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The observation of X-rays is a relatively new branch of Astronomy. As every new field of science this astronomy had an extremely fast and fruitful development. After twenty years of X ray exploration it is time to summarize the results and to envisage the future. This is the aim of this expose. After a brief review of the birth conditions of X-ray astronomy, a description of the main space missions is done. The most important results are explained and help to define the fields to be covered by the future missions. This leads to the description of the instruments which will be flown on future spacecrafts planned for the next ten years.

The salient features of X-80, a European X-ray Astrophysics Mission, and a candidate for selection as the next satellite in the European Space Agency's scientific programme, is described.

THIS communication presents the final results of an analysis of the X-ray emission from the Crab Nebula in the 15 to 150 keV energy range. We carried out two balloon flights (on September 15 and September 22, 1970) and have already presented preliminary results from the first flight1. We have now analysed both flights, in which a total of 70,000 photons from the Crab Nebula were detected (about 15,000 photons from the pulsar).

Three 2.104s observations were carried out with EXOSAT on three fields following the shock from the north to the east of the Cygnus Loop supernova remnant. Due to the softness of the source, most of the photons were collected with the LE package (CMA). For each exposure we used at least three filters (4000 A lexan, Al-Par, boron) in order to extract the maximum spectral information from the data. The few photons gathered with the boron filter are particularly important in that respect. The total count number collected with the other filters allows a statistically significant overall mapping of the fields with 1’x1’ pixels, but a better resolution can be achieved on the brightest areas of the Loop. Interesting details are revealed, such as bright small spots. Irregularities are also evident both in the shock front and inside the remnant, specially in the northern and eastern fields.

The X-ray properties of the supernova remnant G 29.7 – 0.3 are discussed based on spectral data from the EXOSAT satellite. In the 2 to 10 keV range a featureless power-law spectrum is obtained, the best-fit parameters being: energy spectral index α -0.77, hydrogen column density on the line of sight NH= 2.3.1022 cm-2. The incident X-ray flux from the source is (3.6 ± 0.1) 10-11 erg cm-2 s-1 in the 2 to 10 keV range corresponding to an intrinsic luminosity of about 2.1036 erg s-1 for a distance of 19 kpc. The source was not seen with the imaging instrument thus constraining the hydrogen column density to be NH = (3.3 ± 0.3) 1022 cm-2 and the energy spectral index α = 1.0±0.15. This new observation is consistent with emission by a synchroton nebula presumably fed by an active pulsar. An upper limit of ~ 1.5% for the pulsed fraction in the range of periods 32ms to 104 s has been obtained.

THIS report presents the final results of observations of pulsations in the X-ray flux in the 2.5 to 30 keV range coming from the direction of the Crab Nebula.

The North Polar Spur (or Loop I) is a very large feature appearing in large scale radio maps. Soft X-ray emission from this object stands out in soft X-ray maps between 0.5 and 1 keV. The brightest region lies around 1II ~ 25°, bII ~ 25°. We discuss a soft X-ray observation of the Spur obtained with solid state detectors [1]. This spectrum is decomposed in emission lines and the uncertainty on each line is estimated. The derived abundances are compatible with normal galactic abundances, in contradiction with previously published results [2].

A proposal for a European X-ray astronomy satellite includes a lunar occultation technique for the more accurate observation of new and existing X-ray sources.

The medium-energy experiment on board Exosat was used to measure the X-ray spectrum of the Kepler supernova remnant over the range 1.5-10 keV. An Fe emission line was clearly resolved with an energy of about 6.5 keV and equivalent width of about 1.8 keV. This was superposed on a continuum with a temperature of 5.0(+3.8, -1.9) keV. The medium-energy spectrum is shown to be consistent with a model in which the Kepler SNR is presently in a Sedov phase of evolution, the 5 keV continuum arises from the shocked interstellar/circumstellar medium, and thermal (but not ionization) equilibrium exists between electrons and ions behind the primary shock front. However, in this case, an overabundance of iron by more than 6 times cosmic is required. 28 refs.

The effects of Compton interactions of X- and ..gamma..-ray spectra with low-energy electrons are examined. It is demonstrated that the interactions can explain spectral singularities around 25 keV and 1 MeV observed in the X- and ..gamma..-ray diffuse-background spectrum. These interactions very likely occur in the source of radiation. Then most of the X- and ..gamma..-ray diffuse background would be due to a large number of unresolved, dense, extragalactic objects which might be galactic nuclei or quasars. Observations of some extragalactic X-ray objects confirm the existence of such sources. (AIP)