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Seyfert galaxies are the closest active galactic nuclei. As such, we can use
them to test the physical properties of the entire class of objects. To investigate
their general properties, I took advantage of different methods of data analysis. In
particular I used three different samples of objects, that, despite frequent overlaps,
have been chosen to best tackle different topics: the heterogeneous BeppoS AX
sample was thought to be optimized to test the average hard X-ray (E above 10 keV)
properties of nearby Seyfert galaxies; the X-CfA was thought the be optimized to
compare the properties of low-luminosity sources to the ones of higher luminosity
and, thus, it was also used to test the emission mechanism models; finally, the
XMM–Newton sample was extracted from the X-CfA sample so as to ensure a
truly unbiased and well defined sample of objects to define the average properties
of Seyfert galaxies.
Taking advantage of the broad-band coverage of the BeppoS AX MECS and
PDS instruments (between ~2-100 keV), I infer the average X-ray spectral propertiesof nearby Seyfert galaxies and in particular the photon index (~1.8), the
high-energy cut-off (~290 keV), and the relative amount of cold reflection
(~1.0). Moreover the unified scheme for active galactic nuclei was positively
tested. The distribution of isotropic indicators used here (photon index, relative
amount of reflection, high-energy cut-off and narrow FeK 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 sources with type
II objects displaying larger absorbing columns. Taking advantage of the XMM–Newton and X–CfA samples I also deduced from measurements that 30 to 50%
of type II Seyfert galaxies are Compton thick.
Confirming previous results, the narrow FeK line is consistent, in Seyfert 2
galaxies, with being produced in the same matter responsible for the observed obscuration.
These results support the basic picture of the unified model. Moreover,
the presence of a X-ray Baldwin effect in type I sources has been measured using
for the first time the 20-100 keV luminosity (EW proportional to L(20-100)^(−0.22±0.05)). This finding
suggests that the torus covering factor may be a function of source luminosity,
thereby suggesting a refinement of the baseline version of the unifed model itself.
Using the BeppoSAX sample, it has been also recorded a possible correlation
between the photon index and the amount of cold reflection in both type I
and II sources. At a first glance this confirms the thermal Comptonization as the
most likely origin of the high energy emission for the active galactic nuclei. This
relation, in fact, naturally emerges supposing that the accretion disk penetrates, depending
to the accretion rate, the central corona at different depths (Merloni et al. 2006): the higher accreting systems hosting disks down to the last stable orbit while
the lower accreting systems hosting truncated disks. On the contrary, the study of
the well defined X–C f A sample of Seyfert galaxies has proved that the intrinsic X-ray
luminosity of nearby Seyfert galaxies can span values between 10^(38−43) erg s^−1,
i.e. covering a huge range of accretion rates. The less efficient systems have been
supposed to host ADAF systems without accretion disk. However, the study of the
X–CfA sample has also proved the existence of correlations between optical emission
lines and X-ray luminosity in the entire range of L_(X) covered by the sample.
These relations are similar to the ones obtained if high-L objects are considered.
Thus the emission mechanism must be similar in luminous and weak systems.
A possible scenario to reconcile these somehow opposite indications is assuming
that the ADAF and the two phase mechanism co-exist with different relative
importance moving from low-to-high accretion systems (as suggested by the Gamma vs.
R relation). The present data require that no abrupt transition between the two
regimes is present.
As mentioned above, the possible presence of an accretion disk has been tested
using samples of nearby Seyfert galaxies. Here, to deeply investigate the flow patterns
close to super-massive black-holes, three case study objects for which enough
counts statistics is available have been analysed using deep X-ray observations
taken with XMM–Newton. The obtained results have shown that the accretion
flow can significantly differ between the objects when it is analyzed with the appropriate
detail. For instance the accretion disk is well established down to the
last stable orbit in a Kerr system for IRAS 13197-1627 where strong light bending
effect have been measured. The accretion disk seems to be formed spiraling in
the inner ~10-30 gravitational radii in NGC 3783 where time dependent and recursive modulation
have been measured both in the continuum emission and in the broad emission line
component. Finally, the accretion disk seems to be only weakly detectable in rk
509, with its weak broad emission line component.
Finally, blueshifted resonant absorption lines have been detected in all three
objects. This seems to demonstrate that, around super-massive black-holes, there is
matter which is not confined in the accretion disk and moves along the line of sight
with velocities as large as v~0.01-0.4c (whre c is the speed of light). Wether this
matter forms winds or blobs is still matter of debate together with the assessment
of the real statistical significance of the measured absorption lines. Nonetheless,
if confirmed, these phenomena are of outstanding interest because they offer new
potential probes for the dynamics of the innermost regions of accretion flows, to
tackle the formation of ejecta/jets and to place constraints on the rate of kinetic
energy injected by AGNs into the ISM and IGM. Future high energy missions
(such as the planned Simbol-X and IXO) will likely allow an exciting step forward
in our understanding of the flow dynamics around black holes and the formation of
the highest velocity outflows.
