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Results are reported on hard X-ray and soft gamma-ray observations of Centaurus A during 1991-1994 with the Oriented Scintillation Spectrometer Experiment (OSSE) on NASA's Compton Gamma Ray Observatory (CGRO). Long-term intensity changes by a factor of 2-3 were measured over this period. Short-term variations by as much as 25% on timescales as short as 12 hr were detected in most of the observations. No evidence for gamma-ray line emission was detected at sensitivities much improved over previous observations. An intensity-dependent evolution of the 50-1000 keV spectral shape was observed which can be characterized as an exponentially cut-off power law with an intensity-dependent cutoff energy ranging from ∼300 keV at the highest observed level to ∼700 keV at the lowest levels. A broken power-law model with an intensity- correlated break above ∼120 keV describes the observed spectral changes equally well. Best-fit spectral breaks vary from ΔΓ 0.25±0.1 at the lowest levels to ΔΓ 0.7±0.15 at the highest observed intensity. Below 120 keV the spectra at all intensities can be fitted with photon spectral index Γ 1.70±0.08, in agreement with previous X-ray measurements. Contemporaneous measurements by the EGRET instrument on CGRO require additional spectral softening in the MeV region relative to broken power-law extrapolations of the OSSE measurements. At the maximum observed intensity, the luminosity of Cen A appears to peak at ∼200 keV with Lγ(50-1000 keV) ≈7 × 1042 ergs s-1, implying a minimum black hole mass of only ≈5 × 104 solar masses for Eddington-limited accretion.
Kinzer et al. (Tue,) studied this question.