BeppoSAX observations of synchrotron X-ray emission from radio quasars

We present new BeppoSAX Low Energy Concentrator Spectrometer ( LECS), Medium Energy Concentrator Spectrometer (MECS), and Phoswich Detector System (PDS) observations of four flat-spectrum radio quasars (FSRQs) having effective spectral indices alpha(ro) and alpha(ox) typical of high-energy peaked BL Lac objects. Our sources have X-ray to radio flux ratios on average similar to70 times larger than classical FSRQs and lie at the extreme end of the FSRQ X-ray-to-radio flux ratio distribution. The collected data cover the energy range 0.1-10 keV (observer's frame), reaching similar to100 keV for one object. The BeppoSAX band in one of our sources, RGB J1629+4008, is dominated by synchrotron emission peaking at similar to2 x 10(16) Hz, as also shown by its steep (energy index alpha(X) similar to 1.5) spectrum. This makes this object the first known FSRQ whose X-ray emission is not due to inverse Compton radiation. Two other sources display a at BeppoSAX spectrum (alpha(X) similar to 0.7), with weak indications of steepening at low X-ray energies. The combination of BeppoSAX and ROSAT observations, (nonsimultaneous) multifrequency data, and a synchrotron inverse Compton model suggests synchrotron peak frequencies approximate to10(15) Hz, although a better coverage of their spectral energy distributions is needed to provide firmer values. If confirmed, these values would be typical of intermediate BL Lac objects for which the synchrotron and inverse Compton components overlap in the BeppoSAX band. Our sources, although firmly in the radio-loud regime, have powers more typical of high-energy peaked BL Lac objects than of FSRQs, and indeed their radio powers put them near the low-luminosity end of the FSRQ luminosity function. We discuss this in terms of an anticorrelation between synchrotron peak frequency and total power, based on physical arguments, and also as possibly due to a selection effect.