What Types of Jets Does Nature Make? A New Population of Radio Quasars

We use statistical results from a large sample of about 500 blazars, based on two surveys, the Deep X-Ray Radio Blazar Survey (DXRBS), nearly complete, and the ROSAT All-Sky Survey-Green Bank Survey (RGB), to provide new constraints on the spectral energy distribution of blazars, particularly flat-spectrum radio quasars (FSRQs). This reassessment is prompted by the discovery of a population of FSRQs with spectral energy distribution similar to that of high-energy-peaked BL Lac objects. The fraction of these sources is sample dependent, being 10% in DXRBS and 30% in RGB (and reaching 80% for the Einstein Medium Sensitivity Survey). We show that these X-ray-strong radio quasars, which had gone undetected or unnoticed in previous surveys, indeed are the strong-lined counterparts of high-energy-peaked BL Lac objects and have synchrotron peak frequencies, nupeak, much higher than classical FSRQs, typically in the UV band for DXRBS. Some of these objects may be 100 GeV to TeV emitters, as are several known BL Lac objects with similar broadband spectra. Our large, deep, and homogeneous DXRBS sample does not show anticorrelations between nupeak and radio, broad-line region, or jet power, as expected in the so-called blazar sequence scenario. However, the fact that FSRQs do not reach X-ray-to-radio flux ratios and nupeak values as extreme as BL Lac objects and the elusiveness of high-nupeak high-power blazars suggest that there might be an intrinsic, physical limit to the synchrotron peak frequency that can be reached by strong-lined, powerful blazars. Our findings have important implications for the study of jet formation and physics and its relationship to other properties of active galactic nuclei.