X-ray properties of the Parkes sample of flat-spectrum radio sources: dustin radio-loud quasars?

We investigate the X-ray properties of the Parkes sample of Bat-spectrum ra dio sources using data from the ROSAT All-Sky Survey and archival pointed P SPC observations. In total, 163 of the 323 sources are detected. For the re maining 160 sources, 2 sigma upper limits to the X-ray flux are derived. We present power-law photon indices in the 0.1-2.4 keV energy band for 115 so urces, which were determined either with a hardness ratio technique or from direct fits to pointed PSPC data if a sufficient number of photons were av ailable. The average photon index is <Gamma > = 1.95(-0.12)(+0.13) for flat -spectrum radio-loud quasars, <Gamma > = 1.70(-0.24)(+0.23) for galaxies, a nd <Gamma > = 2.40(-0.31)(+0.12) for BL Lac objects. The soft X-ray photon index is correlated with redshift and with radio spectral index in the sens e that sources at high redshift and/or with flat (or inverted) radio spectr a have flatter X-ray spectra on average. The results are in accord with ori entation-dependent unification schemes for radio-loud active galactic nucle i. Webster et al. discovered many sources with unusually red optical continua among the quasars of this sample, and interpreted this result in terms of e xtinction by dust. Although the X-ray spectra in general do not show excess absorption, we find that low-redshift optically red quasars have significa ntly lower soft X-ray luminosities on average than objects with blue optica l continua. The difference disappears for higher redshifts, as is expected for intrinsic absorption by cold gas associated with the dust. In addition, the scatter in log(f(x)/f(o)) is consistent with the observed optical exti nction, contrary to previous claims based on optically or X-ray selected sa mples. Although alternative explanations for the red optical continua cannot be ex cluded with the present X-ray data, we note that the observed X-ray propert ies are consistent with the idea that dust plays an important role in some of the radio-loud quasars with red optical continua.