One hundred fifty-four discrete non-nuclear ultraluminous X-ray ( ULX) sources, with spectroscopically determined intrinsic X-ray luminosities greater than 10 39 ergs s À1 , are identified in 82 galaxies observed with Chandra's Advanced CCD Imaging Spectrometer. Source positions, X-ray luminosities, and spectral and timing characteristics are tabulated. Statistical comparisons between these X-ray properties and those of the weaker discrete sources in the same fields (mainly neutron star and stellar-mass black hole binaries) are made. Sources above $10 38 ergs s À1 display similar spatial, spectral, color, and variability distributions. In particular, there is no compelling evidence in the sample for a new and distinct class of X-ray object such as the intermediate-mass black holes. Eighty-three percent of ULX candidates have spectra that can be described as absorbed power laws with index hÀi ¼ 1:74 and column density hN H i ¼ 2:24 ; 10 21 cm À2 , or $5 times the average Galactic column. About 20% of the ULXs have much steeper indices indicative of a soft, and likely thermal, spectrum. The locations of ULXs in their host galaxies are strongly peaked toward their galaxy centers. The deprojected radial distribution of the ULX candidates is somewhat steeper than an exponential disk, indistinguishable from that of the weaker sources. About 5%-15% of ULX candidates are variable during the Chandra observations (which average 39.5 ks). Comparison of the cumulative X-ray luminosity functions of the ULXs to Chandra Deep Field results suggests $25% of the sources may be background objects, including 14% of the ULX candidates in the sample of spiral galaxies and 44% of those in elliptical galaxies, implying the elliptical galaxy ULX population is severely compromised by background active galactic nuclei. Correlations with host galaxy properties confirm the number and total X-ray luminosity of the ULXs are associated with recent star formation and with galaxy merging and interactions. The preponderance of ULXs in star-forming galaxies as well as their similarities to less-luminous sources suggest they originate in a young but short-lived population such as the high-mass X-ray binaries with a smaller contribution ( based on spectral slope) from recent supernovae. The number of ULXs in elliptical galaxies scales with host galaxy mass and can be explained most simply as the high-luminosity end of the low-mass X-ray binary population.

The core-dominated radio-loud quasar PKS 0637-752 (z = 0.654) was the first celestial object observed with the Chandra X-ray Observatory, offering the early surprise of the detection of a remarkable X-ray jet. Several observations with a variety of detector configurations contribute to a total exposure time with the Chandra Advanced CCD Imaging Spectrometer (ACIS; Garmire et al. 2000, in preparation) of about 100 ks. A spatial analysis of all the available X-ray data, making use of Chandra's spatial resolving power of about 0.4 arcsec, reveals a jet that extends about 10 arcsec to the west of the nucleus. At least four X-ray knots are resolved along the jet, which contains about 5% of the overall X-ray luminosity of the source. Previous observations of PKS 0637-752 in the radio band (Tingay et al. 1998) had identified a kpc-scale radio jet extending to the West of the quasar. The X-ray and radio jets are similar in shape, intensity distribution, and angular structure out to about 9 arcsec, after which the X-ray brightness decreases more rapidly and the radio jet turns abruptly to the north. The X-ray luminosity of the total source is log L X ≈ 45.8 erg s −1 (2 − 10 keV), a and appears not to have changed since it was observed with ASCA in November 1996. We present the results of fitting a variety of emission models to the observed spectral distribution, comment on the non-existence of emission lines recently reported in the ASCA observations of PKS 0637-752, and briefly discuss plausible X-ray emission mechanisms. a We use H0 = 50 km s −1 Mpc −1 and q0 = 0 throughout

The quasar PKS 0637-752, the first celestial X-ray target of the Chandra X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to 12 arcsec west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z = 0.651, the jet is the largest (∼ 100 kpc) and most luminous (∼ 10 44.6 ergs s −1 ) of the few so far detected in X-rays. This letter presents a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-752 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new ATCA radio images at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space VLBI mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. HST images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet, and a radio component to the east, show no X-ray emission to a limit of about 100 times lower flux.The X-ray emission is difficult to explain with models that successfully account for extra-nuclear Xray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton (SSC) model, but this would imply extreme departures from the conventional minimum-energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked. 11 We use H 0 = 50 km s −1 Mpc −1 and q 0 = 0 throughout

A Chandra X-Ray Observatory ACIS-S imaging observation is used to study the population of Xray sources in the nearby Sab galaxy M81 (NGC 3031). A total of 177 sources are detected with 124 located within the D 25 isophote to a limiting X-ray luminosity of ∼3 × 10 36 ergs s −1 . Source positions, count rates, luminosities in the 0.3 -8.0 keV band, limiting optical magnitudes, and potential counterpart identifications are tabulated. Spectral and timing analysis of the 36 brightest sources are reported including the low-luminosity active galactic nucleus, SN 1993J, and the Einstein-discovered ultra-luminous X-ray source X6. The nucleus accounts for ∼86%, or 5×10 40 ergs s −1 , of the total X-ray emission from M81. Its spectrum is well-fit by an absorbed power law with photon index 1.98±0.08 consistent with previous observations (average index 1.9). SN 1993J has softened and faded since its discovery. At an age of 2594 days, SN 1993J displayed a complex thermal spectrum from a reverse shock rich in Fe L and highly-ionized Mg, Si, and S but lacking O. A hard X-ray component, emitted by a forward shock, is also present. X6 is spatially-coincident with a stellar object with optical brightness and colors consistent with an O9 -B1 main sequence star. It is also coincident with a weak radio source with a flux density of ∼95 µJy at λ = 3.6 cm. The continuum-dominated X-ray spectrum of X6 is most closely reproduced by a blackbody disk model suggesting the X-ray source is an ∼18 M ⊙ object accreting at nearly its Eddington limit.The non-nuclear point source population of M81 accounts for 88% of the non-nuclear X-ray luminosity of 8.1×10 39 ergs s −1 . The remaining (unresolved) X-ray emission is confined within ∼2 kpc of the galactic center. The spatial distribution of this emission and of the resolved X-ray bulge sources closely follows that of the bulge optical light. In particular, there is no evidence for an X-ray signature accompanying the filamentary Hα or excess UV emission seen in the central < ∼ 1.0 kpc of the galaxy. The shape of the luminosity function of the bulge sources is a power law with a break at ∼ 4 × 10 37 ergs s −1 ; suggesting the presence of an aging (∼400 Myr) population of low-mass X-ray binaries. Extrapolating this luminosity function to lower luminosities accounts for only ∼10% of the unresolved X-ray emission. Spectroscopically, the unresolved emission can be represented as a combination of soft, kT ∼0.3 keV, optically-thin plasma emission and of a Γ = 1.6 power law. The unresolved bulge X-ray emission is therefore most likely a combination of hot gas and of one or more large and distinct populations of lowluminosity X-ray sources confined in the gravitational potential and tracing the old population of bulge stars. The distribution of disk sources shows a remarkably strong correlation with spiral arms with the brightest disk sources located closest to spiral arms. The luminosity function of sources near the spiral arms is a pure power law (slope −0.48 ± 0.03) while that of sources further away exh...