The aim of this short paper is to motivate and encourage research in the field of Active Galactic Nuclei (AGN). Here we summarize the main open questions concerning the central engine. Is the central black hole rapidly spinning and can we prove this?
What is the dominant accretion mechanism in AGN? Why do some AGN form jets while others dont and how do the jets originate? What keeps jets collimated out to distances of 100 kpc? Is the emission of blazars dominated rather by synchrotron self-Compton or by external Compton processes? Which parameters are important in the unified model? We outline the status of related research, formulate the questions and try to hint at research projects able to tackle these fundamental topics. Deep surveys, polarization measurements, improved models, faster and more accurate simulations as well as bridging the gap in the MeV range can be part of the tools to bring us closer to an understanding of AGN.
NGC 4945 has an outstanding role among the Seyfert 2 active galatic nuclei (AGN) because it is one of the few non-blazars which have been detected in the gamma-rays. Here, we analyse the high energy spectrum using Suzaku, INTEGRAL and Fermi data. We
reconstruct the spectral energy distribution in the soft X-ray to gamma-ray domain in order to provide a better understanding of the processes in the AGN. We present two models to fit the high-energy data. The first model assumes that the gamma-ray emission originates from one single non-thermal component, e.g. a shock-induced pion decay caused by the starburst processes in the host galaxy, or by interaction with cosmic rays. The second model describes the high-energy spectrum by two independent components: a thermal inverse Compton process of photons in the non-beamed AGN and a non-thermal emission of the gamma-rays. These components are represented by an absorbed cut-off power law for the thermal component in the X-ray energy range and a simple power law for the non-thermal component in the gamma-rays. For the thermal process, we obtain a photon index of Gamma=1.6, a cut-off energy of Ec ~ 150 keV and a hydrogen column density of NH = 6e24 1/cm**2. The non-thermal process has a photon index of Gamma=2.0 and a flux of F(0.1-100 GeV) = 1.4e-11 erg/cm**2/sec. The spectral energy distribution gives a total unabsorbed flux of F(2 keV - 100 GeV) = 5e-10 erg/cm**2/sec and a luminosity of L(2 keV - 100 GeV) = 9e41 erg/sec at a distance of 3.7 Mpc. It appears more reasonable that the gamma-ray emission is independent from the AGN and could be caused e.g. by shock processes in the starburst regions of the host galaxy.
The AGN NGC 2110 presents a peculiar case among the Seyfert 2 galaxies, as it displays also features of radio-loud objects and is classified as FR-I radio galaxy. Here we analyse simultaneous INTEGRAL and Swift data taken in 2008 and 2009. We reconst
ruct the spectral energy distribution in order to provide further insight. The combined X-ray spectrum is well represented by an absorbed cut-off power law model plus soft excess. Combining all available data, the spectrum appears flat (photon index 1.25 +- 0.04) with the high-energy cut-off being at 82 +- 9 keV. The intrinsic absorption is moderate (NH = 4E22 1/cm**2), the iron K-alpha line is weak (EW = 114 eV), and no reflection component is detected in the INTEGRAL spectrum. The data indicate that the X-ray spectrum is moderately variable both in flux and spectral shape. The 2008 spectrum is slightly steeper (photon index 1.5, Ec = 90 keV) with the source being brighter, and flatter in 2009 (photon index 1.4, Ec = 120 keV) in the lower flux state. The spectral energy distribution gives a bolometric luminosity of L = 2E44 erg/sec. NGC 2110 appears to be a borderline object between radio loud narrow line Seyfert 1 and radio quiet Seyfert 2. Its spectral energy distribution might indeed be dominated by non-thermal emission arising from the jet.
The INTEGRAL satellite, which studies the Universe in the hard X-ray and soft Gamma-ray domain, has been operational for 5 years now. The X-ray telescopes, which use the coded mask technique, provide unprecedented spectral and imaging resolution. Thi
s led to a number of discoveries, such as the distribution of diffuse emission in the Galaxy, the discovery of highly absorbed sources and fast X-ray transients in the Galactic Plane, localization of ~50 Gamma-ray bursts, and the resolution of the cosmic X-ray background around its peak at 30 keV. About 300 previously known X-ray sources have been detected and in addition more than 200 new sources have been discovered. INTEGRAL provides spectra starting at 3 keV and ranging up to several hundred keV. This article gives a brief overview about the major discoveries of INTEGRAL.
The Seyfert 2 galaxies NGC 2992 and NGC 3081 have been observed by INTEGRAL and Swift. We report about the results and the comparison of the spectrum above 10 keV based on INTEGRAL IBIS/ISGRI, Swift/BAT, and BeppoSAX/PDS. A spectrum can be extracted
in the X-ray energy band ranging from 1 keV up to 200 keV. Although NGC 2992 shows a complex spectrum below 10 keV, the hard tail observed by various missions exhibits a slope with photon index = 2, independent on the flux level during the observation. No cut-off is detectable up to the detection limit around 200 keV. In addition, NGC 3081 is detected in the INTEGRAL and Swift observation and also shows an unbroken Gamma = 1.8 spectrum up to 150 keV. These two Seyfert galaxies give further evidence that a high-energy cut-off in the hard X-ray spectra is often located at energies E_C >> 100 keV. In NGC 2992 a constant spectral shape is observed over a hard X-ray luminosity variation by a factor of 11. This might indicate that the physical conditions of the emitting hot plasma are constant, while the amount of plasma varies, due to long-term flaring activity.
