A&A 485, 417-424 (2008)
DOI: 10.1051/0004-6361:20077569
M. Dadina1,2
1 - INAF/IASF-Bo, via Gobetti 101, 40129 Bologna, Italy
2 -
Dipartimento di Astronomia dell'Università degli Studi di Bologna,
via Ranzani 1, 40127 Bologna, Italy
Received 29 March 2007 / Accepted 6 February 2008
Abstract
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 analyzed in a homogeneous way.
Taking advantage of the broad-band coverage of the BeppoSAX MECS and PDS
instruments (2-100 keV), the X-ray data so collected allow us
to infer the average spectral properties of nearby
Seyfert galaxies included in the original sample and, most notably the
photon index (
), the high-energy cut-off (
keV),
and the relative amount of reflection (
).
The data collected have been used to test some basic assumptions of
the unified scheme for active galactic nuclei. The
distributions of
the 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 active galactic nuclei with type II objects displaying larger columns (
and
cm-2 for type I and II objects respectively) and equivalent width (
and 690 eV for type I and II
sources respectively).
Confirming previous results, the narrow FeK
line is consistent, in
Seyfert 2, 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 Seyfert 1 has been measured using the 20-100 keV
luminosity (
(20-100)
). Finally, the possible presence of a correlation
between the photon index and the amount of reflection is confirmed thus
indicating thermal Comptonization as the most likely origin of the high energy
emission for the active galactic nuclei included in the original sample.
Key words: X-rays: galaxies - galaxies: Seyfert: - galaxies: active
The high energy emission from active galactic nuclei (AGN) is thought to come from the innermost regions of accreting systems that are centered around super-massive black-holes (SMBH). For this reason, X-rays are expected to be tracers of the physical conditions experienced by matter before disappearing into SMBH. Moreover, thanks to their high penetrating power, energetic photons, escaping from the nuclear zones, test the matter located between their source and the observer. Thus, they offer powerful diagnostics to understand the geometry and the physical conditions of the matter surrounding the SMBH.
The broad-band of BeppoSAX offered for the first time the opportunity to measure with a remarkable sensitivity the spectral shape of AGN in the 0.1-200 keV range. This potential had been previously exploited to study in detail a number of sample selected in different manners (see for example Maiolino et al. 1998; Malizia et al. 2003; Perola et al. 2002). These studies were fundamental in making important steps forward in the comprehension of the emitting mechanism at work in the production of X-rays (Perola et al. 2002) and to partially reveal the geometry of the cold matter surrounding the central SMBH (Maiolino et al. 1998; Bassani et al. 1999; Risaliti et al. 1999).
The BeppoSAX database full potential, however, was never exploited
before. In a previous paper, the entire catalog of the Seyfert galaxies
at
contained in the BeppoSAX archive has been presented
(Dadina 2007). This sample was selected starting
from the catalog of Seyfert galaxies contained in the Véron-Cetty & Véron (2006) sample of AGN and contains 13 radio-loud objects and 7 narrow-line
Seyfert 1.
The spectral analysis was performed in the 2-100 keV band whenever possible and the data were fit with a set of template models to obtain a homogeneous dataset. Here the X-ray data so collected are statistically inspected to infer the average characteristics of the nearby Seyfert galaxies contained in this sample in the 2-100 keV band. Finally, the present dataset is used to perform simple tests on the unified scheme (UM) for the AGN (Antonucci 1993) and on the emission mechanism acting in the core of the Seyfert galaxies. More detailed analysis on this latter topic will be presented in another paper (Petrucci et al., submitted) where detailed thermal Comptonization models (Poutanen & Svensson 1996; Haardt & Maraschi 1993) will be used to fit the BeppoSAX data to study the dependence of the spectral properties in the ``two phase'' scenario (Haardt 1991; Haardt & Maraschi 1993) assuming different geometries of the corona.
The scope of this section is to determine and study the mean X-ray spectral properties of the sample and to use these values to test the UM model for AGN (Antonucci 1993). The key parameters are the ones that describe the continuum and the absorption properties. In the framework of the UM models for AGN, the continuum shape is expected to be independent to the orientation angle under which the source is observed. Thus, no difference should be measured in the parameters describing the continuum between type I and II objects. On the contrary, the absorbing column intervening in Seyfert 2 should be the principal discriminator between the two classes of objects. Thus measuring the mean X-ray properties is a test of the basic assumptions of the UM.
The origins of the X-ray photons from AGN are thought to be due to
Comptonization of optical-UV radiation, coming from the accretion disk
and Comptonized by the e- in the hot corona that sandwiches the disk (Haardt & Maraschi 1991; Haardt 1993; Poutanen & Svensson 1996; Czerny et al. 2003). The mechanism is assumed to
be, at least at the zero-th order, very similar in each Seyfert galaxy.
Under this hypothesis, the differences
between the X-ray spectra of different objects are supposed to be mainly due
to two kind of factors: i) the time-dependent state of the emitting
source; ii) the intervening matter that imprints on the emerging spectrum the
features typical of its physical state. In such a scenario, the observations
of
many sources can be regarded as the long-term monitoring of a single source.
On the other hand, it is also true that the contrary has some comparison in the
literature: e.g. time sparse observations of a single source in different states can resemble observations of objects with completely different spectral properties. This is the case, for example, of the narrow line Seyfert 1 NGC 4051
that displayed variations in flux/luminosity by a factor of 100
(Guainazzi et al. 1998) associated with strong variations of the spectral shape
(
-2.4; Guainazzi et al. 1998; Turner et al. 1999; Ponti et al.
2006; but see also Crenshaw & Kraemer 2007, for a different
interpretation of this spectral behavior in terms of variable ionized
absorption). To calculate the mean X-ray
properties of the sources included in this work I treated the multiple
observations of single sources separately: i.e. I assumed different
observations of the same source as if they were observations of different
sources.
This method, in principle, is expected to work properly for all those
quantities which are supposed to vary in accordance with the state of activity
of the central nucleus. For example, the
photon index
is known to vary with
the AGN flux state (Lee et al. 2000; Shih et al. 2002;
Ponti et al. 2006) and the high-energy
cut-off (
)
is linked with the temperature of the corona and thus
expected to be variable (Haardt et al. 1997). On the contrary
this method is not expected to work
properly when constant components are considered. This could be the case,
for instance, of the cold absorption assumed to be due to the putative
dusty torus (Antonucci 1993). Thus, for this component, the average value
recorded for each source should be used. Nonetheless, these components also
were observed to vary in a number of objects (see for example the case of
NGC 4151, De Rosa et al. 2006; or Risaliti et al. 2002). Moreover, the
constancy of the properties of the cold absorption is predicted under the
hypothesis of a continuous distribution of the matter that forms the torus. On the contrary, if the torus is formed of blobs/clouds
(Elitzur & Shlosman 2006), variability in the measurements of the
absorbing column is expected.
For both these observational and theoretical reasons the
measured
in each single observation were treated separately.
In between these two cases are the properties of the emission
FeK
line. BeppoSAX had a too low sensitivity to detect the
relativistically broadened component of this feature in a large number
of sources. Thus only the narrow line has been detected in the vast
majority of the objects included in the original catalog.
This component is supposed to originate
far from the SMBH, at least at
1000 Schwarzschild radii
(Mattson & Weaver 2004), i.e. very likely at the inner edge of the torus
(Nandra 2006). At these distances from the SMBH, the relativistic
effects are negligible.
Thus, in principle, the parameters that describe the line should be
regarded as stable. Nonetheless the properties of the narrow
FeK
line were also observed to vary with its line energy
centroid changes between
6.4 keV (neutral iron) and
6.9 keV
(H-like iron) showing different
ionization states. Moreover, the equivalent width (EW) of
this component is not only a function of the intensity of the line itself, but
it is also directly linked to the underlying variable continuum emitted
in the regions close to the SMBH.
This makes EW a variable quantity. The parameters of the emission
FeK
line were thus treated as variable ones in order to test its
origin, and not averaged when objects were observed more than once.
Finally, in a number of cases it was necessary
to deal with censored data (see Table 1).
To manage these data properly, the ASURV software
(Feigelson & Nelson 1985; Isobe et al. 1986) was used.
In particular, to establish if the distributions of parameters of type I
and type II objects were drawn from different parent populations, the
Peto & Prentice generalized Wilcoxon test (Feigelson & Nelson 1985) has been
used while to calculate the mean values considering also the censored data
the Kaplan-Meyer estimator was used. To establish the presence of
correlations between different quantities, both the Spearman's
and the
generalized Kendall's
methods were applied. The linear regressions were
calculated using the Bukley-James and Schmitt`s methods. In the following,
two quantities are considered as drawn from different parent populations
when the probability of false rejection of the null
hypothesis (same parent population) is
%.
Similarly, one accepts that there is a correlation between two given quantities
when the probability of an absence of correlation remains lower than 1%.
Table 1: General characteristics of the data analyzed in this work. The number of detections and censored data are reported for the interesting parameters for the whole sample of objects (Cols. 2 and 3), for the Seyfert 1 galaxies (Cols. 4 and 5), and for the Seyfert 2 objects (Cols. 6 and 7). The 90% confidence interval limits were used for censored data and the detected values were defined if determined with a 99% confidence level (Dadina 2007)
The X-ray continua of the sources have been modeled using a cut-off power-law
that describes the primary emission from the hot corona plus a
reflection component (PEXRAV model in Xspec, Magdziarz & Zdziarski 1995)
to account for the contribution expected to be due to the disk.
However, an additional reflection component could rise from the
torus (Ghisellini et al. 1994) and the disentangling between the two
reflection components is impossible with the quality of the available data.
Wherever its origin, the reflection has been assumed to be due to cold matter.
The interesting parameters
are the photon index (), the relative amount of reflection
(R), and the high-energy cut-off (
).
In Table 2 the results for the whole set of observations and for the two classes of Seyferts are reported as well as the probability that Seyfert 1 and 2 are drawn from the same parent populations. The histograms of the distributions of the interesting parameters for the entire sample of objects (first column) and for type I (second column) and type II (third column) are reported in Fig. 1.
Table 2: Mean spectral properties. Col. I: Spectral parameter; Col. II: Seyfert 1 mean value; Col. III: Seyfert 2 mean value; Col. IV: Probability that Seyfert 1 and Seyfert 2 are drawn from the same parent populations.
![]() |
Figure 1:
Photon index ![]() ![]() ![]() |
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As previously said, the UM for AGN (Antonucci 1993) predicts that ,
R,
and
are observables independent of the inclination angle, thus the two
classes of Seyfert galaxies should display very similar characteristics. This
is confirmed by the analysis of the present sample.
In particular, there are no hints that the distributions of photon
index
for the two types of Seyfert are drawn from different
parent populations (
%).
Moreover, the photon-index peaks, for both classes,
between 1.8-1.9 in agreement with the two-phase models for the production
of the X-ray in Seyfert galaxies that predict
-2.5
(Haardt & Maraschi 1991; Haardt 1993; Haardt et al. 1997).
Few objects have extremely flat spectra with
.
Type II objects that show such hard X-ray spectra are supposed to be
Compton-thick sources for which, in the 2-10 keV band, only the reflected/flat
spectrum is observable. This is the case for NGC 2273 which displays the harder
X-ray spectrum. This source was first classified as a Compton-thick object
by Maiolino et al. (1998). The Seyfert 1s with flattest spectra are NGC 4151
that is known to have a hard spectrum with complex and variable absorption
(De Rosa et al. 2007) and Mrk 231 (classified as a type I AGN by Farrah et al. 2003). The latter source shows a very hard X-ray spectrum with
.
This source is also classified as a BAL QSO
(Smith et al. 1995). A recent spectral analysis in the X-ray
band of this source was presented in Braito et al. (2004). By combining
XMM-Newton and BeppoSAX data, these authors speculated that
the spectrum of the source below
10 keV is reflection dominated, thus
presenting a case that is difficult to reconciled with the UM of AGN. Moreover,
Mrk 231 is an ultra-luminous infra-red galaxy. These sources display strong
starburst activity that can dominate the total X-ray luminosity
of the galaxy (see Franceschini et al. 2003; Ptak et al. 2003, for details
on this topic) although in Mrk 231 at least 60% of the observed
0.5-10 keV flux seems to be due to the AGN component (Braito et al. 2004).
Less conclusive results are obtained for R and .
In both cases, the
probability of false rejection of the null hypothesis (the two
distributions are drawn from same parent populations,
in accordance with the UM predictions), is
%. However,
both type I and II objects lie in the same
range of R and
.
In particular, the distribution of R for type I objects is
dominated by a peak of detections between
-1.2 due to the contribution
of a few sources observed several times (e.g. NGC 5548, IC 4329a). This has
probably introduced a systematic effect not smeared-out by the relatively
small number of useful observations (i.e, the ones for which R and
have
been estimated). On the other hand, the result on
is most probably
polluted by the large number of lower limits for Seyfert 2. If the same test
is performed only on the detections it is obtained that
%, making impossible to assess that the
two samples are drawn from two different populations.
![]() |
Figure 2:
Distribution of the Fek![]() |
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The result is unambiguously in accordance with UM predictions
for the absorbing column. In this
case the statistical tests confirm that Seyfert 1 and Seyfert 2 display
very different absorption characteristics with the type II objects being
more heavily absorbed than Seyfert 1. Again, it is noticeable that a few
type I objects show high column densities (up to 1024 cm-2) and
some Seyfert 2 have low columns (down to 1021 cm-2). These are not
new results: the high column of NGC 4151 (
,
De Rosa et al. 2007) is well known. The highest column
measured in a Seyfert 1 is detected during the June 9, 1998 observation of
NGC 4051. During this observation the source appeared ``switched-off'' and only a pure reflection component was measured (Guainazzi et al. 1998). The spectral
fit in Dadina (2007) degenerated between two solutions, one in which the source was purely reflection-dominated (
)
and a second one in which a direct
component was visible but highly absorbed. The latter scenario was slightly
preferred from a pure statistical point of view when the 2-200 keV band is
condidered, and, for homogeneity, entered in
the catalog (Dadina 2007). Nonetheless, when the entire BeppoSAX band
(0.1-200 keV) is considered, the reflection scheme is preferred (Guainazzi et al. 1998).
Finally, a number of objects show
upper-limits to the absorbing column of the order of 1022
cm-2. This is a selection effect induced by the energy band
considered using only MECS and PDS data (
2-100 keV, Dadina 2007). The
low-energy
cut-off due to such a column (1022 cm-2) peaks at
keV
and only for those objects with good statistics it is possible
to infer upper limits on the
below 1022 cm-2. This is most
probably responsible for the high value obtain for the average
in
type I objects.
The FeK
line is produced by reprocessing the primary X-ray emission
in matter surrounding the source of hard photons. In the framework of the UM, the origin
of this component can be placed in a number of regions such as the accretion
disk, the dusty torus, and the broad-line regions
(even if this last hypothesis is disfavored by the recent results
obtained with the XMM-Newton and Chandra observatories and
presented in Nandra 2006). If the line originates in the disk close to
the SMBH, relativistic effects that broaden the resulting line are expected. For the vast majority of the
sources included in the original sample, only narrow components of such
features were detected and only in a few cases broad emission lines (e.g. IC 4329a) or
relativistically blurred features were detected (for example in MCG-6-30-15).
Thus, the results presented here are essentially based
on the measured properties of the narrow features.
As shown in Fig. 2 (first row), the line energy centroid is peaked
at 6.4-6.5 keV (see also Table 3) in both type I and II objects. The
centroid is slightly
above 6.4 keV but, considering the energy
resolution of the instrument at these energies
(
200 eV FWHM, Boella et al. 1997), the results obtained here
are in agreement with the line being mainly produced in cold or
nearly cold matter (ionization state below FeXVII), i.e. in both type I and II objects, by matter
in the same physical state. However, a well known difference between the two
classes of objects is the EW of the narrow FeK
line in type II
objects, which shows stronger features than type I objects (see second row of
Fig. 2 and Table 3, Bassani et al. 1999; Risaliti et al. 1999; Cappi et al. 2006; Panessa
et al. 2006). As shown in the
central panel of Fig. 2, the Seyfert 1s peak at
-200 eV while
the type II have a broader distribution with a hard tail that reaches values
well above 1 keV. Also few Seyfert 1 have large values of EW of the
FeK
line (above 300-400
eV). The large EW tail of the Seyfert 1 distribution is composed mainly of
objects in which the broad components of the FeK
line are detected such
as in MCG-6-30-15, Mrk 841 and IC 4329a. The Seyfert 1 with largest EW is NGC
4051 during the June 9, 1998 observation when its spectrum was due to pure
reflection (Guainazzi et al. 1998).
Table 3:
Mean properties of the FeK
emission line. Column I: spectral parameter; Column II mean value for the whole sample; Column III: mean value for Seyfert 1; Column IV: mean value for Seyfert 2. Column V: Probability that the parameters of type I and type II objects are drawn from the same parent population.
![]() |
Figure 3:
Left panel: Log (
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The larger FeK
EW in Seyfert 2 galaxies is in agreement with the
UM (Antonucci 1993). If the origin of this component is indeed located
in the dusty torus, than the line EW has to be correlated with
the absorber column density. This is indeed what is observed also in this
sample (see Fig. 3, left panel). Moreover, the Spearman
and
Kendall's
tests indicate that a correlation between the FeK
EW
and the
is highly probable for type II objects (
%),
i.e. for that sources for which we can have direct evidence of the torus
absorbing column.
The robustness of the
estimates have been tested by correlating it with the model independent
indicator offered by the ratio of the observed fluxes at 2-10 and 20-100 keV
respectively (center panel of Fig. 3). The two
quantities are strongly correlated (generalized Spearman
and
Kendal
tests give
%) with only a few exceptions:
the pure Compton-thick sources which
are located in the diagram below the majority of the sources.
This effect is expected since the
absorbing column can affect the X-ray radiation below
10 keV while at
harder energies the radiation pierces the matter for columns
3-
cm-2. Otherwise, the Compton absorption
dominates and also photons with energy above 10 keV are stopped since the
Klein-Nishina regime is reached.
![]() |
Figure 4:
Panel a): Log (
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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As stated above, when the EW of the FeK
emission line is tested
against the measured
(left panel of Fig. 3) a result in good
agreement with what is predicted by theoretical models is obtained
(Makishima 1986; Leahy & Creighton 1993). The majority of the sources,
in fact, behave as expected if the line is produced by the absorbing matter
that depresses the direct continuum (Makishima 1986).
All the known Compton-thick sources are located at low
and high EW, in accordance with previous results (Bassani et al. 1999; Risaliti et al. 1999). As an additional test, the EW
of the FeK
line has been plotted
versus the
/
ratio. As expected (see right panel of Fig. 3), a good correlation (
% according to generalized Spearman
and Kendall
tests) is obtained.
These results thus confirm that the properties of the FeK
line
agree with the expectations of the UM for AGN.
Nonetheless, this is not the only information we have about the iron line.
In recent papers (Iwasawa & Taniguchi 1993; Page et al. 2004; Grandi et al. 2006; Bianchi et al. 2007) it has been claimed that a X-ray ``Baldwin effect'' (or Iwasawa-Taniguchi effect) is present in AGN when the FeK
intensity
is probed against the 2-10 keV luminosity. Here this effect is tested
considering for the first time both the 2-10 keV and the 20-100 keV
luminosities.
A strong correlation (
% using Spearman
and generalized Kendal
test) is found when the EW of the FeK
line is plotted both against the observed 2-10 keV (panel (a) of Fig. 4) and 20-100 keV (panel (c) of Fig. 4) keV luminosities.
The nature of these correlations, however, is not straightforward, especially
for the 2-10 keV luminosity. In this energy band the effect of the absorber is
very important. As previously demonstrated, the EW of the iron line correlates
with
,
but stronger absorbing columns imply lower fluxes.
Moreover, when the relation between the FeK
line EWand the 2-10 keV flux is investigated (panel (b) of Fig. 4), it is found
that the two quantities are correlated (
%).
This is explainable only in terms of instrumental effects, since the
sensitivity of the instruments to narrow features decreases with the source's
flux. Thus, to be detected in faint objects, the
FeK
line must be strong enough. Since the original sample
is limited to the local Universe, this correlation in flux acts, at least
partially, also in the EW vs.
relation.
![]() |
Figure 5:
Left panel: photon index ![]() ![]() ![]() ![]() ![]() |
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This
effect should be negligible when the 20-100 keV band is
considered.
In fact, in this case, one expects to find a correlation between the observed
20-100 keV luminosity and the EW of the FeK
line only in the most
extreme cases, i.e. for the ``pure Compton thick'' objects. Apart from
NGC 1068, these sources are too weak to be detected by the PDS, thus
unable to drive the relation observed in plot (c) of Fig. 4.
Moreover, panel (e) of Fig. 4 indicates that the EW of the iron line is not
related to the 20-100 keV flux thus excluding that the ``Baldwin effect''
measured using the 20-100 keV band is due to instrumental selection effect as
it happens for the
.
On the other hand, in the 20-100 keV band the reflection-hump
at
30-40
keV contributes to the observed flux. If the origin of the FeK
line is
due to the same
matter responsible of the reflection, one should expect that the EW of the
iron line should increase as the reflection component augments the 20-100 keV
flux (i.e. the iron line EW should correlate with the 20-100 keV flux).
Thus, the net contribution of the reflection component should act in the
opposite direction to that observed (i.e. larger iron line EW at smaller
20-100 keV flux). If the two classes of Seyfert galaxies are analyzed
separately it is obtained that a strong correlation is found for Seyfert 1
(
% using both Spearman
and generalized Kendal
tests, panel (d) of Fig. 4) while no correlation
is evident for type II objects (
%).
This result is not unexpected since the EW of the obscured sources are
boosted by the suppression of part of the underlying continuum. To conclude,
the presence of a X-ray Baldwin effect for Seyfert 1 is unambiguously
confirmed by the present data if the
is considered and it has
the following relation:
![]() |
(1) |
The origin of the X-ray ``Baldwin effect'' is unclear. In the light bending
scenario (Miniutti & Fabian 2004) the height of the source above the
accretion disk determines the degree of beaming along the equatorial plane
of the high energy emission. Because of relativistic effects, the closer the
source is to the disk, the greater will be the fraction of
X-rays beamed in the equatorial plane (i.e. towards the disk) and
correspondingly lower will be the observed flux.
Thus, the EW of the relativistically blurred FeK
line produced
in the inner regions of the disk and the EW of the narrow iron line produced
in the outer parts of the disk would appear enhanced in low-state sources.
On the contrary, Page et al. (2004)
speculated that this effect could indicate that luminous
sources are surrounded by dusty tori with lower covering factor thus
pointings towards a torus origin of most of the narrow FeK
lines.
The present work supports this view. The FeK
line EW of the iron line
correlates with the observed
as predicted by theory (Makishima 1988;
Lehay & Creighton 1993; Ghisellini et al. 1994).
Moreover, the case ofthe extremely low state of NGC 4051
(Guainazzi et al. 1998) included in this dataset also seems to point in this
direction. The very large EW of the narrow FeK
line recorded in this
observation is typical of Compton-thick type II objects, but the
line does not show evidence of relativistic
broadening due to the contribution of the inner orbits of the accretion disk.
In the fitting procedure some parameters may degenerate given the
interdependence among them. This is the case, for example of the photon index
with the column density for low statistics observations. The same
could happen for the determination of R and ,
since R introduces in the
spectrum a bump peaking between 20-40 keV and declining at higher energies
where the
may appear.
To check if the results presented here are affected by such effects,
the correlations between these parameters have been studied and the results
are presented in Fig. 5.
The left panel of Fig. 5 shows how, on average, the estimate
of
is not affected by the simultaneous determination of the absorbing
column. No trend is observed between
and
.
Obviously,
this does not imply that this is true for each single source included in the
original sample. On the other hand, this is an expected result since
the broad band of BeppoSAX should reduce this spurious effect.
Similar results are obtained also when the
vs.
,
and R vs.
(center panel of Fig. 5) relations are
investigated. In these cases the Spearman
and
Kendall's
tests do not support the existence of
a relation between these quantities (
%).
All these indications suggest that, if any, the possible
degeneracy in the fitting procedure did not introduce strong
spurious relations between the spectral parameters.
However a strong correlation (
%) is recorded
between
and R (that are linked by the following relation):
![]() |
(2) |
A similar relation was previously found using
Ginga and RXTE data (Zdziarski et al. 1999; Gilfanov et al. 1999). Zdziarski et al. (1999)
interpreted this as evidence of thermal Comptonization as the origin of
X-rays providing
that the optical-UV seed photons were mainly produced by the same material
responsible for the reflected component. In this case the cooling
rate of the hot corona is directly linked to the power-law slope. But the
cooling rate is also related to the angle subtended by the reflector. This
result is also in agreement with predictions of models that consider mildly
relativistic outflows driven by magnetic flares (Beloborodov 1999).
In general, Merloni et al. (2006) demonstrated that any geometry in which
the hot, X-ray emitting
plasma is photon starved (i.e. geometries of the accreting systems in
which the accretion disk is only partially covered by the Comptonizing
plasma such as patchy coronae, inner ADAF plus outer disks etc.) will produce
hard X-ray spectra, little soft thermal emission and a
weak reflection component. On the other hand, geometries
corresponding to a very
large covering fractions of the cold phase have strong soft emission,
softer spectra and strong reflection fractions (Collin-Souffrin et al. 1996).
Thus, moving along the
vs. R relation implies moving from lower to
higher accreting systems.
The average properties of Seyfert galaxies in the local
Universe ()
as seen by BeppoSAX was investigated, analyzing
the sample of
objects presented in Dadina (2007). Multiple observations of single objects
were treated independently, i.e. the multiple measurements of parameters were not
averaged for statistical purposes. This method has been chosen since, in the
framework of the simplest version of UM for AGN (Antonucci 1993) the
AGN are thought to be very similar to each other and only
orientation/absorption effects and the activity-level of the targets could
introduce observational
differences between different objects.
In this scenario, the monitoring of a single source could be reproduced by the
observations of many sources in different states and vice versa.
BeppoSAX offered the advantage of a useful X-ray broad energy band. Data studied here fall in the 2-100 keV band for the majority of objects. This advantage has been used to investigate the properties of the high-energy continuum of Seyfert galaxies. As stated in a previous paper (Dadina 2007), the basic template was a power-law with a high energy cut-off plus a reflection component (namely PEXRAV model in XSPEC). The results of this analysis can be summarized as follow:
Acknowledgements
I thank G. G. C. Palumbo and M. Cappi for helpful discussion and for careful reading of the previous versions of the manuscript. I also thank the ASDC staff for their wonderful work in mantaining the BeppoSAXdatabase. I really thank the referee for her/his helpful comments and suggestions that contributed to improve the quality of the manuscript. Financial support from ASI is aknowledged.