Many active galactic nuclei (AGNs) have been observed from Ginga, and their spectra as well as the time variations have been studied. Study of the sky brightness fluctuation has given a new result related to the cosmic X-ray background.

A series of 20 deep pointed observations with the ROSAT PSPC is discussed. 530 X-ray sources with 0.5-2 keV fluxes down to 3·10-15erg cm-2 s-1 have been discovered in 5.9 deg2. The N(>S) relation of the sources selected in the 0.5-2 keV band shows a density in excess of 200 deg-2 at the faintest fluxes and a flattening below 2·10-14erg cm-2 s-1. The average spectrum of those sources is a power law with energy index 1.2 ±0.1. The absorption column densities are consistent with the galactic HI columns. More than 50% of the 1-2 keV background has been resolved into discrete sources in the deepest field. The total background spectrum shows an emission line feature around 0.65 keV, most probably due to OVII-OVIII from a 2·106K plasma. Above ~ 1 keV the background is dominated by a power law spectrum with a normalization of 13.4 ± 0.2 keV cm-2 s-1 sr-1 keV-1 and a slope 1.2 ± 0.1, i.e. considerably steeper than the extrapolation from higher energies.

The ROSAT observatory with its high spatial resolution X-ray telescope is an ideal instrument for the study of clusters of galaxies. In the first part of the mission an All Sky X-ray Survey was conducted with ROSAT. Here we present first results of observations of Virgo, Perseus and some other clusters of galaxies from the All Sky Survey. These data illustrate the capabilities of the ROSAT observatory for studies of the morphological structure and physical properties of galaxy clusters.

Based on the recent Ginga results, following topics on X-ray binaries are briefly discussed: The cyclotron resonnance features observed from several X-ray pulsars, and related problem of the magnetic field decay. Search for millisec. pulsations from LMXRBs. Very bright transients which are suspected to be new black hole candidates, and an estimation of the number of such black hole sources in our galaxy.

After the first year of performance of SIGMA, more than 390 hours of data on the Galactic Centre region have been recorded. This survey establishes the extremely variable nature of this region. During a period of 6 months, 3 sources have dominated successively: 1E 1740.7-2942, GRS 1758-258, GX1 + 4. Concerning the two first sources, their spectral shape and their variability suggest to include them in the list of the black hole candidates.

Spectrally resolved X-ray images of the Cygnus Loop and the Vela supernova remnant have been obtained during the ROSAT all sky survey. The remnants show highly filamentary brightness distributions. Temperatures between 1·106 K and 10·106 K have been measured. Both remnants show significant deviation from thermal pressure equilibrium. The highest pressure excursions of up to a factor of 50 are associated with the X-ray brightest filaments. Pressure variations are expected for very recently shocked clouds, on a smaller scale though. In a limited search of the all sky survey data 14 extended objects have been found, which have tentatively been identified as previously unknown supernova remnants. A total number of ~ 60 new remnants are expected from the analysis of the full sky.

The BBXRT observed nine supernova remnants during its nine-day flight. We present preliminary results from some of these observations, emphasizing the ability of BBXRT to perform spatially resolved spectroscopy. The improved spectral resolution and efficiency over previous instruments makes possible measurements of previously undetectable lines, and the broad bandpass allows simultaneous measurement of lines from oxygen through iron.

The ROSAT all-sky survey has now been completed and the analysis of this huge body of data is in progress. While final results on the number of detected X-ray sources and their distribution in flux are not yet available, the total number of X-ray sources is expected to be around 60 000. Preliminary results from optical identifications of selected fields show that about one quarter of the X-ray sources discovered at high galactic latitudes come from by comparison nearby stellar sources (Fleming 1991), while at lower galactic latitudes up to one half of the detected X-ray sources are of stellar origin; in areas occupied by star forming regions (for example, Orion) or open clusters (for example, Hyades or Pleiades) a large number of the detected X-ray sources can be identified with young stars, yielding up to 80 percent of the total source count as galactic stars. For the whole of the ROSAT all-sky survey we may therefore expect about one third of the total sources to be of stellar origin. The vast majority of these stellar X-ray sources is of coronal origin (i.e., late-type low mass stars). Only a relatively small number of stellar X-ray sources will be associated with early-type massive stars where the X-ray emission is thought to arise from instabilities in their radiatively driven winds or metal-poor degenerate stars where the X-ray emission comes from portions of the atmosphere considerably hotter than the optically visible photosphere. From the preliminary analyses performed so far it is already clear now that supersoft sources such as white dwarfs do not constitute a major fraction of the X-ray source population found in the ROSAT all-sky survey and the number of newly X-ray discovered white dwarfs will certainly be considerably less than one thousand. The X-ray emitting late-type stars are commonly referred to as “active” stars, and the ROSAT all-sky survey catalog will comprise the most extensive list of such objects.

Preliminary results from the survey carried out with the Wide Field Camera (WFC) on ROS AT are presented. 732 sources were detected, of which 230 were observable in both the S1 and S2 filters. The distribution of sources with galactic longitude, near the galactic plane, shows the greatest number in the quadrant where H I absorption is least. The identifiable sources include a high proportion of hot white dwarfs and ‘normal’ late-type stars. The optical follow-up programme has identified over 40 new white dwarfs and over 60 late-type stars including RS CVn systems and cataclysmic variables. Emission from some A stars detected appears to come from white dwarf companions. Individual sources of particular interest are discussed. The systematic analysis of a sample of late-type stars has begun.

The Extreme Ultraviolet Explorer mission is described. For the first six months, an all-sky survey will be carried out covering 90 to 750 Å, or essentially the entire extreme ultraviolet (EUV) bandpass. This EUV survey will be made in four bands, or colors: λλ 90-150 Å, 170-250 Å, 400-600 Å, and 550-750 Å. A portion of the sky which is free from the normally intense 304 Å geocoronal helium background will be surveyed at greater sensitivity; the wavelength coverage of this band is from 90 to 400 Å. Following the sky survey portion of the mission, spectroscopy of individual sources will be carried out. Three spectrometers employing novel variable line-space gratings will provide spectra with ~1 Å resolution over the band from 70 to 760 Å. This spectroscopy will be carried out by guest observers chosen by NASA in a manner roughly analogous to the International Ultraviolet Explorer (IUE) guest observer program.

The Gamma Ray Observatory was launched on April 7, 1991 for a mission expected to last several years. Its four instruments are designed to study gamma ray emission over a wide energy regime from 0.02 to over 30,000 MeV. Following a brief description of the capabilities of each instrument and their status, the viewing program and is discussed, and some very early results are presented.

The following papers formed a Joint Discussion sponsored by Commissions 40, 44, 47 and 48 of the International Astronomical Union. The topic was Cosmic Backgrounds. The plural “backgrounds” is important. Speakers considered electromagnetic radiation fields ranging from radio wavelengths to X-rays. Because other sessions at this General Assembly treated higher energy backgrounds, in particular the X-ray region, emphasis in this Joint Discussion was on radio and infrared backgrounds. In addition to the wide range of wavelengths covered, the session also dealt with a wide range of models for various backgrounds. For instance, at some wavelengths (e.g., λ ~ 1 cm) the background is truly a background, generated at very high redshifts—in this case a thermal radiation field at ~ 3 K. At other wavelengths (e.g., in the X-ray region) the “background” may consist in whole or in part of the summed emission from many unresolved or subliminal sources.

Preliminary analyses of COBE/FIRAS data have already produced a spectacularly accurate determination of the microwave background spectrum for 1 cm ≤ λ ≤ 500 μm. The absence of detectable deviations from a blackbody spectrum sets strong constraints on physical conditions of the intergalactic plasma and, in particular, has ruled out the possibihty of a truly diffuse thermal bremsstrahlung origin of the X-ray background. General arguments suggest that comptonization distortions due to heating of the intergalactic medium associated with the formation of cosmic structures, with hot protogalactic winds, or with the ionizing flux from AGNs, are likely to be very small (comptonization parameter y ≲, 10-4). A larger signal is expected from the integrated re-radiation from dust in external galaxies; to what extent this may conceal possible comptonization distortions depends on the maximum redshift at which galaxies contain substantial amounts of dust and on the temperature distribution of dust grains. In any case, a precise determination of either the y parameter or the background from distant galaxies requires a careful subtraction of the emission from the Milky Way.

The great success of COBE strengthens the need for a parallel improvement in the accuracy of spectral measurements in the Rayleigh-Jeans region, where imprints of physical processes occurring at very early epochs (such as, e.g., the dissipation of small scale density inhomogeneities) may show up.

Ever since the discovery in 1964 of the Cosmic Microwave Background (CMB), scientists have tried to make accurate measurements of its spectrum and anisotropie3. With the successful COBE mission, major advances in our understanding of the very early universe have been achieved.

COBE’s complement of instruments are the Far Infrared Absolute Spectrophotometer (FIRAS), the Differential Microwave Radiometers (DMR), and the Diffuse Infrared Background Experiment (DIRBE). FIRAS and DIRBE are located inside a 4He dewar to operate at 1.5 K. DMR receivers are located around the outside of the dewar. The instruments and mission plan have been described by Gulkis et al. (1990). Essential for the cosmological objectives are the all-sky observing strategy, periodic absolute calibrations of the instruments, high sensitivity, and extensive care to minimize potential systematic errors.

Recent precise observations of the microwave and submillimeter cosmic background radiation are summarized, including rocket experiments, the FIRAS (Far InfraRed Absolute Spectrophotometer) on the COBE, CN results, and microwave measurements. Theoretical implications are summarized.

Recent precise observations of the cosmic microwave background (CMB) isotopy and experiments in progress are summarized. The current measurements on all angular scales are approaching sensitivities of the order of ΔT/T ≈ 10-5. Nearly all experiments are observing positive detections. Most assume the detection to be due to galactic emission or other foregrounds or systematics and not due to primordial anisotropies in the CMB. The context and significance of these observations are discussed.

Why do we need to think about any alternatives when the primordial interpretation of the microwave background radiation (MBR) has been accepted by so many for so long? The answer is that the primordial interpretation, in spite of its successes has manifest shortcomings in spite of attempts to remove them by so many for so long. To mention a few:

1. a) Why is the MBR temperature 2.7 K? The value is taken as a parameter in all early universe calculations; it is not predicted by the hot big bang theory with or without inflation.

2. b) There are other astrophysical processes of comparable energy density and other radiation backgrounds that have no primordial origin; why should MBR alone stand out as the odd one out just at this epoch?

3. c) Why are there no signatures of structure formation on the MBR; why is it so smooth?

4. d) The hot big bang model relates to the universe in the first three minutes while the MBR is observed in the more recent past; are we not making too long a jump across from the one to the other?

Cosmic infrared background radiation, expected to carry the radiative record of luminous objects since they first formed, has yet to be detected. The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) satellite is designed to search for this primeval background over the spectral range from 1 to 300 μm. Initial examination of data from this experiment show that foreground radiations from the solar system and Galaxy dominate the diffuse infrared sky brightness, with relative minima near 3.5 μm and in the submillimeter wavelength range. DIRBE measurements do not confirm some previous data from rockets and IRAS. Preliminary upper limits on the cosmic infrared background are generally above theoretical expectations for pregalactic and protogalactic sources, and substantially above estimated lower limits based upon observations of external galaxies. Careful foreground modeling is required to reduce these limits or identify an isotropic residual.

We are carrying on measurements of the absolute temperature of the CBR at various frequencies near and below 1 GHz, looking for so far undetected deviations from a planckian spectrum. The amplitude and frequency of those distortions can give precious information about the history of the Universe.