be -121.2 +/-0.3 km/s. We have used an improved Bayesian analysis to determine the velocity dispersion profile of M92. The most probable profile is a cored power-law with a scale radius of 2', velocity dispersion at 1' of 6.3km/s and outer power-law with slope -0.6. We have also reanalyzed the M15 radial velocities of Drukier et al. (1998) and find that a pure power-law with a 1' velocity dispersion of 8 km/s and slope -0.5, and the combination of a power-law with slope -0.4 and scale of 7.5 km/s inside 9' and a dispersion of 4 km/s outside, are equally likely. In both clusters there is evidence that the samples include escaping stars. We present results from a GRAPE-based N-body simulation of an isolated cluster that demonstrates this effect. We suggest additional tests to determine the relative importance of tidal heating and stellar ejection for establishing the velocity field in globular cluster halos.
12/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: 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*10^{31} ergs/s within the half-mass radius of the cluster. Thirty-three of these have L_X>10^{32} ergs/s, 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.
07/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: We have detected 300 X-ray sources within the half-mass radius (2.79') 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 et al. The radial distributions of detected sources imply roughly 70 are background sources of some kind. Most source spectra are well-fit by thermal plasma models, except for quiescent low-mass X-ray binaries (qLMXBs, containing accreting neutron stars) and millisecond pulsars (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 hour 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 N_H columns of order 10^{21} 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 \~25-30 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. (Abstract truncated.)
04/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: Chandra observations of globular clusters provide insight into the formation, evolution, and X-ray emission mechanisms of X-ray binary populations. Our recent (2002) deep observations of 47 Tuc allow detailed study of its populations of quiescent LMXBs, CVs, MSPs, and active binaries (ABs). First results include the confirmation of a magnetic CV in a globular cluster, the identification of 31 additional chromospherically active binaries, and the identification of three additional likely quiescent LMXBs containing neutron stars. Comparison of the X-ray properties of the known MSPs in 47 Tuc with the properties of the sources of uncertain nature indicates that relatively few X-ray sources are MSPs, probably only ~30 and not more than 60. Considering the \~30 implied MSPs and 5 (candidate) quiescent LMXBs, and their canonical lifetimes of 10 and 1 Gyr respectively, the relative birthrates of MSPs and LMXBs in 47 Tuc are comparable. Comment: 6 pages, 2 figures (needs AIP style file). To appear in the conference proceedings of `Interacting Binaries: Accretion, Evolution & Outcomes' (Cefalu, July 4-10 2004)
03/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: We present initial results of WIYN spectroscopic observations of selected objects detected in the Chandra Multiwavelength Plane (ChaMPlane) Survey in fields towards the Galactic anti-center. ChaMPlane is designed to identify low luminosity X-ray sources, both accretion-powered and stellar coronal, in the Galaxy. It also includes a wider-fields optical imaging Survey conducted with the NOAO Mosaic cameras to identify optical counterparts as well as H alpha-selected objects in the ~5 times larger field. We report spectra for 1048 objects in galactic anti-center (i.e. northern) fields, resulting in 609 type determinations. These include 5 new cataclysmic variables, 4 Be stars, 14 lithium-absorption stars, 180 stellar coronal sources (primarily dMe stars), and 29 new quasars. Bright optical counterparts of Chandra sources in this sample are most frequently dMe stars whereas a majority of the faintest (R ~21) spectroscopically classified Chandra source counterparts are quasars. The bulk of H alpha-selected sources appears to be roughly evenly divided between dMe stars and M stars at all magnitudes.
11/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: We have created a set of dynamically evolving Fokker-Planck models for
the globular star cluster 47 Tucanae. The models are able to reproduce
the observed stellar mass function, star-count and velocity-dispersion
profiles, the millisecond pulsar accelerations, and the radial
distribution of neutron stars within the cluster. The best fitting model
indicates that the cluster has not yet undergone core collapse but is
likely to do so in the near future. The ratio of the core radius to the
half-mass radius is 5% for the best fitting model. We find that roughly
1% of the total mass of the clusters is in the form of 1.4 solar mass
objects. If all of these are in the form of neutron stars, it
corresponds to a neutron star retention fraction of 20%. The models also
contain a substantial population of massive white dwarfs that range in
mass up to 1.1 solar masses. The combined contribution by the massive
white dwarfs and neutron stars provides the gravitational potential
needed to reproduce the velocity dispersion profile and millisecond
pulsar accelerations.
11/2003; 35:1289.
-
[show abstract]
[hide abstract]
ABSTRACT: Quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars have been identified in several globular clusters using Chandra or XMM X-ray observations, using their 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 CV, population of low-luminosity X-ray sources with 10^31<L_X<10^32.5 ergs/s. The radial distribution of our qLMXB sample suggests an average system mass of 1.5^{+0.3}_{-0.2} Msun, 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 10^32 ergs/s 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 seven times more qLMXBs than bright LMXBs reside in globular clusters. The distribution of harder sources (primarily CVs) has a weaker dependence upon density than that of the qLMXBs. Finally, we discuss possible effects of core collapse and globular cluster destruction upon X-ray source populations. Comment: Submitted to ApJ. 22 pages, seven figures
05/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: We report our analysis of a Chandra X-ray observation of the rich globular cluster M80, in which we detect some 19 sources to a limiting 0.5-2.5 keV X-ray luminosity of 7*10^30 ergs/s within the half-mass radius. X-ray spectra indicate that two of these sources are quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars. We identify five sources as probable cataclysmic variables (CVs), one of which seems to be heavily absorbed, implying high inclination. The brightest CV may be the X-ray counterpart of Nova 1860 T Sco. The concentration of the X-ray sources within the cluster core implies an average mass of 1.2+/-0.2 Msun, consistent with the binary nature of these systems and very similar to the radial distribution of the blue stragglers in this cluster. The X-ray and blue straggler source populations in M80 are compared to those in the similar globular cluster 47 Tuc. Comment: Submitted to ApJ. 15 pages, 6 figures
05/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: We have developed a new set of dynamically evolving Fokker-Planck models
for the collapsed-core globular star cluster M15, which directly address
the issue of whether a central black hole is required to fit recent
Hubble Space Telescope (HST) observations of the stellar spatial
distribution and kinematics. As in our previous work reported by Dull et
al., we find that a black hole is not needed. Using local mass-function
data from recent HST studies, we have inferred the global stellar mass
function. As a consequence of extreme mass segregation, the local mass
functions differ from the global mass function at every location. In
addition to reproducing the observed mass functions, the models also
provide good fits to the star-count and velocity-dispersion profiles,
and to the millisecond pulsar accelerations. We address concerns about
the large neutron star populations adopted in our previous Fokker-Planck
models for M15. We find that good model fits can be obtained with as few
as 1000 neutron stars; this corresponds to a retention fraction of 20
percent of the initial population. The models contain a substantial
population of massive white dwarfs, that range in mass up to 1.1 solar
masses. The combined contribution by the massive white dwarfs and
neutron stars provides the gravitational potential needed to reproduce
recent HST measurements of the central velocity dispersion profile.
04/2003; 35:735.
-
[show abstract]
[hide abstract]
ABSTRACT: 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 August 2000 observation, complicating the analysis. Nevertheless nine additional sources clearly associated with the cluster are also detected, ranging from L_X(0.5-2.5 keV)=5.6*10^{32} down to 8.6*10^{31} ergs/s. Their X-ray colors and 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 L_X(0.5-2.5 keV)=10^{32} and 10^{33} ergs/s as 11.4^{+4.7}_{-1.8} 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 0.2'' of the boresighted LMXB position. Simultaneous Rossi X-ray Timing Explorer (RXTE) 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. Comment: 18 pages, 8 figures, accepted to ApJ
03/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: We present Chandra ACIS-I observations of the accreting binary
populations in the rich globular clusters Terzan 5 and M80. We detect
nine low-luminosity X-ray sources in Terzan 5 besides the known bright
low-mass X-ray binary (LMXB), which entered a rare high state during the
observation. Of these, four are likely quiescent LMXBs containing
neutron stars, and five are probable cataclysmic variables (CVs), on the
basis of X-ray colors, luminosities, and simple spectral fitting. We
detect >16 faint X-ray sources in M80, of which two are probable
quiescent LMXBs and most of the rest probable CVs. The identified
probable quiescent LMXB population in globular clusters is now 20, large
enough to study their luminosity function and distribution within the
Galactic globular cluster system, and for simple comparative spectral
analysis. This work was supported in part by Chandra grant GO0-1034A.
11/2002; 34:1313.
