High Resolution X-ray Imaging of a Globular Cluster Core: Compact Binaries in 47Tuc

We have obtained high resolution (<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 kilosec exposure of the cluster reveals a centrally concentrated population of faint (Lx ~10^{30-33} erg/s) x-ray sources, with at least 108 located within the central 2arcmin X 2.5arcmin and at least half with Lx <10^{30.5} erg/s. All 15 millisecond pulsars (MSPs) recently located precisely by radio observations are identified, though two are unresolved by Chandra. The x-ray spectral and temporal characteristics, as well as initial optical identifications with the Hubble Space Telescope, suggest that >~50% are millisecond pulsars, ~30% are accreting white dwarfs, ~15% are main sequence binaries in flare outbursts and only 2 to 3 are quiescent low mass x-ray binaries containing neutron stars, the conventional progenitors of MSPs. An approximate upper limit of ~470Msun for the mass of an accreting central black hole in the cluster is derived. These observations provide the first x-ray "color-magnitude" diagram for a globular cluster and census of its compact object and binary population.Comment: 19 pages, 7 figures, 1 table (as supplementary file). Published online in Science Express on May 17, 2001 (reference: 10.1126/science.1061135). To appear in Science (print version) in late June, 200

X-ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State

We report spectral and variability analysis of two quiescent low mass X-ray binaries (X5 and X7, previously detected with the ROSAT HRI) in a Chandra ACIS-I observation of the globular cluster 47 Tuc. X5 demonstrates sharp eclipses with an 8.666+-0.01 hr period, as well as dips showing an increased N_H column. The thermal spectra of X5 and X7 are well-modeled by unmagnetized hydrogen atmospheres of hot neutron stars. No hard power law component is required. A possible edge or absorption feature is identified near 0.64 keV, perhaps an OV edge from a hot wind. Spectral fits imply that X7 is significantly more massive than the canonical 1.4 \Msun neutron star mass, with M>1.8 \Msun for a radius range of 9-14 km, while X5's spectrum is consistent with a neutron star of mass 1.4 \Msun for the same radius range. Alternatively, if much of the X-ray luminosity is due to continuing accretion onto the neutron star surface, the feature may be the 0.87 keV rest-frame absorption complex (O VIII & other metal lines) intrinsic to the neutron star atmosphere, and a mass of 1.4 \Msun for X7 may be allowed.Comment: 16 pages, 7 figures, accepted by Ap

An Extensive Census of HST Counterparts to Chandra X-ray Sources in the Globular Cluster 47 Tucanae. I. Astrometry and Photometry

We report the largest number of optical identifications of X-ray sources yet obtained in a single globular cluster. Using deep Chandra/ACIS-I imaging and extensive HST studies with WFPC2, we have detected optical counterparts to at least 22 cataclysmic variables (CVs) and 29 chromospherically active binaries (BY Dras and RS CVns) in 47 Tuc. These identifications are all based on tight astrometric matches between X-ray sources and objects with unusual (non main sequence) optical colors and/or optical variability. In the U vs U-V color magnitude diagram (CMD), the CVs all show evidence for blue colors compared to the main sequence, but most of them fall close to the main sequence in the V vs V-I CMD, showing that the secondary stars dominate the optical light. The X-ray detected active binaries have magnitude offsets above the main sequence that are indistinguishable from those of the much larger sample of optical variables (eclipsing and contact binaries and BY Dras) detected in the WFPC2 studies of Albrow et al. (2001). We also present the results of a new, deeper search for optical companions to MSPs. One possible optical companion to an MSP (47 Tuc T) was found, adding to the two optical companions already known. Finally, we study several blue stars with periodic variability from Albrow et al. (2001) that show little or no evidence for X-ray emission. The optical colors of these objects differ from those of 47 Tuc (and field) CVs.Comment: Accepted for publication in Astrophysical Journal. 25 pages, 12 figures, 3 tables, abridged abstract. Some figures at reduced resolutio

Transient Extremely Soft X-ray Emission from the Unusually Bright CV in the Globular Cluster M3: a New CV X-ray Luminosity Record?

We observed the accreting white dwarf 1E1339.8+2837 (1E1339) in the globular cluster M3 in Nov. 2003, May 2004 and Jan. 2005, using the Chandra ACIS-S detector. The source was observed in 1992 to possess traits of a supersoft X-ray source (SSS), with a 0.1-2.4 keV luminosity as large as 2x10^{35} erg/s, after which time the source's luminosity fell by roughly two orders of magnitude, adopting a hard X-ray spectrum more typical of CVs. Our observations confirm 1E1339's hard CV-like spectrum, with photon index Gamma=1.3+-0.2. We found 1E1339 to be highly variable, with a 0.5-10 keV luminosity ranging from 1.4+-0.3x10^{34} erg/s to 8.5+4.9-4.6x10^{32} erg/s, with 1E1339's maximum luminosity being perhaps the highest yet recorded for hard X-ray emission onto a white dwarf. In Jan. 2005, 1E1339 displayed substantial low-energy emission below 0.3 keV. Although current Chandra responses cannot properly model this emission, its bolometric luminosity appears comparable to or greater than that of the hard spectral component. This raises the possibility that the supersoft X-ray emission seen from 1E1339 in 1992 may have shifted to the far-UV.Comment: ApJ in press, 6 pages, 5 figure

The High Energy Telescope on EXIST

The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed next generation multi-wavelength survey mission. The primary instrument is a High Energy telescope (HET) that conducts the deepest survey for Gamma-ray Bursts (GRBs), obscured-accreting and dormant Supermassive Black Holes and Transients of all varieties for immediate followup studies by the two secondary instruments: a Soft X-ray Imager (SXI) and an Optical/Infrared Telescope (IRT). EXIST will explore the early Universe using high redshift GRBs as cosmic probes and survey black holes on all scales. The HET is a coded aperture telescope employing a large array of imaging CZT detectors (4.5 m^2, 0.6 mm pixel) and a hybrid Tungsten mask. We review the current HET concept which follows an intensive design revision by the HET imaging working group and the recent engineering studies in the Instrument and Mission Design Lab at the Goddard Space Flight Center. The HET will locate GRBs and transients quickly (<10-30 sec) and accurately (< 20") for rapid (< 1-3 min) onboard followup soft X-ray and optical/IR (0.3-2.2 micron) imaging and spectroscopy. The broad energy band (5-600 keV) and the wide field of view (~90 deg x 70 deg at 10% coding fraction) are optimal for capturing GRBs, obscured AGNs and rare transients. The continuous scan of the entire sky every 3 hours will establish a finely-sampled long-term history of many X-ray sources, opening up new possibilities for variability studies.Comment: 10 pages, 6 figures, 3 tables, SPIE conference proceedings (UV, X-ray, and Gamma-Ray Space Instrumentation for Astronomy XVI, 7435-9