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

Abstract

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.