Hard X-ray properties of blazars

We have considered all blazars observed in the X-ray band and for which the slope of the X-ray spectrum is available. We have collected 421 spectra of 268 blazars, including 12 archival unpublished ASCA spectra of 7 blazars w hose analysis is presented here. The X-ray spectra of blazars show trends a s a function of their power, confirming that the blazar overall energy dist ribution can be parameterized on the basis of one parameter only, i.e. the bolometric luminosity. This is confirmed by the relatively new hard (2-10 k eV) X-ray data. Our results confirm the idea that in low power objects the X-ray emission mechanism is the synchrotron process, dominating both the so ft and the hard X-ray emissions. Low energy peaked BL Lac objects are inter mediate, often showing harder spectra in the hard X-ray band, suggesting th at the synchrotron process dominates in the soft band, with the inverse Com pton process dominating at high energies. The most powerful objects have X- ray spectra that are at both in the soft and in the hard band, consistent w ith a dominating inverse Compton component.