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.

We have obtained high-resolution (approximately 1") deep x-ray images of the globular cluster 47Tucanae (NGC 104) with the Chandra X-ray Observatory to study the population of compact binaries in the high stellar density core. A 70-kilosecond exposure of the cluster reveals a centrally concentrated population of faint (Lx approximately 10(30-33) ergs per second) x-ray sources, with at least 108 located within the central 2' x 2.5' and greater, similar half with Lx approximately 10(30.5) ergs per second. All 15 millisecond pulsars (MSPs) recently located precisely by radio observations are identified, though 2 are unresolved by Chandra. The x-ray spectral and temporal characteristics, as well as initial optical identifications with the Hubble Space Telescope, suggest that greater, similar50 percent are MSPs, about 30 percent are accreting white dwarfs, about 15 percent are main-sequence binaries in flare outbursts, and only two to three are quiescent low-mass x-ray binaries containing neutron stars, the conventional progenitors of MSPs. An upper limit of about 470 times the mass of the sun is derived for the mass of an accreting central black hole in the cluster. These observations provide the first x-ray "color-magnitude" diagram for a globular cluster and census of its compact object and binary population.

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 report results from a large Hubble Space Telescope project to observe a significant (∼34,000) ensemble of main-sequence stars in the globular cluster 47 Tucanae with a goal of defining the frequency of inner orbit, gas giant planets. Simulations based on the characteristics of the 8.3 days of time series data in the F555W and F814W Wide Field Planetary Camera 2 (WFPC2) filters show that ∼17 planets should be detected by photometric transit signals if the frequency of hot Jupiters found in the solar neighborhood is assumed to hold for 47 Tuc. The experiment provided high-quality data sufficient to detect planets. A full analysis of these WFPC2 data reveals ∼75 variables, but no light curves resulted for which a convincing interpretation as a planet could be made. The planet frequency in 47 Tuc is at least an order of magnitude below that for the solar neighborhood. The cause of the absence of close-in planets in 47 Tuc is not yet known; presumably the low metallicity and/or crowding of 47 Tuc interfered with planet formation, with orbital evolution to close-in positions, or with planet survival.