Are the X-ray Spectra of Flat-spectrum Radio Quasars and BL Lacertae   Objects Different?
  We 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. 
Paolo Padovani (1), Paolo Giommi (2) and Fabrizio Fiore (2,3) ((1)   Dip. di Fisica, II Universita' di Roma, (2) SAX/SDC, AS...