X-ray surveys of quasars with the Einstein observatory

The contribution of the HEAO 2 satellite (Einstein Observatory) to the understanding of quasars is summarized. Einstein Observatory data confirm that quasars are, as a class, powerful X-ray emitters and are major contributors to the diffuse X-ray background. A large number of quasars was discovered by the Observatory during the optical identification process of serendipitous X-ray sources detected with the Imaging Proportional Counter and High Resolution Imager. Before the launch of the Einstein Observatory, quasars were basically radio and optically selected quasi-stellar objects, very luminous and at fairly high red shifts. X-ray selection favors the detection of low red shift, low luminosity objects, so that many of the X-ray selected objects come from the boundary region between classical quasars and Seyfert galaxies. In this respect the Observatory contributed to further crumbling the distinction between quasars and Seyfert galaxies and to promoting use of the term active galactic nuclei to refer to these objects in general.