On the Soft X‐Ray Spectra of γ‐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 energydistribution with particular emphasis on the relation between X-ray andgamma-ray properties. A significant anti-correlation between X-ray andgamma-ray spectral shapes of flat radio spectrum quasars (FSRQ) and BL Lacs hasbeen discovered. A different shape in the overall energy distributions fromradio to gamma-ray energies between FSRQ and BL Lacs is also implied by thecorrelation of their broad-band spectral indices $\alpha_{ro}$ and $\alpha_{x\gamma}$. Both the above correlations can be explained if both the IR to UVemission and the hard X-ray to gamma-ray emission originate from the sameelectron population, via, respectively, the synchrotron process and the inverseCompton mechanism. We suggest that a key parameter for understanding theoverall energy distributions of both classes of objects is the energy at whichthe synchrotron emission peaks in a $\nu-\nu F(\nu)$ representation.