1994A&A...287..436B

We present ROSAT soft X-ray spectra for the members of a complete sample of 13 core-dominated, flat radio spectrum sources. The sample comprises all radio sources from a flux-limited radio catalog (S_5 GHz_ > 1 Jy; Kuehr et al.) which are north of δ= 70deg, at galactic latitudes b > 10deg, and have a flat radio spectrum between 1.4 and 5 GHz (α_r<0.5; f~ν^–α). The sources have already undergone much study at radio and optical wavelengths and are classified in broad terms as quasars (8 sources) and BL Lac objects (5 sources). We find mean X-ray power-law energy indices of α_x=0.59±0.19 for the quasars and 1.36±0.27 for the BL Lac objects (68 % confidence range for two parameters of interest as determined by a maximum likelihood method), supporting earlier Einstein Observatory results for heterogeneous samples of sources (Worrall & Wilkes). A non-zero dispersion on α_x is found for both the quasars and the BL Lac objects. The quasar X-ray spectra are harder than the interpolated spectral index between the optical and X-ray bands, α_ox, and they cluster tightly around <α_ox-α_x≥∼0.6. In contrast, the BL Lac objects give <α_ox-α_x≥∼0, but with a relatively large dispersion (σ∼0.5) which is similar to that on α_x. The BL Lac objects separate into a group of three sources with α_x<1.0 and two sources with α_x>1.7. When we incorporate published radio, mm, and optical measurements and compare the X-ray and broad-band spectral indices α_x, α_rx, α_mm,x, and α_ox, the most obvious difference between the quasar and BL Lac subsamples lies within the X-ray band. We have fitted the multi-wavelength data to inhomogeneous synchrotron-self-Compton models and find that, for the BL Lac objects with steep X-ray spectra, synchrotron emission can account for the radio to soft X-ray measurements, whereas the BL Lac objects with hard X-ray spectra and the quasars require significant Compton emission to model the spectral flattening indicated by α_x<α_ox.