High‐Energy Break and Reflection Features in the Seyfert Galaxy MCG +8‐11‐11

Abstract

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 ~10 ks each, indicate a modest flux increase (~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 (Γ ~ 1.64 ± 0.03) absorbed by a column density slightly larger than the Galactic value. An iron line at 6.4 keV of EW ~ 400 eV is also required by the data. The simultaneous OSSE data are characterized by a much softer power law with photon index Γ=3.0+ 0.9−0.8 , strongly suggesting the presence of a spectral break in the hard X-ray/soft γ-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 ~300 keV (e-folding energy), as well as a quite strong reflection component. MCG +8-11-11 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/γ-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.