We investigate models for the class of ultraluminous non-nuclear X-ray sources (ULXs) seen in a number of galaxies and probably associated with star-forming regions. Models where the X-ray emission is assumed to be isotropic run into several difficulties. In particular formation of sufficient numbers of the required ultramassive black-hole X-ray binaries is problematic, and the likely transient behaviour of the resulting systems is not in good accord with observation. The assumption of mild X-ray beaming suggests instead that ULXs may represent a shortlived but extremely common stage in the evolution of a wide class of X-ray binaries. The best candidate for this is the phase of thermal-timescale mass transfer inevitable in many intermediate and high-mass X-ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high-mass X-ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive black holes.Comment: 4 pages, no figures; accepted for ApJ Letter

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The Chandra Source Catalog (CSC) is a general purpose virtual X-ray astrophysics facility that provides access to a carefully selected set of generally useful quantities for individual X-ray sources, and is designed to satisfy the needs of a broad-based group of scientists, including those who may be less familiar with astronomical data analysis in the X-ray regime. The first release of the CSC includes information about 94,676 distinct X-ray sources detected in a subset of public Advanced CCD Imaging Spectrometer imaging observations from roughly the first eight years of the Chandra mission. This release of the catalog includes point and compact sources with observed spatial extents 30. The catalog (1) provides access to the best estimates of the X-ray source properties for detected sources, with good scientific fidelity, and directly supports scientific analysis using the individual source data; (2) facilitates analysis of a wide range of statistical properties for classes of X-ray sources; and (3) provides efficient access to calibrated observational data and ancillary data products for individual X-ray sources, so that users can perform detailed further analysis using existing tools. The catalog includes real X-ray sources detected with flux estimates that are at least 3 times their estimated 1σ uncertainties in at least one energy band, while maintaining the number of spurious sources at a level of 1 false source per field for a 100 ks observation. For each detected source, the CSC provides commonly tabulated quantities, including source position, extent, multi-band fluxes, hardness ratios, and variability statistics, derived from the observations in which the source is detected. In addition to these traditional catalog elements, for each X-ray source the CSC includes an extensive set of file-based data products that can be manipulated interactively, including source images, event lists, light curves, and spectra from each observation in which a source is detected.

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.5 sq.deg of the COSMOS field (centered at 10 h , +02 o ) with an effective exposure of ∼160 ksec, and an outer 0.4 sq.deg. area with an effective exposure of ∼80 ksec. The limiting source detection depths are 1.9×10 −16 erg cm −2 s −1 in the Soft (0.5-2 keV) band, 7.3×10 −16 erg cm −2 s −1 in the Hard (2-10 keV) band, and 5.7×10 −16 erg cm −2 s −1 in the Full (0.5-10 keV) band. Here we describe the strategy, design and execution of the C-COSMOS survey, and present the catalog of 1761 point sources detected at a probability of being spurious of <2×10 −5 (1655 in the Full, 1340 in the Soft, and 1017 in the Hard bands). By using a grid of 36 heavily (∼50%) overlapping pointing positions with the ACIS-I imager, a remarkably uniform (±12%) exposure across the inner 0.5 sq.deg field was obtained, leading to a sharply defined lower flux limit. The widely different PSFs obtained in each exposure at each point in the field required a novel source detection method, because of the overlapping tiling strategy, which is described in a companion paper. This method produced reliable sources down to a 7-12 counts, as verified by the resulting logN-logS curve, with sub-arcsecond positions, enabling optical and infrared identifications of virtually all sources, as reported in a second companion paper. The full catalog is described here in detail and is available on-line.

We have derived bias-corrected X-ray luminosity functions ( XLFs) of sources detected in a uniformly selected sample of 14 E and S0 galaxies observed with Chandra ACIS-S3. The entire sample yields 985 pointlike X-ray sources, with typical detections of 30-140 sources per galaxy. After correcting for incompleteness, the individual XLFs are statistically consistent with a single power law of a (differential) XLF slope ¼ 1:8 2:2 (with a typical error of 0.2-0.3). A break at or near L X, Edd , as reported in the literature for some of these galaxies, is not required in any case. Given the uniform XLF shape, we have generated a combined, higher statistics XLF, representative of X-ray sources in elliptical galaxies. Although the combined XLF is marginally consistent with a single power law (with ¼ 2:1 AE 0:1), a broken power law gives an improved fit. The best-fit slope is ¼ 1:8 AE 0:2 in the low-luminosity range L X ¼ a few ; 10 37 to 5 ; 10 38 ergs s À1 . At higher luminosities, the slope is steeper, ¼ 2:8 AE 0:6. The break luminosity is (5 AE 1:6) ; 10 38 ergs s À1 (with an error at 90%), which may be consistent with the Eddington luminosity of neutron stars with the largest possible mass (3 M ), He-enriched neutron star binaries, or low-mass stellar-mass black holes. If the change in XLF slope at high luminosities is real and does not mask a step in the XLF, our result would imply a different population of high-luminosity sources, instead of a beaming effect. This high-luminosity portion of the XLF must reflect the mass function of black holes in these galaxies. We note that this high-luminosity population does not resemble that of the ultraluminous X-ray sources detected in star-forming galaxies, where no break in the XLF is present and the XLF is much flatter than in the older stellar system we are studying here. We use our results to derive the integrated X-ray luminosity of accreting low-mass X-ray binaries (LMXBs) in each sample galaxy. We confirm that the total X-ray luminosity of LMXBs is correlated with the optical and more tightly with the near-IR luminosities, but in both cases the scatter exceeds that expected from measurement errors. We find that the scatter in L X (LMXB)/L K is marginally correlated with the specific frequency of globular clusters.