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3.4.2. The medium energy component

At energies higher than about 0.5keV, the spectrum is well described by a single power law extending to the MeV region. There is no hint of any curvature in this spectrum, contrary to predictions of models in which the X-ray flux is in part reflected by the surface of a cool disc. These latter models predict the presence of a so-called reflection hump corresponding to the Compton reflection of the primary component. This hump is observed in many Seyfert galaxies [Mushotzky Done & Pounds 1993], but not in 3C 273 [Maisack et al. 1992], [Grandi et al. 1997]. This result is surprising because 3C 273 has a bright blue bump. Indeed, reprocessing models (models in which the blue bump emission is due to a disc heated by an external X-ray source) predict that an important blue bump would be linked to the presence of re-processing signatures also in the X-ray domain.

The spectral slope of this component is typically 0.5 and shows evidence for some variations (see below). As a result, the energy radiated per logarithmic energy interval (nu . fnu) peaks at the energy at which the spectral break is observed, i.e. around 1MeV.