Synchrotron X-ray Emission from Flat-spectrum Radio Quasars

We present new BeppoSAX observations of four flat-spectrum radio quasars (FSRQ) with spectral energy distributions similar to those of high-energy peaked BL Lacs. In one of our sources the BeppoSAX band is dominated by synchrotron emission peaking at ˜ 2 10<SUP>16</SUP> Hz, as also shown by its steep (energy index alpha<SUB>x</SUB> ˜ 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 X-ray spectrum (alpha<SUB>x</SUB> ˜ 0.7) but with indications of steepening at low energies. In these objects, the combination of BeppoSAX and ROSAT observations, (non-simultaneous) multifrequency data, and a synchrotron inverse Compton model suggest synchrotron peak frequencies ≈ 10<SUP>15</SUP> Hz, although a better coverage of the spectral energy distributions is needed to provide firmer values. Our sources, although firmly in the radio--loud regime, have powers more typical of high--energy peaked BL Lacs than of FSRQ, and indeed their radio powers put them near the low--luminosity end of the FSRQ luminosity function. We discuss this in terms of an anti-correlation between synchrotron peak frequency and total power, based on physical arguments, and also as possibly due to a selection effect.