I review some of the major achievements of the ROSAT extreme-ultraviolet all-sky survey, including the results of the recently completed 2RE source catalogue and associated study of temporal variability in EUV sources.

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.

The Array of Low Energy X-ray Imaging Sensors (ALEXIS) satellite is Los Alamos’ pathfinding small space mission achieving low cost and rapid development time for its technology demonstration and science goals. The ALEXIS satellite contains the ALEXIS telescope array, which consists of six EUV/ultrasoft X-ray telescopes utilizing normal incidence multilayer mirrors, microchannel plate detectors, and thin UV rejecting filters. Each telescope is tuned to a relatively narrow bandpass centered at either 130, 171, or 186 angstroms. Each telescope has a 33° field-of-view, and a resolution of ~ 0.25°. With each 50 s rotation of the satellite, the telescopes scan most of the anti-solar hemisphere of the sky. The spacecraft is controlled exclusively from a ground station located at Los Alamos.

This paper discusses the characteristics and performance of the ALEXIS telescopes and the results from the mission in spite of the damage incurred to the spacecraft at launch.

From a total catalogue of 479 2RE sources, 31 have been found to be variable. All the variable sources are optically identified, with a breakdown by source type as follows: cataclysmic variables 9, late-type (F-M) stars 18, the high-mass X-ray binary Her X-1, the eclipsing binaries Algol and V471 Tau, and the active galaxy Mkn 478. The most highly variable objects in the EUV band are cataclysmic variables. The survey was sensitive to timescales from ~ 1.5 h to ~ 5 d, and observed variability ranges from flare-like events lasting < 1.5 h to irregular and periodic flux changes over ~ 0.5–2 d. With the exception of the cataclysmic variables, and possibly a few late-type dK-dMe stars with large flares, the observed variability levels should not substantially affect EUV luminosity functions

The EUV emission from hot interstellar plasmas is observed by normal incidence telescopes on board a sounding rocket. It was performed on January 29, 1995, to observe the sky area around the HZ 43 close to the north galactic pole. The wave bands (δλ ~ 10 Å) are provided at 130 and 170 Å by the multilayer coating on the reflectors of 20 cm in diameter and of 30 cm in focal length. The focal plane images are detected by Csl coated MCP’s. The observed flux of HZ 43 is 1.5 counts s−1 at 130 Å and 3 counts s−1 at 170 Å. The diffuse emission is 27 c/s/deg2 at 130 Å and 20 c/s/deg2 at 170 Å. Those preliminary numbers are subjects to change along with the data analysis.

EUV observations could provide valuable and unique information about the nature of active galactic nuclei. I discuss their potential usefulness and review some of the main results obtained so far with the ROSAT WFC and the EUVE instruments. About three dozen sources, mostly Seyfert galaxies and BL Lacertae objects, have already been identified, and spectroscopic measurements of several of them have yielded a number of surprises. However, more observations are clearly needed to confirm some of the reported spectral features and to validate their interpretation.

During the EUVE 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 × 106K (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.

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.

EUVE observations of the Seyfert 1 galaxy NGC 5548 have shown the presence of line emission features identified as a Ne VII/Ne VIII blend at 88 Å and Si VII emission at 70 Å. The lines show significant broadening (FWHM 3800 km s−1) placing the emitting region at the same distance as the inner broad-line region. A fit to a thermal plasma yields a temperature of 6 × 105 K. The line emission can be attributed to the warm absorbing material discovered before from oxygen and iron absorption features, which appears to be an optically thin (for the Lyman continuum), highly ionized phase of the broad-line region.

The Seyfert 1 galaxy Mrk 478, observed during the EUVE all-sky survey, is the brightest EUV source among its class. The SW spectrum of this object shows evidence of discrete emission, although this interpretation is tentative, since the source spectrum must be extracted against a bright background. If the EUV flux is, in fact, composed partly of line emission, we consider the implications if this is the result of emission from a collision-driven plasma at temperatures ≳ 106 K. In this context, we discuss some of the constraints imposed on the emission-line region by this observation.

Data from several collaborations will be shown which demonstrate the utility of EUVE observations. For Mk 478, a Seyfert 1 galaxy, the rapidly variable EUV flux is shown to have a steep, featureless continuum. The EUV data are combined with UV and optical data to form an overall spectrum that is consistent with an accretion disk model; slight temperature variations in the innermost regions could cause the large EUV flux changes. EUVE data for other sources are presented: NGC 5548, which shows significant variations and has an EUV spectrum that shows no emission lines, contrary to a previous report; 3C 273, which did not vary much; and PKS 2155-304, which was observed simultaneously with ASCA and IUE when a hard X-ray flare was detected as an EUV polarization measurement was being attempted.

Using data from the public archive of the Extreme Ultraviolet Explorer (EUVE) all-sky survey, we have systematically searched for extreme ultraviolet (EUV) emission (58-174 Å, 0.07-0.21 keV) around approximately 2500 distinct positions in the sky corresponding to known X-ray emitting extragalactic sources. We find that 20 X-ray galaxies are EUV bright and were detected with significance above 4 σ during the EUVE survey: 8 are reported here for the first time (MS 0037.7−0156, Mrk 142, M 65, EXO 1128.1+6908, M 87, Mrk 507, PKS 2005–489 and 1H 2351−315.A). 68 additional galaxies are detected with a lower significance (3 < σ < 4), but the list is affected by a high percentage of spurious sources.

We report EUVE spectral and photometric data of the Seyfert 1 galaxy MRK 279. The photometric data show large amplitude variations over time scales less than 10,000 s. The spectrum is characterized by several features between 80 and 100 Å. We compare the observed data with several models. We can rule out the possibility that the EUV emission is from a diffuse corona or intercloud medium. Models that assume the soft X-ray/EUV emission results from reprocessing in an optical BLR region are also inconsistent with the data. A collisional excitation model is consistent with the observations but requires a cloud density ≥ 1011 cm−3.

Following a summary of early solar EUV spectroscopy the spectra of some late-type stars obtained with the Extreme Ultraviolet Explorer (EUVE) are briefly surveyed. Some transitions which are not included in current emissivity codes but could lead to numerous weak lines, and an apparent continuum in the EUVE short wavelength region, are discussed. The importance of the geometry adopted when interpreting the emission measure distribution is stressed, since radial factors can lead to an apparent emission measure distribution gradient that is steeper than the value of 3/2 expected in plane parallel geometry.

EUV observations of RS CVn binaries and M dwarf flare stars are used to illustrate the variability of coronal emission seen in EUVE spectra and photometry. The EUVE emission line spectra show that the quiescent coronae of active stars are extremely hot (≥10 million K) and flares, which are even hotter, are common. Spectral sequences showing the evolution of flare temperature and density are presented. Very high coronal electron densities (log Ne ~ 13) have been detected during M dwarf flares. EUVE data for the RS CVn binary HR1099 are compared with those from simultaneous observations in other spectral regions, including X-ray (ASCA), ultraviolet (IUE), and radio (VLA, AT) data. These combined data show that the peak coronal temperature is above the sensitivity of EUVE and that the time of maximum flare brightness is spectral region dependent.

“Yes, it will be a long time before people know what I know. How much of iron and other metal there is in the sun and the stars is easy to find out, but anything which exposes our swinishness is difficult, terribly difficult” (Tolstoy 1889).

In the solar corona, the abundances of elements appear to differ from the photospheric values in a manner related to the element first ionization potential (FIP): species with FIP ≤ 10 eV are observed to be enhanced relative to the photosphere by factors of ~3−4. The first studies of stellar coronal composition with EUVE suggest that some stars exhibit a solar-like FIP effect, whereas others do not. We briefly review the latest results, and we argue that element abundance anomalies, such as the FIP effect, can provide potentially powerful new coronal diagnostics. Moreover, knowledge of the composition of a stellar corona is crucial for interpreting its spectrum—for understanding its structure and energy balance, and for testing its possible heating mechanisms: We must begin to understand coronal abundance anomalies and the compositions of active stars in order to begin to understand their coronal physics.