High-energy break and reflection features 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 1995 August-September. The ASCA observations, split into two pointings of similar to 10 ks each, indicate a modest flux increase (similar to 20%) in 3 days, possibly correlated to a softening of the 0.6-9 keV spectrum. The spectrum obtained combining data from both the observations is well described by a hard power law (Gamma similar to 1.64 +/- 0.03) 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-ray/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. A joint fit to the complete OSSE data sets and our ASCA data clearly shows an exponential cutoff of the power law at similar to 300 keV (e-folding energy), as well as a quite strong reflection component. MCG fg-ll-ll is then one of the few Seyfert galaxies for which a spectral break in the underlying continuum is detected unambiguously, fitting well in the X-ray/gamma-ray description of co-added spectra of Seyfert galaxies. Though the break could be due to downscattering of X-rays on cold e(+)e(-) pairs in a nonthermal plasma, the inferred low compactness of this source favors instead thermal or quasi-thermal electron Comptonization in a structured corona as the leading process of high-energy radiation production.