We have detected 300 X-ray sources within the half-mass radius (2A79) 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 AE 0:19 M . Most source spectra are well fitted by thermal plasma models, except for quiescent lowmass 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 N H columns of order 10 21 cm À2 , 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.

Quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars have been identified in several globular clusters using Chandraor 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 −1 . The radial distribution of our qLMXB sample suggests an average system mass of 1.5 +0.3 −0.2 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 10 32 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 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.

We report Chandra observations of the complete sample of millisecond pulsars (MSPs) with precise radio positions in the globular clusters 47 Tuc (NGC 104) and NGC 6397. The x-ray luminosities and colors are derived or constrained and compared to x-ray MSPs previously detected in the field as well as one previously detected in a globular cluster (M28). The 47 Tuc MSPs are predominantly soft sources suggestive of thermal emission from small (r x < 0.6km) polar caps on the neutron star rather than magnetospheric emission and are a relatively homogeneous sample, with most x-ray luminosities in a surprisingly narrow range (L x ∼1-4 ×10 30 erg s −1 ). We use previously derived intrinsicṖ values and find a new relation between L x and spindown luminosity,Ė : L x ∝ E β , with β ∼0.5±0.2 vs. ∼1.0 for both pulsars and MSPs in the field. Adding the single MSP in NGC 6397 constrains β = 0.5 ± 0.15. This L x -Ė relation and also the L x /Ė vs. spindown age are each similar to that found by for thermal emission from polar cap heating. However, the cluster MSPs are relatively longer-lived (in thermal x-rays) than either the models or field MSPs, which may have additional magnetospheric (non-thermal) components. We suggest the cluster MSPs may have altered surface magnetic field topology (e.g. multipole) or their neutron stars are more massive from repeated accretion episodes due to encounters and repeated exchange interactions. MSP binary companions on or just off the main sequence (e.g. NGC 6397) are likely to have been re-exchanged and might show anomalousṖ andĖ values due to relaxation of misaligned core-crust spins. The radial distribution of ∼40 soft Chandra sources in 47 Tuc is consistent with a ∼ 1.4 M ⊙ component in a multi-mass King model and with the identified MSP sample. The implied total MSP population in 47 Tuc with L x > ∼ 10 30 erg s −1 is ∼35-90, and can constrain the relative beaming in radio vs. soft x-rays. NGC 6397 is relatively deficient in MSPs; its single detected example may have been re-exchanged out of the cluster core.

We have searched for optical identifications for 79 Chandra X-ray sources that lie within the halfmass radius of the nearby, core-collapsed globular cluster NGC 6397, using deep Hubble Space Telescope Advanced Camera for Surveys Wide Field Channel imaging in Hα, R, and B. Photometry of these images allows us to classify candidate counterparts based on color-magnitude diagram location. In addition to recovering nine previously detected cataclysmic variables (CVs), we have identified six additional faint CV candidates, a total of 42 active binaries (ABs), two millisecond pulsars (MSPs), one candidate active galactic nucleus, and one candidate interacting galaxy pair. Of the 79 sources, 69 have a plausible optical counterpart.The 15 likely and possible CVs in NGC 6397 mostly fall into two groups: a brighter group of six for which the optical emission is dominated by contributions from the secondary and accretion disk, and a fainter group of seven for which the white dwarf dominates the optical emission. There are two possible transitional objects that lie between these groups. The faintest CVs likely lie near the minimum of the CV period distribution, where an accumulation is expected. The spatial distribution of the brighter CVs is much more centrally concentrated than those of the fainter CVs and the active binaries. This may represent the result of an evolutionary process in which CVs are produced by dynamical interactions, such as exchange reactions, near the cluster center and are scattered to larger orbital radii, over their lifetimes, as they age and become fainter.

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 1.0Y6 keV X-ray luminosity of 3 ; 10 31 ergs s À1 within the half-mass radius of the cluster. Thirty-three of these have L X > 10 32 ergs s À1 , 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 lowluminosity X-ray binaries in Galactic globular clusters, but a metallicity dependence of the form suggested by Jordán et al. for extragalactic LMXBs is consistent with our data.