Padovani, P.P.PadovaniGhisellini, G.G.GhiselliniGiommi, PaoloPaoloGiommi2020-09-172020-09-172003-07-01https://hdl.handle.net/20.500.13025/182High Energy Blazar Astronomy, ASP Conference Proceedings, Vol. 299, held 17-21 June 2002 at Tuorla Observatory, Piikkio, Finland. Edited by Leo O. Takalo and Esko Valtaoja. ISBN 1-58381-146-X. San Francisco Astronomical Society of the Pacific, 2003. p.63.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 &tilde; 2 10<SUP>16</SUP> Hz, as also shown by its steep (energy index alpha<SUB>x</SUB> &tilde; 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> &tilde; 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 &ap; 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.preprinthttp://adsabs.harvard.edu/abs/2003ASPC..299...63P2003ASPC..299...63P