From an extremely long hard X-ray look at AGN we found unexpected differences amongst different classes of objects. These differences could be explained by supermassive black holes living in different environments, and might help shading light on the origin of the peak of the cosmic X-ray background. |
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ABSTRACT
Artist's impression of the different environments surrounding the supermassive black hole in different classes of objects This image illustrates two different configurations of the absorbing material surrounding Active Galactic Nuclei (AGN) of different types. In the left of the two upper panels, the black hole and the accretion disc feeding it are surrounded by a patchy and sparse distribution of gas clouds. The clouds absorb only slightly the radiation emitted by the central source and give rise to little reflection (or none) at hard X-ray wavelengths. In the right of the two upper panels, instead, the black hole and accretion disc are embedded in a much denser environment of clouds. This different morphology results in both stronger reflection at hard X-ray wavelengths and stronger absorption at shorter wavelengths. Such a scenario could explain the excess of hard X-ray emission detected in a sample of AGN observed with INTEGRAL. The excess emission characterises the AGN affected by stronger absorption at shorter wavelengths (from the infrared to the soft X-ray bands). The left upper panel corresponds to the active nucleus of a Lightly Obscured (LOB) Seyfert 2 galaxy, such as NGC 4258 (also known as M 106) which is shown in the lower panel to the left; the right upper panel corresponds to the active nucleus of a Mildly Obscured (MOB) Seyfert 2 galaxy, such as NGC 4941 which is shown in the lower panel to the right.
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Ratio between the average spectra of Seyfert 2s and Seyfert 1/1.5s Ratio between the normalized average hard X-ray spectra of Seyfert 1/1.5s and Seyfert 2s. The average spectrum of Seyfert 2s shows a larger reflection component than that of Seyfert 1/1.5s. This cannot be easily explained by the unified model of AGN, which predicts that at these energies all the classes have the same average emission. Credits: ISDC/C. Ricci |
Ratio between the average spectra of MOB Sy2s and Sy1s Ratio between the normalized average hard X-ray spectra of mildly obscured (MOB) Seyfert 2s and Seyfert 1s. Most of the reflection observed in the average spectrum of Seyfert 2s is due to the contribution of MOB Seyfert 2s, while less obscured objects have an average emission consistent with that that of Seyfert 1s and Seyfert 1.5s. Credits: ISDC/C. Ricci |
Fit to the spectrum of the Cosmic X-ray Background Fit to the spectrum of the Cosmic X-ray Background (CXB) measured by INTEGRAL and Swift/BAT. The model used is a combination of our best-fit models for the average spectra of the different classes of objects, normalizing their ratio using the column density distribution of Treister et al. (2009). The large reflection of MOB Seyfert 2s reproduces well the peak at ~30 keV of the CXB. Credits: ISDC/C. Ricci |
by Claudio Ricci on 2011-08-02 | >> INTEGRAL science results | >> All science results |