On the Soft X‐Ray Spectra of γ‐Loud Blazars

ROSAT observations of a large sample of bright γ-ray ( E > 100 MeV) blazars are presented. Results of a detailed spectral analysis in the soft approximately 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 γ-ray properties. A significant anticorrelation between X-ray and γ-ray spectral shapes of flat-spectrum radio quasars (FSRQs) and BL Lac objects has been discovered. A different shape in the overall energy distributions from radio to γ-ray energies between FSRQs and BL Lac objects is also implied by the correlation of their broadband spectral indices α ro and α x γ . Both the above correlations can be explained if both the infrared to ultraviolet emission and the hard X-ray to γ-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 ν-ν F (ν) representation.