James D. Lewis

Results from the Second EUVE Source Catalog

We present the results of the Second EUVE Source Catalog, including all detections from the EUVE all-sky survey, the EUVE deep survey, and sources detected during dedicated instrument pointings. Where available, we furnish identifications of these objects and statistics with regard to type of stellar or extragalactic object.

EUV and Soft X-ray Evidence for Partially Ionized Gas in Active Galactic Nuclei

We present a synoptic study of active galactic nuclei (AGN) detected by EUVE. We also present complementary ROSAT PSPC spectra for these sources and for other AGN in directions of low galactic absorption. It is found that the best-fit power-law photon indices of the X-ray spectra at 0.1-2.4 keV are anti-correlated with their galactic hydrogen columns. The indices for the 0.9-2.4 keV range do not show such a correlation, and are considerably smaller (i.e. flatter). We discuss a number of possible interpretations of this correlation but only one of these, the presence of a partially ionized absorbing gas in the AGN, explains the observations satisfactorily. The ubiquity of this effect suggests that this component be may very common in AGN.

Searching EUVE Data for Transient/Flaring Extreme Ultraviolet Sources

The Extreme Ultraviolet Explorer (EUVE), because of its sky survey strategy, performed two observations of each point along a 180° by 1° strip of the ecliptic during the initial survey phase of the mission. one observation used the deep survey telescope, and another, 90 days earlier or later, used the all sky scanner telescopes, two of which have a nearly identical passband to that of the ecliptic deep survey. Since the completion of the initial sky survey, EUVE has been used to carry out deep, pointed observations of selected targets. Many areas of the sky have therefore been observed two or more times, allowing us to compare count rates for some objects over a long temporal baseline. Objects with significantly varying count rates for widely separated times are of particular astrophysical interest.

Discovery of Warm Gas in the Virgo Cluster

During the EU VE sky survey of the Virgo region, a central source positionally coincident with the X-ray emitting galaxy M87, and a surrounding halo of extended emission, were detected in the 0.065–0.248 keV band. A detailed comparison of these data with the ROSAT PSPC data of M87 revealed an excess flux at energies < 0.4 keV within the central 30′ radius which cannot be associated with the well-known cluster gas at X-ray temperatures (kT ≥ a few keV). Instead, it is necessary to introduce a second gas component, of temperature T ∼ 5 × 106 K (kT ∼ 0.1 keV). The resulting two-component model (warm + hot) can account for all the data. The origin and stability of the warm component, with a temperature near the peak of the thermal plasma cooling curve, is unclear. Both the temperature and spatial extent argue against cooling flow as the primary process responsible for its production. Other mechanisms, such as a galactic wind and heating by galaxy motions, must be considered.