All X-ray instruments taken to space were essentially sky-scanning until 1978. For the first time, the Einstein Observatory could operate in a pointed mode. It could be pointed anywhere in the sky on command. It detected X-ray sources 1,000 times fainter than those observed by Uhuru and ten million times fainter than SCO X-1, the first source found outside the solar system. For the first time, X-ray astronomers had an instrument with a resolution comparable with that of the optical telescope.
The Einstein Observatory has revealed a wide range of data on celestial objects. Perhaps the most fundamental discovery is that X-rays are quite a standard phenomenon rather than an exception. In fact, this finding is against the traditional theories that predict that only stars whose temperature is between 5,500 and 10,000 degrees (K) would emit X-rays. It is now known that X-radiation is quite common, occurring in a wide range of stars, thousands of times fainter or brighter than the Sun.
Only a limited number of medium and high-resolution spectroscopic observations (up to about 4 Ke V) were made with Einstein, which ended its mission in 1981. In order to continue the research, ESA designed EXOSAT and launched it in May 1983 with an American rocket. It had a highly eccentric orbit with an apogee of 191,000 km, taking four days to complete it, thereby avoiding the Earth’s radiation belts. The orbit made it possible to observe the X-ray sources from interplanetary space using the Moon to occult them. The orbit also allowed real-time data recovery to proceed continuously for some 80 hours of the 91- hour orbital period. EXOSAT re-entered the Earth’s atmosphere in 1986, after making nearly 2,000 pointed observations of the X-ray Universe.
Among the findings of EXOSAT is double star system with a neutron star which pulses every 1.24 seconds. Another X-ray satellite, ROSAT, was launched in 1990. It is a joint effort by Germany, UK and the USA. Its notable achievement was its ability to see through gaps in the hydrogen gas between stars and look at the first-ever X-ray source outside our galaxy. The findings suggest that the number of quasar would be much more than found by the Einstein Satellite. All quasars are extremely active in putting out X-rays and probably account for one-half of the X-rays from beyond the Milky Way. The ROSAT all-sky survey expanded the catalogue to include nearly 100,000 X-ray sources.
Two Japanese X-ray satellites, Ginga (1987-1991) and ASCA (1993), have given interesting results. ASCA is the first X-ray observatory capable of simultaneous imaging and spectroscopic observations over the range of 0.5-10 KeV.
When two or more atomic nuclei collide and fuse into one, they release tremendous energy (e.g. in the Sun). Far more efficient energy-producing processes are at work in X-ray-emitting stars, known as X-ray binaries. In these binaries, gas flows from the normal star to a neutron star. In the process, enormous energy, much more than the output of hydrogen fusion, is released. One of the binaries than swallows the matter from the other and ends up in a supernova, while the famished star may become a pulsar. X-rays are produced, when gas collects on the surface of an old neutron star and explodes.
Since the 1970s, astronomers have discovered about 30 pulsating X-ray binaries. X-ray bursters provide clues to understand neutron stars and black holes. The Hubble Space Telescope has detected optically an unusual double star which puts out X-rays. One kind of X-ray stars, known as low-mass binaries, is believed to consist of a neutron star that snatches material from its companion and gradually becomes a millisecond pulsar (that rotates in milliseconds) giving off radio waves but no X-rays. Another type of X-ray binaries is believed to be a bright blue star (with a high mass) and a neutron star.
After the blue star expands and swallows the neutron star, two white dwarfs (remnants of a burnt-out Sun-like star) may evolve. The X-ray binaries rotate very fast (some at 160 rotations a second) as otherwise they may lose their mass to their companion neutron stars quickly. White dwarfs, neutron stars or black holes are illuminated in binary systems by the transfer of gas from their companions.