2011A&A...531A..70B

The radio galaxy Cen A has been detected all the way up to the TeV energy range. This raises the question about the dominant emission mechanisms in the high-energy domain. Spectral analysis allows us to put constraints on the possible emission processes. Here we study the hard X-ray emission, in order to distinguish between a thermal and a non-thermal inverse Compton process. Using hard X-ray data provided by INTEGRAL, we determined the cut-off of the power-law spectrum in the hard X-ray domain (3-1000keV). In addition, INTEGRAL data are used to study the spectral variability. The extended emission detected in the gamma-rays by Fermi/LAT is investigated using the data of the spectrometre SPI in the 40-1000keV range. The hard X-ray spectrum of Cen A shows a significant cut-off at energies E_C=434⁺106^_–73keV with an underlying power-law of photon index Γ=1.73±0.02. A more physical model of thermal Comptonisation (compPS) gives a plasma temperature of kT_e=206±62keV within the optically thin corona with Compton parameter y=0.42⁺0.09^_–0.06. The reflection component is significant at the 1.9σ level with R=0.12⁺0.09^_–0.10, and a reflection strength R>0.3 can be excluded on a 3σ level. Time resolved spectral studies show that the flux, absorption, and spectral slope varied in the range f_3–30keV=1.2-9.2x10^–10erg/cm2/s, N_H=7-16x10²²cm^–2, and Γ=1.75-1.87. Extending the cut-off power-law or the Comptonisation model to the gamma-ray range shows that they cannot account for the high-energy emission. On the other hand, a broken or curved power-law model can also represent the data, therefore a non-thermal origin of the X-ray to GeV emission cannot be ruled out. The analysis of the SPI data provides no sign of significant emission from the radio lobes and gives a 3σ upper limit of f_40–1000keV≲1.1x10^–3ph/cm2/s. While gamma-rays, as detected by CGRO and Fermi, are caused by non-thermal (jet) processes, the main process in the hard X-ray emission of Cen A is still not unambiguously determined, since it is either dominated by thermal inverse Compton emission or by non-thermal emission from the base of the jet.