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Published online by Cambridge University Press: 04 August 2017
The compact objects which are the subject of this talk are essentially quasars and Seyfert galaxies; I will only briefly mention a couple of results about BL Lac objects. Before describing the X-ray properties of these objects, it is useful to introduce a “working” definition of radio-quiet and radio-loud quasars: I will call radio-loud quasars all the objects which have been detected at radio frequencies and have a spectral index between radio (5 GHz) and optical frequencies (2500 A) greater than 0.35 (Zamorani et al. 1981); all the other objects will be considered radio-quiet. Note that this definition is independent of distance and is a function only of the relative importance of radio and optical emission.