Soft X-ray properties of flat-spectrum radio quasars

We report the soft X-ray properties of 41 flat-spectrum radio quasars (FSRQs), derived by cross-correlating literature blazar samples with the ROSAT PSPC archive of pointed observations. The redshifts of the sources span the range z = 0-3. The 0.1-2.4 keV spectra of FSRQs are well described by a single power law modified by column densities of neutral gas consistent with Galactic values. Spectral flattening/steepening at the lower energies is present in only 4/41 and 2/41 sources, respectively. The ROSAT photon indices of FSRQs span a wide range, 0.5 less than or similar to Gamma less than or similar to 2.5, and have a measurable intrinsic dispersion; only in part is the spread of slopes due to a redshift effect. The rest-frame ROSAT spectral indices of FSRQs are statistically indistinguishable from low-energy peaked BL Lacs and flatter than high-energy peaked BL Lacs, although with several FSRQs having soft X-ray spectra as steep as HBLs. Similarly, while the bulk of FSRQs are intrinsically X-ray brighter than both types of BL Lacs, the X-ray luminosity distributions of the three classes have a large overlap. The ROSAT spectra of 13 FSRQs are steeper than measured with the Einstein IPC (0.3-4.5 keV), indicating a flat tail emerging at approximate to 1-2 keV at rest-frame energies. We find a trend for blazars with stronger optical emission lines to be more gamma-ray-loud and to have flatter X-ray-to-gamma-ray continua, possibly supporting external Compton scattering models as the primary generation mechanisms of the high-energy continuum in these objects. For several FSRQs repeatedly observed with ROSAT on timescales of weeks to years, flux variations not larger than a factor of 2 have been detected, with more typical variability amplitude on the order of 10%-30%, and with no accompanying spectral changes. This appears to be in contrast to the less luminous BL Lacs, which are generally characterized by large-amplitude X-ray flux changes, usually accompanied by correlated spectral variations.