BeppoSAXObservations of Synchrotron X‐Ray Emission from Radio Quasars

Abstract

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 αro and αox typical of high-energy peaked BL Lac objects. Our sources have X-ray-to-radio flux ratios on average ~70 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 ~100 keV for one object. The BeppoSAX band in one of our sources, RGB J1629+4008, is dominated by synchrotron emission peaking at ~2 × 1016 Hz, as also shown by its steep (energy index αX ~ 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 flat BeppoSAX spectrum (αX ~ 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 ≈1015 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.