3.4.2 The medium energy component

At energies higher than about 0.5 keV, 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 et al. 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 ( . f) peaks at the energy at which the spectral break is observed, i.e. around 1 MeV.