OSSE observations of gamma-ray emission from Centaurus A

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 prevous 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 L_y(50-1000 keV) ≈ 7 × 10⁴² ergs s^-1, implying a minimum black hole mass of only ≈ 5 × 10⁴ solar masses for Eddington-limited accretion.