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 RASS-Green Bank survey (RGB), to provide new constraints on the spectral energy distribution of blazars, particularly flat-spectrum radio quasars (FSRQ). This reassessment is prompted by the discovery of a population of FSRQ with spectral energy distribution similar to that of high-energy peaked BL Lacs. 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 Lacs and have synchrotron peak frequencies, νpeak, much higher than “classical” FSRQ, typically in the UV band for DXRBS. Some of these objects may be 100 GeV – TeV emitters, as are several known BL Lacs with similar broadband spectra. Our large, deep, and homogeneous DXRBS sample does not show anti-correlations between νpeak and radio, broad line region, or jet power, as expected in the so-called “blazar sequence” scenario. However, the fact that FSRQ do not reach X-ray-to-radio flux ratios and νpeak values as extreme as BL Lacs and the elusiveness of high νpeak–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. Subject headings: galaxies: active — BL Lacertae objects: general — quasars: general — radiation mechanisms: non-thermal — radio continuum: galaxies — X-rays: galaxies 1. a phenomenological description of the