X-RAY-SPECTRA OF A COMPLETE SAMPLE OF EXTRAGALACTIC CORE-DOMINATED RADIO-SOURCES

We present ROSAT soft X-ray spectra for the members of a complete samp le of 13 core-dominated, flat radio spectrum sources. The sample compr ises all radio sources from a flux-limited radio catalog (S5GHz > 1 Jy ; Kuhr et al. 1981) which are north of delta = 70-degrees, at galactic latitudes b > 10-degrees, and have a flat radio spectrum between 1.4 and 5 GHz (alpha(r) < 0.5; f is similar to nu(-alpha)). The sources ha ve already undergone much study at radio and optical wavelengths and a re classified in broad terms as quasars (8 sources) and BL Lac objects (5 sources). We find mean X-ray power-law energy indices of alpha(x) = 0.59 +/- 0.19 for the quasars and 1.36 +/- 0.27 for the BL Lac objec ts (68 % confidence range for two parameters of interest as determined by a maximum likelihood method), supporting earlier Einstein Observat ory results for heterogeneous samples of sources (Worrall & Wilkes 199 0). A non-zero dispersion on alpha(x) is found for both the quasars an d the BL Lac objects. The quasar X-ray spectra are harder than the int erpolated spectral index between the optical and X-ray bands, alpha(ox ), and they cluster tightly around < alpha(ox) - alpha(x) > congruent- to 0.6. In contrast, the BL Lac objects give < alpha(ox) - alpha(x) > congruent-to 0, but with a relatively large dispersion (sigma is simil ar to 0.5) which is similar to that on alpha(x). The BL Lac objects se parate into a group of three sources with alpha(x) < 1.0 and two sourc es with alpha(x) > 1.7. When we incorporate published radio, mm, and o ptical measurements and compare the X-ray and broad-band spectral indi ces alpha(x), alpha(rx), alpha(mm,x), and alpha(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 sy nchrotron-self-Compton models and find that, for the BL Lac objects wi th steep X-ray spectra, synchrotron emission can account for the radio to soft X-ray measurements, whereas the BL Lac objects with hard X-ra y spectra and the quasars require significant Compton emission to mode l the spectral flattening indicated by alpha(x) < alpha(ox).