Ground based observations of X-ray sources—a short review

Abstract

Forty-five X-ray sources have been announced up to October 1968; five (possibly seven) of these have been identified convincingly with optical objects; seven more have very well determined positions and it may be hoped that they will be identified in the near future; of the remaining objects, eight have been observed in several flights and their existence is beyond doubt, although their location is poorly known, 23 have been observed only in one flight and for some of them it is still uncertain whether they are real or not. The identified sources belong to three categories: A, supernova remnants; B, sources whose optical counterparts are starlike objects having spectra resembling old novae; C, radiogalaxies (only Vir A = M87 belongs to this class). After a short critical discussion concerning the distribution of X-ray sources upon the sky and a comparison with the distribution of other known objects, a short review is given of optical observations of objects of class B.