Feroci, M.M.FerociAntonelli, L. A.L. A.AntonelliGuainazzi, M.M.GuainazziMuller, J. M.J. M.MullerCosta, E.E.CostaPiro, L.L.PiroButler, Reginald ChristopherReginald ChristopherButlerFrontera, F.F.FronteraDal Fiume, D.D.Dal FiumeNicastro, L.L.NicastroOrlandini, M.M.OrlandiniPalazzi, E.E.PalazziZavattini, G.G.ZavattiniGiommi, PaoloPaoloGiommiParmar, A. N.A. N.ParmarOwens, A.A.OwensCastro-Tirado, A. J.A. J.Castro-TiradoMaccarone, M. C.M. C.Maccarone2020-09-172020-09-171998-04-01https://hdl.handle.net/20.500.13025/418We study the X-ray spectra of 114 flat-spectrum radio quasars (FSRQ) using the hardness ratios as given in the WGA catalogue of ROSAT sources. This sample includes all WGA FSRQ with high-quality data and comprises about 20 per cent of presently known such objects, which makes this the largest FSRQ sample ever studied in the X-ray band. We find that FSRQ have a distribution of energy spectral indices ranging between 0 and 3 with a mean value alpha_x 1. This is consistent with that of low-energy cutoff BL Lacs (LBL; alpha_x 1.1), generally found in radio surveys, but significantly different from that of high-energy cutoff BL Lacs (HBL), normally selected in the X-ray band, which display steeper X-ray spectra (alpha_x 1.5). The shape of the optical-to-X-ray continuum is concave (that is alpha_x < alpha_ox) for the majority of FSRQ, as found for LBL, supporting a dominance of inverse Compton emission in the X-ray band in most objects. Our results are at odds with previous studies of the X-ray spectra of FSRQ, which were however plagued by low spectral resolution and/or small number statistics and selection effects, and have important implications for the proposed connections between FSRQ and BL Lacs.gamma rays: burstsX-rays: burstsAstrophysicsletter to the editorhttp://arxiv.org/abs/astro-ph/9803015https://ui.adsabs.harvard.edu/abs/1998A%26A...332L..29F/abstract54dcce078580fe1368eeaeba