A High Energy Study of Active Galactic Nuclei

This thesis contains a study of the hard X-ray properties of active galactic nuclei. The results have been obtained from the analysis of data sets from two different satellites: ComptonGRO-BATSE and BeppeSAX-PDS. The source sample used for this investigation is the Piccinotti sample of AGN, which is an X-ray selected sample and up to now the only complete sample in 2-10 keV band. Nearly four years of BATSE observations have been analyzed to search for emission in the 20-100 keV band from all the sources and for the first time the whole sample was observed at higher energies, with most giving significant detections. Average fluxes have been estimated over the observing period while long term variability has been investigated for the brightest sources. The high energy spectral characteristics of the Piccinotti sample have been studied using the BeppoSAX-PDS data in 20-200 keV band. With this analysis both the Compton reflection and the high energy cut-off have been investigated: both components are required by the high energy data; they are well constrained in the brightest sources (NGC4151 and IC4329A). Finally BeppoSAX MECS data together with the PDS data have been used to study the average 3-200 keV spectra of Seyfert and Seyfert 2 galaxies in order to investigate their average spectral characteristics and the possible differences between the two classes. If an extra absorber that partially covers the source is added to the Seyfert 2 average spectrum then the primary continuum of both type of Seyferts is found to be similar: a power law with photon index 1.9 a reflection of ~1 and a high energy cut-off at around 200 keV.