On the soft X-ray spectra of gamma-loud blazars
  ROSAT observations of a large sample of bright gamma-ray (E > 100 MeV) blazars are presented. Results of a detailed spectral analysis in the soft $\sim$ 0.1-2.0 keV energy range are discussed in relation to the overall energy distribution with particular emphasis on the relation between X-ray and gamma-ray properties. A significant anti-correlation between X-ray and gamma-ray spectral shapes of flat radio spectrum quasars (FSRQ) and BL Lacs has been discovered. A different shape in the overall energy distributions from radio to gamma-ray energies between FSRQ and BL Lacs is also implied by the correlation of their broad-band spectral indices $\alpha_{ro}$ and $\alpha_{x \gamma}$. Both the above correlations can be explained if both the IR to UV emission and the hard X-ray to gamma-ray emission originate from the same electron population, via, respectively, the synchrotron process and the inverse Compton mechanism. We suggest that a key parameter for understanding the overall energy distributions of both classes of objects is the energy at which the synchrotron emission peaks in a $\nu-\nu F(\nu)$ representation. 
A. Comastri, G. Fossati, G. Ghisellini and S. Molendi