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We study the X-ray spectra of 114 flat-spectrum radio quasars (FSRQ) usingthe hardness ratios as given in the WGA catalogue of ROSAT sources. This sampleincludes all WGA FSRQ with high-quality data and comprises about 20 per cent ofpresently known such objects, which makes this the largest FSRQ sample everstudied in the X-ray band. We find that FSRQ have a distribution of energyspectral indices ranging between 0 and 3 with a mean value alpha_x ~ 1. This isconsistent with that of low-energy cutoff BL Lacs (LBL; alpha_x ~ 1.1),generally found in radio surveys, but significantly different from that ofhigh-energy cutoff BL Lacs (HBL), normally selected in the X-ray band, whichdisplay steeper X-ray spectra (alpha_x ~ 1.5). The shape of theoptical-to-X-ray continuum is concave (that is alpha_x < alpha_ox) for themajority of FSRQ, as found for LBL, supporting a dominance of inverse Comptonemission in the X-ray band in most objects. Our results are at odds withprevious studies of the X-ray spectra of FSRQ, which were however plagued bylow spectral resolution and/or small number statistics and selection effects,and have important implications for the proposed connections between FSRQ andBL Lacs.

Are the X-ray spectra of flat-spectrum radio quasars and BL Lacertae objects different?