Spectral Properties of Blazars. II. an X-Ray Observed Sample

Abstract

All the blazars observed in X-rays in various reference lists and all those belonging to X-ray selected samples are considered. It is found that X-ray selected objects have, on average, radio (5 GHz) to ultraviolet (2500 Å) and ultraviolet to X-ray (2 keV) spectral indices significantly flatter than radio-selected objects. The monochromatic luminosity distributions show that X-ray selected and radio-selected blazars have the same average X-ray luminosity, and that the X-ray selected ones are underluminous at ultraviolet and radio frequencies. It is argued that the objects with flat overall spectrum, which may be called radio-weak, discovered from X-ray searches are the dominant members of the blazar population in terms of space density, while radio-loud blazars with the same X-ray luminosity are a minority. The results are interpreted in the framework of a model where the X-ray emission is isotropic, while that at lower frequencies is relativistically beamed. Radio-loud blazars should be those with the beam pointing towards us.