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Abstract

The results obtained with INTEGRAL on Seyfert galaxies are in agreement with those of previous missions. For some of the studied sources, an exponential cut-off at > 50 keV in the X-ray spectrum represents the signature of Comptonisation processes. For the brightest AGN at hard X-rays, the Seyfert NGC 4151, we were able to constrain the different physical parameters of the Comptonisation process, such as the electron temperature, the optical depth of the corona, and the fraction of emission reflected by cold matter. In the frame of a multiwavelength collaboration for the study of blazars in outburst, INTEGRAL has observed the two blazars S0716+714 and 3C 454.3. For the latter, the INTEGRAL spectra that we extracted, together with the other multiwavelength data, have shown that the high-energy emission is mainly produced through synchrotron self-Compton of the electrons in the jet rather than through external Compton processes.

Our understanding of the cosmic X-ray background (CXB) is strongly connected to our knowledge of the AGN spectrum in this energy domain, since AGN are most likely at the origin of this apparently diffuse emission. Thanks to its large field of view and its angular resolution, INTEGRAL/IBIS has provided us with the first opportunity to make a large-area AGN survey above 20 keV. Our study of the early IBIS/ISGRI data on a local and bright sample of Seyfert galaxies showed that these objects account for 1% of the CXB in the hard X-ray band and that the fractions of absorbed and Compton thick AGN over unabsorbed AGN, and their dependence on the energy band considered, are consistent with the predictions of the most recent CXB models.

In order to analyze also the timing properties of AGN, particularly in the X-rays, we studied the multiwavelength variability of the bright quasar 3C 273. We updated the 3C 273 database (created in 1999) to include more than 40 years of observations. This variability study revealed a further proof of the presence of dust in 3C 273, thanks to the correlation found between the heating radiation (in the UV and optical) and the emitted near-IR component. As another result of the inter-band correlation analysis, we suggested that the hard X-rays are produced by inverse Compton processes by the same electrons that are emitting in the optical band through synchrotron. Furthermore, the different timing properties of the X-ray emission below and above 20 keV could be explained either by two separated components or by a single component described by at least two independent parameters.

With the goal of studying the timing properties of the hard X-ray emission of AGN, we present a study of the first 9 months of Swift/BAT data performed on a sample of hard X-ray selected AGN. The most important result of this work is that the more absorbed sources turned out to be more variable than less absorbed ones. This is likely connected with the observation that the more absorbed sources are also less luminous.