Multifrequency observations of blazars. III - The spectral shape of the radio to X-ray continuum

Multifrequency, quasi-simultaneous spectra for a sample of 11 blazars are presented. The spectral shape of the violently variable millimeter to ultraviolet flux is consistent with emission from a very compact single component which becomes self-absorbed at wavelengths longer than about 3 mm. The centimeter emission can be attributed to a separate, more slowly varying component. Three out of four optically violent variable quasars also exhibit evidence of a UV excess component. Values for the size of the flaring regions of 0.001-0.1 pc and magnetic fields of order 1 G are deduced. It is found that photon energy densities may dominate over magnetic field energy densities, in which case inverse Compton scattering may be the dominant energy loss mechanism in flaring components.