X-Ray Emission in Gamma-Ray Blazars
  Although the gamma-ray emission in blazars dominates the power output, there are crucial informations carried by the X-rays. Indeed, their paucity, together with the short variability timescales observed both at X and gamma-ray energies constrains the location, along the jet, of the active region responsible for most of the emission. Furthermore, the knowledge of the X-ray flux and spectrum helps in defining the overall spectral energy distribution (SED) of blazars, allowing to discriminate between models. 
Gabriele Ghisellini