0520037

We propose to monitor NGC 4151 with INTEGRAL to accurately and simultaneously measure the spectral shape up to above 200 keV and to study large amplitude variations with an unprecedented sensitivity and time coverage at high energy. We will correlate the high energy spectral cutoff with other continuum parameters to confirm thermal comptonisation and determine the main physical mechanism driving the variability. We will also disentangle the reflection hump and its variations from the primary continuum and intrinsic absorption and correlate those with the narrow iron line variability to determine the nature of the thick reprocessing medium and the geometry of cold matter surrounding the source. We will finally probe for the existence of a hard non thermal tail which will constrain the electrons acceleration mechanisms. We request 4 observations of 400 ksec each. [truncated! Please see actual data for full text]