BeppoSAXObservations of Centaurus A: The Hard Continuum and the Iron‐Line Feature

The radio galaxy Centaurus A was observed by the BeppoSAX satellite fivetimes from 1997 to 2000. From July 6 1999 to August 17 1999, the source wasalso simultaneously pointed by COMPTEL on-board of the satellite CGRO.Centaurus A has a complex spectrum with multiple extended components and astrongly absorbed (N_H\sim 10^{23}$ cm${-2}) nucleus well fitted by a power law(\Gamma \sim 1.8) which bends at high energies. When the BeppoSAX and COMPTELobservations are combined together, an exponential cutoff with e-folding energy\sim 1000 keV gives an adequate description of the spectral steepening. Acomplex feature in emission at 6-7 keV is resolved into two Fe components, onenarrow cold line and an ionized line centred at 6.8 keV. Significant variationshave been observed in the iron feature, with the less prominent ionized lineseemingly being the only one responsible for them: its variations do not appearto correlate with the strength of the continuum. The high energy cutoff and theFe feature suggest the presence of an accretion flow in the Centaurus Anucleus. However the absence of a significant reflection, the narrowness of thecold line as well as the lack of correlation between the continuum and 6.8 keVline variations disfavour a standard cold/ionized thin disk (at least in theinner regions). A more plausible configuration might be a hot thick opticallythin accretion flow surrounded by material with different opacities. Finally, we note that high energy break observed by BeppoSAX and COMPTELcould be also reasonably explained by Inverse Compton radiation from a jet. Ifthis is the case, a structured jet with outer slow layers surrounding a beamedinner region is necessary to explain the strong Fe feature observed byBeppoSAX.Comment: 21 pages, 8 figures, accepted for publication in Ap