Extreme flux states of NGC 4151 observed with INTEGRAL

We present a comprehensive spectral analysis of all INTEGRAL data obtained so far for the X-ray--bright Seyfert galaxy NGC 4151. We also use all contemporaneous data from RXTE, XMM, Swift and Suzaku. We find a linear correlation between the medium and hard-energy X-ray fluxes measured by INTEGRAL, which indicates an almost constant spectral index over six years. The majority of INTEGRAL observations were made when the source was either at a very bright or very dim hard--X-ray state. We find that thermal Comptonization models applied to the bright state yields the plasma temperature of 50--70 keV and its optical depth of 1.3--2.6, depending on the assumed source geometry. For the dim state, these parameters are in the ranges of 180--230 keV and 0.3--0.7, respectively. The Compton parameter is y = 1 for all the spectra, indicating a stable geometry. Using this result, we can determine the reflection effective solid angles associated with the close and distant reprocessing media as = 0.3 x 2pi and 0.2 x 2pi, respectively. The plasma energy balance, the weak disc reflection and a comparison of the UV fluxes illuminating the plasma to the observed ones are all consistent with an inner hot accretion surrounded by an outer cold disc. The disc truncation radius can be determined from an approximate equipartition between the observed UV and X-ray emission, and from the fitted disc blackbody model, as 15 gravitational radii. Alternatively, our results can be explained by a mildly relativistic coronal outflow.