ABSTRACT. The Chandra X-Ray Observatory (CXO), the X-ray component of NASA's Great Observatories, was launched on 1999 July 23 by the space shuttle Columbia. After satellite systems activation, the first X-rays focused by the telescope were observed on 1999 August 12. Beginning with the initial observation it was clear that the telescope had survived the launch environment and was operating as expected. Despite an initial surprise due to the discovery that the telescope was far more efficient for concentrating CCD-damaging low-energy protons than had been anticipated, the observatory is performing well and is returning superb scientific data. Together with other space observatories, most notably XMM-Newton, it is clear that we have entered a new era of discovery in high-energy astrophysics.

Abstract. We present the high resolution spectrum of the isolated neutron star RX J0720.4-3125, obtained with the Reflection Grating Spectrometer on XMM-Newton, complemented with the broad band spectrum observed with the EPIC PN camera. The spectrum appears smooth, with no evidence for strong photospheric absorption or emission features. We briefly discuss the implications of our failure to detect structure in the spectrum.

We compare X-ray and UV spectroscopic observations of NGC 5548. Both data sets show O vi absorption troughs associated with the active galactic nuclei outflow from this galaxy. We find that the robust lower limit on the column density of the O vi X-ray trough is 7 times larger than the column density found in a study of the O vi UV troughs. This discrepancy suggests that column densities inferred for UV troughs of Seyfert galaxy outflows are often severely underestimated. We identify the physical limitations of the UV Gaussian modeling as the probable explanation of the O vi column density discrepancy. Specifically, Gaussian modeling cannot account for a velocity-dependent covering fraction, and it is a poor representation for absorption associated with a dynamical outflow. Analysis techniques that use a single covering fraction value for each absorption component suffer from similar limitations. We conclude by suggesting ways to improve the UV analysis.

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