BeppoSAX Observations of Centaurus A: The Hard Continuum and the Iron-Line Feature

The radio galaxy Centaurus A was observed by the BeppoSAX satellite five times from 1997 to 2000. From 1999 July 6 to 1999 August 17, the source was also simultaneously observed by COMPTEL on board the Compton Gamma Ray Observatory. Centaurus A has a complex spectrum with multiple extended components and a strongly absorbed (NH ~ 1023 cm-2) nucleus well fitted by a power law (Γ ~ 1.8) that bends at high energies. When the BeppoSAX and COMPTEL observations are combined together, an exponential cutoff with an e-folding energy of ~600 keV gives an adequate description of the spectral steepening. A complex feature in emission at 6-7 keV is resolved into two Fe Kα components, one narrow cold line and an ionized line centered at 6.8 keV. Significant variations have been observed in the iron feature, with the less prominent ionized line seemingly being the only one responsible for them: its variations do not appear to correlate with the strength of the continuum. The high-energy cutoff and the Fe feature suggest the presence of an accretion flow in the Centaurus A nucleus. However, the absence of a significant reflection and the narrowness of the cold line, as well as the lack of correlation between the continuum and 6.8 keV line variations, disfavor a standard cold/ionized thin disk (at least in the inner regions). A more plausible configuration might be a hot, thick, optically thin accretion flow surrounded by material with different opacities. Finally, we note that the high-energy break observed by BeppoSAX and COMPTEL could be also reasonably explained by inverse Compton radiation from a jet. If this is the case, a structured jet with outer slow layers surrounding a beamed inner region is necessary to explain the strong Fe feature observed by BeppoSAX.