* Astronomy

Blobrana · L

RE: Seyfert galaxies

Blobrana · L

Radio-telescope images have revealed previously-unseen galactic cannibalism -- a triggering event that leads to feeding frenzies by gigantic black holes at the cores of galaxies. Astronomers have long suspected that the extra-bright cores of spiral galaxies called Seyfert galaxies are powered by supermassive black holes consuming material. However, they could not see how the material is started on its journey toward the black hole.
One leading theory said that Seyfert galaxies have been disturbed by close encounters with neighboring galaxies, thus stirring up their gas and bringing more of it within the gravitational reach of the black hole. However, when astronomers looked at Seyferts with visible-light telescopes, only a small fraction showed any evidence of such an encounter. Now, new images of hydrogen gas in Seyferts made using the National Science Foundation's Very Large Array (VLA) radio telescope show the majority of them are, in fact, disturbed by ongoing encounters with neighbor galaxies.

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Blobrana · L

Title: Seyfert galaxies in the local Universe (z <= 0.1): the average X-ray spectrum as seen by BeppoSAX
Authors: Mauro Dadina

The BeppoSAX archive is currently the largest reservoir of high sensitivity simultaneous soft and hard-X ray data of Seyfert galaxies. From this database all the Seyfert galaxies (105 objects of which 43 are type I and 62 are type II) with redshift lower than 0.1 have been selected and analysed in a homogeneous way (Dadina 2007). The X-ray data so collected allow to infer the average spectral properties of nearby Seyfert galaxies included in the original sample and, most notably: the photon index (\Gamma ~1.8), the high-energy cut-off (Ec ~290 keV), and the relative amount of reflection (R ~1.0). The data have been also used to test some assumptions of the unified scheme for the AGN. The distributions of the isotropic indicators (photon index, relative amount of reflection, high-energy cut-off and narrow FeK \alpha energy centroid) are similar in type I and type II objects while the absorbing column and the iron line equivalent width significantly differ between the two classes of active galactic nuclei. Confirming previous results, the narrow FeK \alpha line is consistent, in Seyfert 2, with being produced in the same matter responsible for the observed obscuration. These results, thus, support the basic picture of the unified model. Moreover, the presence of a X-ray Baldwin effect in Seyfert 1 has been here measured using the 20-100 keV luminosity (EW \propto L(20-100)$^{-0.22 ±0.05}). Finally, the possible presence of a correlation between the photon index and the amount of reflection is confirmed thus indicating thermal Comptonisation as the most likely origin of the high energy emission for the active galactic nuclei included in the original sample.

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Blobrana · L

Title: The Seyfert galaxies in the local Universe: from BeppoSAX to Simbol-X
Authors: Mauro Dadina

The operational conditions found by BeppoSAX in observing nearby (z <= 0.1) Seyferts were reproduced for Simbol -X in order to simulate a realistic final database of the mission. The results indicate that, even in the worst conditions, the Simbol - X archive of pointings will allow to fully characterise the high-energy spectrum of nearby Seyferts and, most importantly, to obtain solid results on R and Ec (fundamental to model the cosmic X-Ray background, CXB). The measurement of the inclination angle of the accretion disk will be possible for ~15 objects allowing to directly test the unified models for AGN. Finally, the time-dependent characteristics of the reflected component will be studied in at least ~25 objects.

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