Detection of a high energy break in the Seyfert galaxy MCG+8-11-11

We present the results from ASCA and OSSE simultaneous observations of the Seyfert 1.5 galaxy MCG+8-11-11 performed in. August-September 1995. The ASCA data are well described by a hard power law (Gamma similar to 1.6 - 1.7) absorbed by a column density slightly larger than the Galactic value. An iron line at 6.4 keV of EW similar to 400 eV is also required by the data. The simultaneous OSSE data, are characterized by a much softer power law with photon index Gamma = 3.0(-0.8)(+0.9), strongly suggesting the presence of a spectral break in the hard X/soft gamma-ray band. In order to better investigate the complex hard X-ray spectrum of this source, a total spectrum has been produced by summing over all the observations of MCG+8-11-11 performed by OSSE so far. The joint ASCA+OSSE fit clearly showed a reflection component plus an exponential cutoff of the power law at similar to 300 keV. Although the break could be due to downscattering of X-rays on cold e(+)e(-)-pairs in a non-thermal plasma, the inferred low compactness of this source favours instead thermal or quasi-thermal Comptonization as the likely process of high energy radiation production.