The High Energy Spectrum of NGC 4151

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Abstract
We present the first INTEGRAL observations of the type 1.5 Seyfert galaxy NGC 4151. Combining several INTEGRAL observations performed during 2003, totaling about 400 ks of exposure time, allows us to study the spectrum in the 2 - 300 keV range. The measurements presented here reveal an overall spectrum from X-rays up to soft gamma-rays that can be described by an absorbed (NH = 6.9e22 1/cm**2) model based on a Compton continuum from a hot electron population (kT = 94 keV) from an optically thick (tau = 1.3) corona, reflected on cold material (R=0.7), consistent with earlier claims. The time resolved analysis shows little variation of the spectral parameters over the duration of the INTEGRAL observations. The comparison with CGRO/OSSE data shows that the same spectral model can be applied over a time span of 15 years, with flux variations of the order of a factor of 2 and changes in the underlying continuum reflected by the temperature of the electron population (kT = 50 - 100 keV). When modeled with an exponentially cut-off power law plus Compton reflection this results in photon indices ranging from Gamma = 1.5 to Gamma = 1.9 and a cut-off energy in the range 100 - 500 keV.

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