The hard X-ray emission of Centaurus A

Context. 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. Aims. 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. Methods. Using hard X-ray data provided by INTEGRAL, we determined the cut-off of the power-law spectrum in the hard X-ray domain (3-1000 keV). 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 40-1000 keV range. Results. The hard X-ray spectrum of Cen A shows a significant cut-off at energies E-C = 434(-73)(+106) keV with an underlying power-law of photon index Gamma = 1.73 +/- 0.02. A more physical model of thermal Comptonisation (compPS) gives a plasma temperature of kT(e) = 206 +/- 62 keV within the optically thin corona with Compton parameter y = 0.42(-0.06)(+0.09). The reflection component is significant at the 1.9 sigma level with R = 0.12(-0.10)(+0.09), and a reflection strength R > 0.3 can be excluded on a 3 sigma level. Time resolved spectral studies show that the flux, absorption, and spectral slope varied in the range f(3-30keV) = 1.2-9.2 x 10(-10) erg cm(-2) s(-1), N-H = 7-16 x 10(22) cm(-2), and Gamma = 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 3 sigma upper limit of f(40-1000keV) less than or similar to 1.1 x 10(-3) ph cm(-2) s(-1). Conclusions. 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.