The hard X-ray continuum of Cen A observed with INTEGRAL SPI

Abstract

We revisit the average hard X-ray spectrum from the AGN of Centaurus A (Cen A) using ten years worth of observations with INTEGRAL SPI. This source has the highest flux observed from any AGN in the SPI bandpass (23 keV--8 MeV). The 10 year lightcurve of Cen A is presented, and hardness ratios confirm that the spectral shape changes very little despite the luminosity varying by a factor of a few. Primarily, we establish the presence of a reflection component in the average spectrum by demonstrating an excess between 20-60 keV, from extending the spectral shape observed at low energy to the SPI regime. The excess in Chandra HETGS and INTEGRAL SPI data is well described by reflection of the dominant power law spectrum from a neutral, optically-thick atmosphere. We find that the reprocessed emission contributes 20-25% of the 23-100 keV flux. The existence of a cut-off at tens to hundreds of keV remains controversial. Using simulated spectra, we demonstrate that a high energy cut off reproduces the observed spectral properties of Cen A more readily than a simple power law. However, we also show that such a cut-off is probably underestimated when neglecting (even modest) reflection, and for Cen A would be at energies >700 keV, with a confidence of >95%. This is atypically high for thermal Comptonizing plasmas observed in AGN, and we propose that we are in fact modelling the more gradual change in spectral shape expected of synchrotron self-Compton spectra.