X-ray astronomy: past achievements and future prospects

Abstract

X-ray astronomy can only be carried out from above the Earth atmosphere which absorbs this radiation. Its development could only occur with the advent of instruments carried in space on board of rockets and satellites. The first x-ray star was discovered in June 1962 in a rocket flight and found to emit an extraordinary and unexpectedly large flux of x-rays, equal to 10−7 erg cm−2s−1 at Earth and exceeding by a thousand times the visible light emission. In the subsequent ten years with the advent of "Uhuru", the first orbiting observatory entirely devoted to x-ray astronomy, the physical nature of such celestial sources was revealed. They consist of binary stellar systems containing a normal star and a collapsed companion, a neutron star or a black hole. Gas from the atmosphere of the normal star acquires energy when falling in the deep potential well of the compact companion. The energy which can be acquired and liberated per nucleon is much larger then can be extracted per nucleon during nuclear burning. This source of energy, gravitational infall onto massive compact objects, forms today the generally accepted explanation for the release of the stupendous quantity of energy emitted by the quasars and other active galactic nuclei. Also in the 70's and early 80's, a new type of matter was discovered; a tenuous high temperature plasma filling the spaces between galaxies in galaxy clusters. This gas, which could not be detected except for its x-ray emission, contains as much mass in baryons as that known to exist in the galaxies themselves, thus doubling the luminous mass of clusters. Given the high X luminosity of the clusters, x-ray observations have permitted us to study the dynamic state of evolution of clusters through their morphological appearance. We find that clusters formation has occurred relatively recently and is in fact ongoing at the present epoch. With the advent in the 70's and 80's of focusing x-ray telescopes capable of angular resolutions comparable to thoe obtained in visible light, the scope of x-ray astronomy has expanded to include all known classes of celestial objects. We have observed fluorescent x-rays from the moon; the coronas of main sequence stars; novas and supernovas; binary x-ray systems in our own and external galaxies; normal galaxies of all morphological types; active galactic nuclei in Seyferts, BL Lac, QSO's and radio galaxies; clusters of galaxies and the sources of the x-ray background, possibly early QSO's or Seyferts. In many of these objects the energy loss in x-rays equals that in the visible. In the 30 years since the discovery of the first x-ray stars, the sensitivity of x-ray astronomical observations has improved by more then one hundred million times, comparable to the improvement in the optical from the naked eye to Mt. Palomar which occurred in the last three centuries. In fact in some observations the sensitivity of x-ray detectors has outstripped the capacity of optical ground based telescopes to observe the optical counterparts and the new generation of 8 meter class telescopes will be required to carry out correlated measurements. X-ray astronomy will permit us therefore to study in the next decades the problem of formation of structures in the early universe, their evolution and dynamical development.