Detection of a high energy break 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 August–September 1995. The ASCA data are well described by a hard power law (Γ∼1.6–1.7) 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=3.0−0.8+0.9, strongly suggesting the presence of a spectral break in the hard X/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. The joint ASCA+OSSE fit clearly showed a reflection component plus an exponential cutoff of the power law at ∼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.