X-ray sources of cosmological relevance

The imaging and spectroscopic instruments onboard the Einstein Observatory have been extensively used to study in detail the X-ray properties of a large variety of astronomical objects. In this paper, the authors briefly discuss some of the most relevant results on extragalactic astronomy obtained mainly with the Imaging Proportional Counter. In Section 2 the X-ray data on Active Galactic Nuclei (quasars and Seyfert galaxies) are discussed, BL Lacs are discussed in Section 3, and clusters of galaxies in Section 4. (Auth.)