Multifrequency Spectra of EXOSAT Blazars.

Abstract

Blazars have been divided into two groups: radio-selected blazars (RBLs) and X-ray-selected blazars (XBLs). Results of the X-ray spectral analysis of 28 such blazars which were observed with EXOSAT are presented in this paper, and in four these are new results which were not published earlier. X-ray results suggest that XBLs are steep-spectrum sources and RBLs are relatively flat-spectrum sources. Even though the RBLs are, in general, higher redshift objects than XBLs, they are slightly more luminous (by a factor of 4-7) in X-rays than XBLs. However, the RBLs are much more luminous (by two to three orders of magnitude) in radio than the XBLs. Using simultaneous/quasi-simultaneous and nonsimultaneous observations, we have constructed the multifrequency spectra (radio through X-ray continuum fluxes) of 28 blazars, and they can be well represented with a single parabolic curve for XBLs and with two parabolic curves for RBLs. One of the important differences between the multifrequency spectra of RBLs and XBLs is the spectral discontinuity in the UV-X-ray region of RBLs. This result has been confirmed by only using simultaneous UV and X-ray observations of 50% of the sources of the present sample. Luminosities at different bands have also been computed for all the blazars in the present sample, and a bimodal character of the distribution of these objects has been found in the radio and X-ray luminosity plane. All these results will be discussed in the framework of a "wide-jet" model.