Seyfert galaxies in the local Universe (z$\leq$ 0.1): the average X-ray spectrum as seen by $BeppoSAX$

0.41 MB·English

by  Mauro Dadina

Astronomy & Astrophysics manuscript no. dadina2 February 3, 2008
(DOI: will be inserted by hand later)
Seyfert galaxies in the local Universe (z≤ 0.1): the average
X-ray spectrum as seen by BeppoSAX
Mauro Dadina1,2
8 1INAF/IASF-Bo,viaGobetti 101, 40129 Bologna, Italy
0 2Dipartimento diAstronomia dell’Universit`a degli Studidi Bologna, via Ranzani 1, 40127 Bologna, Italy
0
2
Received date/ Accepted date
n
a
Abstract.
J
The BeppoSAX archive is currently the largest reservoir of high sensitivity simultaneous soft and hard-X ray
8
data of Seyfert galaxies. From this database all the Seyfert galaxies (105 objects of which 43 are type I and 62
2
are type II) with redshift lower than 0.1 have been selected and analyzed in a homogeneous way (Dadina 2007).

] Takingadvantageof thebroad-bandcoverage oftheBeppoSAX MECS andPDSinstruments(∼2-100keV),the
h X-ray data so collected allow to infer the average spectral properties of nearby Seyfert galaxies included in the
p original sample and, most notably: the photon index (Γ∼1.8), the high-energy cut-off (Ec∼290 keV), and the
-
o relative amount of reflection (R∼1.0). The data collected have been also used to test some basic assumptions of
r the unified scheme for the active galactic nuclei. The distributions of the isotropic indicators used here (photon
st index, relative amount of reflection, high-energy cut-off and narrow FeKα energy centroid) are similar in type I
a andtypeIIobjectswhiletheabsorbingcolumnandtheironlineequivalentwidthsignificantlydifferbetweenthe
[ two classes of active galactic nuclei with type II objects displaying larger columns (NH∼3.7×1022 and 6.1×1023
cm−2 fortypeIandIIobjectsrespectively) andequivalentwidth(EW∼220and690 eVfortypeIandIIsources
1
v respectively). Confirming previous results, the narrow FeKα line is consistent, in Seyfert 2, with being produced
8 in the same matter responsible for the observed obscuration. These results, thus, support the basic picture of
3 the unified model. Moreover, the presence of a X-ray Baldwin effect in Seyfert 1 has been here measured using
3 the20-100 keVluminosity (EW∝L(20-100)−0.22±0.05).Finally,thepossible presenceof acorrelation betweenthe
4 photonindexandtheamountofreflectionisconfirmedthusindicatingthermalComptonizationasthemostlikely
1. origin of the high energy emission for the active galactic nuclei included in theoriginal sample.
0
8 Key words.X-rays:galaxies – galaxies: Seyfert: – galaxies: active
0
:
v
i 1. Introduction 2002). These studies were fundamental in making impor-
X
tant steps forward in the comprehension of the emitting
r The high energy emission from active galactic nuclei mechanismatworkinthe productionofX-rays(Perolaet
a
(AGN) is thought to come from the innermost regions of al. 2002) and to partially unveil the geometry of the cold
accrettingsystemsthatarecenteredaroundsuper-massive matter surroundings the central SMBH (Maiolino et al.
black-holes(SMBH). Forthis reason,X-raysareexpected 1998; Bassani et al. 1999; Risaliti et al. 1999).
to be tracers of the physical conditions experienced by The BeppoSAX database full potential, however,was
matterbeforedisappearingintoSMBH.Moreover,thanks neverexploitedbefore.Inapreviouspaper,theentirecat-
to their high penetrating power, energetic photons, es- alog of the Seyfert galaxies at z≤0.1 contained in the
caping from the nuclear zones, test the matter located BeppoSAX archive has been presented (Dadina 2007).
between their source and the observer. Thus, they offer This sample was selected starting from the catalog of
powerful diagnostics to understand the geometry and the Seyfert galaxies contained in the V´eron-Cetty & V´eron
physicalconditionsofthe mattersurroundingthe SMBH. (2006)sample of AGN andcontains13 radio-loudobjects
Thebroad-bandofBeppoSAX offeredfortheveryfirst and 7 narrow-line Seyfert 1.
time the opportunity to measure with a remarkable sen- The spectral analysis was performed in the 2-100 keV
sitivity, the spectral shape of AGN in the 0.1-200 keV bandwheneverpossibleandthedatawerefitwithasetof
range. This potential has been previously exploited to template models to obtain a homogeneous dataset. Here
study in details a number of sample selected in different the X-ray data so collected are statistically inspected in
manners(seeforexampleMaiolinoetal.1998;Perolaetal. order to infer what are the average characteristics of the
2 Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX
nearby Seyfert galaxies contained in this sample in the ior in terms of variable ionized absorption). To calculate
2-100 keV band. Finally, present dataset is used to per- the mean X-ray properties of the sources included in this
formsimpletestsontheunifiedscheme(UM)fortheAGN work, thus, I treated the multiple observations of single
(Antonucci 1993) and on the emission mechanism acting sourcesseparately:i.e.I assumeddifferentobservationsof
in the core of the Seyfert galaxies. More detailed anal- the same source as if they were observations of different
ysis on this latter topic will be presented in another pa- sources.
per(Petrucci,Dadina&Landi,submitted)wheredetailed This method, in principle, is expected to work prop-
thermal Comptonization models (Poutanen & Svensson erly for all those quantities which are supposed to vary
1996, Haardt & Maraschi 1993 ) will be used to fit the in accordance with the state of activity of the central
BeppoSAX data with the main scope to study the depen- nucleus. For example, the photon index Γ is known to
dence of the spectral properties in the “two phase” sce- varyaccordinglywiththeAGNfluxstate(Leeetal.2000;
nario (Haardt 1991, Haardt & Maraschi 1993) assuming Shih, Iwasawa & Fabian 2002, Ponti et al. 2006) and the
different geometries of the corona. high-energy cut-off (Ec) is linked with the temperature
of the corona and thus expected to be variable (Haardt,
Maraschi&Ghisellini1997).Onthecontrarythismethod
2. Mean X-ray Properties
is not expected to work properly when constant compo-
Scope of this section is to determine and study the mean nents are considered.This couldbe the case,for instance,
X-ray spectral properties of the sample and to use these of the cold absorption assumed to be due to the putative
values to test the UM model for AGN (Antonucci 1993). dusty torus (Antonucci 1993). For this component, thus,
The key parameters are the ones that describe the con- theaveragevaluerecordedforeachsourceshouldbeused.
tinuum and the absorption properties. In the framework Nonetheless,alsothesecomponentswereobservedtovary
of the UM models for AGN, the continuum shape is ex- in a number of objects (see for example the case of NGC
pectedtobeindependentfromtheorientationangleunder 4151,DeRosaetal.2006orRisalitietal.2002).Moreover,
which the source is observed. Thus, no difference should the constancy of the properties of the cold absorption is
be measured in the parameters describing the continuum predicted under the hypothesis of a continuous distribu-
betweentypeIandIIobjects.Onthecontrary,theabsorb- tionofthematterthatformsthetorus.Onthecontrary,if
ing column intervening in Seyfert 2 should be the princi- the torus is formed by blobs/clouds (Elitzur & Shlosman
paldiscriminatorbetweenthetwoclassesofobjects.Thus 2006), variability in the measurements of absorbing col-
measuring the mean X-ray properties means to test the umn is naturally expected. For both these observational
basic assumptions of the UM. andtheoreticalreasonsalsothe NH measuredineachsin-
gle observation were treated separately.
In between these two cases, are the properties of the
2.1. Methods
emission FeKα line. BeppoSAX had a too low sensitiv-
TheoriginsoftheX-rayphotonsfromAGNarethoughtto ity to detect the relativistically broadened component of
beduetoComptonizationofoptical-UVradiation,coming this feature in a large number of sources. Thus it turned-
fromtheaccretiondiskandComptonizedbythee− inthe out that only the narrow line has been detected in the
hot corona that sandwiches the disk (Haardt & Maraschi vast majority of the objects included in the originalcata-
1991; Haardt 1993, Poutanen & Svensson 1996, Czerny log.This componentis supposed to originatefar fromthe
et al. 2003). The mechanism is assumed to be, at least SMBH, atleastat∼1000Schwarzschildradii(Mattson&
at the zero-th order, very similar in each Seyfert galaxy. Weaver2004),i.e.verylikelyattheinneredgeofthetorus
Under this hypothesis, the differences between the X-ray (Nandra 2006). At these distances from the SMBH, the
spectraofdifferentobjectsaresupposedtobe mainlydue relativistic effects are negligible. Thus, in principle, the
to two kind of factors: i) the time-dependent state of the parameters that describe the line should be regarded as
emitting source; ii) the intervening matter that imprints stable.NonethelessalsothepropertiesofthenarrowFeKα
onthe emergingspectrumthe featurestypicalofitsphys- line were observed to vary with its line energy centroid
ical state. In such a scenario, the observations of many changes between ∼6.4 keV (neutral Iron) and ∼6.9 keV
sources can be regarded as the long-term monitoring of a (H-likeIron)showingdifferentionizationstates.Moreover,
single source. On the other hand, it is also true that the the equivalent width (EW) of this component is not only
contrary has some comparison in the literature: e.g. time function of the intensity of the line itself, but it is alsodi-
sparse observations of a single source in different states rectlylinkedwiththeunderlyingvariablecontinuumemit-
can resemble observations of objects with completely dif- ted in the regions close to the SMBH. This makes EW a
ferentspectralproperties.Thisisthecase,forexample,of variable quantity. The parameters of the emission FeKα
the narrow line Seyfert 1 NGC 4051 that displayed vari- line were, thus, treated as variable ones in order to test
ations in flux/luminosity by a factor of ∼100 (Guainazzi its origin, and not averaged when objects were observed
et al 1998) associated to strong variations of the spec- more than once.
tralshape(Γ∼0.5-2.4;Guainazzietal.1998;Turneretal. Finally, it is worth noting here that in a number of
1999;Pontietal.2006,butsee alsoCrenshaw& Kraemer cases,it wasnecessaryto dealwithcensoreddata(see ta-
(2007)foradifferentinterpretationofthisspectralbehav- ble 1). To manage these data properly, the ASURV soft-
Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX 3
ware (Feigelson & Nelson 1985; Isobe et al. 1986) has In table 2 the results for the whole set of observations
been used. In particular, to establish if the distributions and for the two classes of Seyferts are reported as well as
of parameters of type I and type II objects were drawn the probability that Seyfert 1 and 2 are drawn from the
from different parent populations, the Peto & Prentice same parent populations. The histograms of the distribu-
generalized Wilcoxon test (Feigelson & Nelson 1985) has tions of the interesting parameters for the entire sample
been used while to calculate the mean values consider- of objects (first column) and for type I (second column)
ing also the censored data the Kaplan-Meyer estimator and type II (third column) are reported in figure 1.
has been used. To establish the presence of correlations
Table 2. Mean spectral properties. Col. I: Spectral pa-
between different quantities, both the Spearman’s ρ and
rameter; Col. II: Seyfert 1 mean value; Col.III: Seyfert 2
the generalized Kendall’s τ methods were applied. The
meanvalue;Col.IV:ProbabilitythatSeyfert1andSeyfert
linearregressionswerecalculatedusingthe Bukley-James
2 are drawn from the same parent populations.
and Schmitt‘s methods. In the following, two quantities
are considered as drawn from different parent population
when the probability of false rejection of the null hypoth-
esis (same parentpopulation) is Pnull≤1%. Similarly,one
Parameter Tot. Seyfert 1 Seyfert 2 Pnull
acceptsthatthereisacorrelationbetweentwogivenquan-
tities when the probability of absence of correlation re-
mains smaller than 1%. Γ 1.84±0.03 1.89±0.03 1.80±0.05 90%
Table 1. General characteristics of the data analyzed in
R 1.01±0.09 1.23±0.11 0.87±0.14 5%
thiswork.Thenumberofdetectionsandcensoreddataare
reportedfortheinterestingparametersforthewholesam-
Ec† 287±24 230±22 376±42 5%
pleofobjects(columns2and3),fortheSeyfert1galaxies
(columns 4 and 5), and for the Seyfert 2 objects (column
NH‡ 31.7±9.1 3.66±2.34 61.3±18.0 ≤0.1%
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) † in unitsof eV;‡ in units of 1022 cm−2
Aspreviouslysaid,theUMforAGN(Antonucci1993)
predicts that Γ, R, and Ec are observables independent
Parameter Tot. Sample Seyfert 1 Seyfert 2 from the inclination angle, thus the two classes of Seyfert
galaxiesshoulddisplayverysimilarcharacteristics.Thisis
Det. Cens. Det. Cens. Det. Cens. confirmedbytheanalysisofthepresentsample.Inpartic-
ular,therearenohintsthatthedistributionsofphotonin-
dexΓforthetwotypesofSeyfertaredrawnfromdifferent
NH 83 80 31 53 52 27 parent populations (Pnull∼90%). Moreover, the photon-
index peaks, for both classes, between 1.8-1.9 well in
EW 129 7 66 4 63 3
agreement with the two-phase models for the production
of the X-ray in Seyfert galaxies that predicts Γ∼1.5-2.5
R 68 18 46 9 22 9
(Haardt&Maraschi1991;Haardt1993;Haardt,Maraschi
&Ghisellini1997).Fewobjectshaveextremelyflatspectra
Ec 33 51 27 26 6 25
with Γ≤1. Type II objects that show so hard X-ray spec-
tra 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
2.2. The X-ray continuum and the cold absorption
the harder X-ray spectrum. This source was first classi-
The X-ray continua of the sources have been modeled us- fied as a Compton-thick object by Maiolino et al. (1998).
ing a cut-off power-law, that describes the primary emis- The Seyfert 1s with flattest spectra are NGC 4151that is
sion from the hot corona plus a reflection component knowntohaveahardspectrumwithcomplexandvariable
(PEXRAV modelinXspec,Magdziarz&Zdziarski1995) absorption (De Rosa et al. 2006) and Mrk 231 (classified
to account for the contribution expected to be due to the as a type I AGN by Farrahet al. 2003).The latter source
disk. It is worth noting, however, that an additional re- showsaveryhardX-rayspectrumwithΓ∼0.7.Thissource
flection component could rise from the torus (Ghisellini, is also classified as a BAL QSO (Smith et al. 1995).A re-
Haardt& Matt 1994)andthat the disentangling between centspectralanalysisintheX-raybandofthissourcewas
the two reflection component is impossible with the qual- presented in Braito et al. (2004). By combining XMM-
ityoftheavailabledata.Whereveritsorigin,thereflection Newton and BeppoSAX data, these authors speculated
has been assumed to be due to cold matter. The inter- that the spectrum of the source below ∼10 keV is reflec-
esting parameters are the photon index (Γ), the relative tiondominated,thuspresentingacasethatcanbe hardly
amountofreflection(R),andthehigh-energycut-off(Ec). reconciled with the UM of AGN. Moreover, Mrk 231 is
4 Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX
Fig.1. Photon index Γ (first row), R (second row), Ec (third row), and NH (fourth row) in units of cm−2 distributions for the
whole dataset (left column), for typeI objects (center column), and for typeII objects (right column).
an ultra-luminous infra-red galaxy. These sources display The result is unambiguously in accordance with UM
strong starburst activity that can dominate the total X- predictionsforthe absorbingcolumn.Inthis casethe sta-
ray luminosity of the galaxy (see Franceschini et al. 2003 tistical tests confirm that Seyfert 1 and Seyfert 2 dis-
and Ptak et al. 2003 for details on this topic) although playverydifferentabsorptioncharacteristicswiththetype
in Mrk 231 at least 60% of the observed 0.5-10 keV flux II objects being more heavily absorbed than Seyfert 1.
seems to be due to the AGN component (Braito et al. Again,itisnoticeablethatafew typeI objectsshowhigh
2004). column densities (up to ∼1024 cm−2) and some Seyfert 2
havelowcolumns(downto1021cm−2).Thesearenotnew
Less conclusive results are obtained for R and Ec. In
results:thehighcolumnofNGC4151(∼5×1022,DeRosa
bothcases,theprobabilityoffalserejectionofthenullhy-
et al. 2006) is well known. The highest column measured
pothesis (the two distributions are drawn from same par-
in a Seyfert 1 is detected during the June 9, 1998 obser-
ent populations, in accordancewith the UM predictions),
vation of NGC 4051. During this observation the source
isPnull∼5%.Itisworthnoting,however,thatbothtypeI
appeared“switched-off”andonlyapurereflectioncompo-
andIIobjectslayinthesamerangeofRandEc.Inpartic-
nent was measured (Guainazzi et al. 1998). The spectral
ular,the distribution of R for type I objects is dominated
fit in Dadina (2007) degenerated between two-solutions,
byapeakofdetectionsbetweenR∼1-1.2duetothecontri-
one in which the source was purely reflection-dominated
bution of a few sources observed several times (e.g. NGC
(R≥7) and a second one in which a direct component
5548, IC 4329a). This has probably introduced a system-
was visible but highly absorbed. The latter scenario was
aticeffectnotsmeared-outbytherelativelysmallnumber
slightly preferredby a pure statistical point of view when
of useful observations (i.e, the ones for which R and Ec
the 2-200 keV band is condidered, and, for homogeneity,
have been estimated). On the other hand, the result on
entered in the catalog (Dadina 2007). Nonetheless, when
Ecismostprobablypollutedbythelargenumberoflower
the entire BeppoSAX band (0.1-200 keV) is considered,
limits for Seyfert 2. In fact if the same test is performed
the reflection scheme is preferred (Guainazzi et al. 1998).
onlyonthedetectionsitisobtainedthatPnull∼35%,mak-
ing impossible to assess that the two samples are drawn Finally, a number of objects show upper-limits of the
from two different populations. order of ∼1022 cm−2 to the absorbing column. This is a
Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX 5
Fig.2.DistributionoftheFekαemissionlineenergycentroid(firstrow)andofitsEW(secondrow).Distributionsforthewhole
sample of observations (left panel), for thetypeI objects (center panel) and for typeII objects (right panel).
selectioneffectinducedbytheenergybandconsideredus- et al. 1999, Cappi et al. 2006 and Panessa et al. 2006).
ingonlyMECSandPDSdata(∼2-100keV,Dadina2007). As shown in the central panel of figure 2, the Seyfert 1
The low-energycut-off due to such a column (1022 cm−2) peak at EWFeKα∼100-200 eV while the type II have a
peaks in fact at E∼2 keV and only for those objects with broaderdistribution with an hardtail that reachesvalues
a good statistics it is possible to infer upper limits on the well above 1 keV. Also few Seyfert 1 have large values of
NH below1022cm−2.Thisismostprobablyresponsibleof EW of the FeKα line (above 300-400 eV). The large EW
thehighvalueobtainfortheaverageNH intypeIobjects. tailoftheSeyfert1distributioniscomposedmainlyofob-
jects in which the broadcomponents of the FeKα line are
3. Probing the origin of the FeKα emission line detected suchasin MCG-6-30-15,Mrk841andIC 4329a.
The Seyfert 1 with largest EW is NGC 4051 during the
TheFeKαlineisproducedbyreprocessingtheprimaryX- June 9, 1998 observation when its spectrum was due to
rayemissioninmattersurroundingthesourceofhardpho-
pure reflection (Guainazzi et al. 1998).
tons. In the framework of the UM, the originof this com-
ponent can be placed in a number of region s such as the The largerFeKα EW in Seyfert 2 galaxies is in agree-
accretiondisk,the dustytorus,andthe broad-lineregions ment with the UM (Antonucci 1993). If the origin of this
(even if this last hypothesis is disfavored by the recent component is indeed located in the dusty torus, than the
results obtained with the XMM-Newton and Chandra line EW has to be correlated with the absorber column
observatories and presented in Nandra 2006). If the line density.Thisisindeedwhatisobservedalsointhissample
originates in the disk close to the SMBH, relativistic ef- (see figure 3, left panel). Moreover, the Spearman ρ and
fects thatbroadenthe resultingline areexpected.Forthe Kendall’s τ tests indicate that a correlation between the
vast majority of the sources included in the original sam- FeKα EW and the NH is highly probable for type II ob-
ple,only narrowcomponentofsuchfeatureweredetected jects (Pnull≤0.1%), i.e. for that sources for which we can
andonlyinafewcasesbroademissionlines(e.g.IC4329a) have direct evidences of the torus absorbing column. The
or relativistically blurred features were detected (for ex- robustnessoftheNH estimateshavebeentestedbycorre-
ample in MCG-6-30-15).Thus, the results presented here lating it with the model independent indicator offered by
are essentially based on the measured properties of the theratiooftheobservedfluxesat2-10and20-100keVre-
narrow features. spectively(centerpaneloffigure3).Thetwoquantitiesare
As shown in figure 2 (first row), the line energy cen- strongly correlated (generalized Spearman ρ and Kendal
troid is peaked at ∼6.4-6.5 keV (see also table 3) in both τ tests give Pnull≤0.1%) with only a few exceptions: the
type I and II objects. The centroid is slightly above 6.4 pure Compton-thick sources which are located in the di-
keV but, considering the energy resolution of the instru- agram below the majority of the sources. This effect is
ment at these energies (∼200 eV FWHM, Boella et al. expected since the absorbing column can affect the X-ray
1997),theresultsobtainedhereareinagreementwiththe radiation below ∼10 keV while at harder energies the ra-
line being mainly produced in cold or nearly cold matter diation pierces the matter for columns ≤3-5×1024 cm−2.
(ionizationstate below FeXVII), i.e. inboth type I andII Otherwise, the Compton absorption dominates and also
objects, by matter in the same physical state. However,a photons with energy above 10 keV are stopped since the
wellknowndifferencebetweenthetwoclassesofobjectsis Klein-Nishina regime is reached.
the EWofthe narrowFeKαline in type II objects,which
shows stronger features than type I objects (see second
row of figure 2 and table 3, Bassani et al 1999, Risaliti
6 Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX
Fig.3. Left panel: Log(EWFeKα) vs. Log(NH). As expected, the sources are divided in two families: the ones that follow the
expectations if the FeKα line is produced in the absorption matter and the candidate Compton-thick ones that display low
absorptionandlargeEW.ThesolidlineindicatesthepredictionbyMakishima(1986).Centerpanel:Log(F2−10keV/F20−100keV)
vs.Log(NH).Thetwoquantitiesarecorrelatedasexpectedsincethe2-10keVbandisstronglyaffectedbytheabsorptionwhile
the20-100bandisalmostfreefromabsorption.ThesolidlineisthebestfitobtainedwithlinearregressionmethodsRightPanel:
Log(EWFeKα)vs.Log(F2−10keV/F20−100keV).Thetwoquantitiesarestronglycorrelated(Pnull≤0.01).TheratiobetweenX-ray
fluxesisamodelindependentindicatoroftheabsorptionaffectingthelowenergyband.Thus,thiscorrelationstronglyindicates
thatthenarrowcomponentofFeKαlineinemissionisindeedproducedbythesamematterresponsiblefortheabsorption.The
solid line is thelinear regression obtained using Bukley-James method (Isobe et al. 1986).
Table 3.MeanpropertiesoftheFeKαemissionline.Col. These results thus confirm that the properties of the
I: spectral parameter; Col. II mean value for the whole FeKα line agree with the expectations of the UM for
sample; Col. III: mean value for Seyfert 1; Col. IV: mean AGN. Nonetheless, this is not the only information we
value for Seyfert 2. Col. V: Probability that the parame- have about the Iron line. In recent papers (Iwasawa &
tersoftypeIandtypeIIobjectsaredrawnfromthesame Taniguchi 1993; Page et al. 2004, Grandi et al. 2006;
parent population. Bianchi et al. 2007) it has been claimed that a X-ray
“Baldwin effect” (or Iwasawa-Taniguchi effect) is present
in AGN when the FeKα intensity is probed against the
Parameter Tot. Seyfert 1† Seyfert 2† Pnull 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.
EFeKα† 6.49±0.02 6.46±0.03 6.51±0.03 32%
A strong correlation ( Pnull≤0.1% using Spearman ρ
EWFeKα‡ 448±67 222±33 693±195 ≤1% and generalized Kendal τ test) is found when the EW of
the FeKα line is plotted both against the observed 2-10
keV(panel(a)offigure4)and20-100keV(panel(c)offig-
† in units of keV;‡ in units of eV
ure 4) keV luminosities. The nature of these correlations,
As stated above, when the EW of the FeKα emission however,isnotstraightforward,especiallyforthe2-10keV
lineistestedagainstthemeasuredNH (leftpanelofFigure luminosity. In this energy band the effect of the absorber
3)aresultingoodagreementwithwhatpredictedbythe- isveryimportant.Aspreviouslydemonstrated,theEWof
oretical models is obtained (Makishima 1986; Leahy & the Iron line correlates with NH, but stronger absorbing
Creighton1993).The majority of the sources,in fact, be- columns imply lower fluxes. Moreover, when the relation
have as expected if the line is produced by the absorb- between the FeKα line EW and the 2-10 keV flux is in-
ingmatterthatdepressthe directcontinuum(Makishima vestigated (panel (b) of figure 4), it is found that the two
1986). All the known Compton-Thick sources are located quantities are correlated (Pnull≤1%). This is explainable
at low NH and high EW, in accordance with previous re- only in terms of instrumental effects, since the sensitiv-
sults (Bassani et al. 1999, Risaliti et al. 1999 ). As an ity of the instruments to narrow features decreases with
additionaltest,the EWofthe FeKαlinehasbeenplotted the source’s flux. Thus, to be detected in faint objects,
versus the F2−10keV/F20−100keV ratio. As expected (see the FeKα line must be strong enough. Since the original
right panel of figure 3), a good correlation ( Pnull≤0.1% sample is limited to the local Universe,this correlationin
accordingtogeneralizedSpearmanρandKendallτ tests) flux acts, at least partially, also in the EW vs L2−10keV
is obtained. relation.
Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX 7
Fig.4. Panel (a): Log(EWFeKα) vs. L2−10,observed. Panel (b): Log(EWFeKα) vs. F2−10,observed. Panel (c): Log(EWFeKα) vs.
L20−100,observed. Panel (d): The same of panel (c) but only for Seyfert 1 objects. Panel (e): Log(EWFeKα) vs. F20−100,observed.
Solid lines in panel (a), (b), and (c) are the linear regressions obtained for the whole sample of observations. The dashed line
in panels (c) and (e) is the linear regression obtained considering only typeI objects. As visible in panel (b) the less scattered
relationisobtainedconsideringthe2-10keVobservedflux.Therelationislinear,asexpectedifthecorrelationisduetoselection
effects, i.e.considering that in faint objects only large EW were detectable bythe MECS instrumentson-board BeppoSAX.
This NH effect should be negligible when the 20-100 20-100keV flux). If the two classesof Seyfert galaxiesare
keV band is considered. In fact, in this case, one expects analyzedseparatelyitisobtainedthatastrongcorrelation
to find a correlationbetween the observed20-100keV lu- isfoundforSeyfert1(Pnull≤0.1%usingbothSpearmanρ
minosity and the EW of the FeKα line only in the most andgeneralizedKendalτ tests,panel(d)offigure4)while
extreme cases, i.e. for the “pure Compton thick” objects. no correlation is evident for type II objects (Pnull∼80%).
Apart from NGC 1068, these sources are too weak to be ThisresultisnotunexpectedsincetheEWoftheobscured
detectedbythePDS,thusunabletodrivetherelationob- sources are boosted by the suppression of part of the un-
servedinplot(c)offigure4.Moreover,panel(e)offigure4 derlying continuum. To conclude, the presence of a X-ray
indicatesthattheEWoftheironlineisnotrelatedtothe Baldwin effect for Seyfert 1 is unambiguously confirmed
20-100 keV flux thus excluding that the “Baldwin effect” by present data if the L20−100keV is consideredand it has
measuredusingthe 20-100keVbandis due to instrumen- the following relation:
tal selectioneffect as it happens for the F2−10keV. Onthe
otherhand,inthe20-100keVbandthereflection-humpat Log(EW)=-(0.22±0.05)×Log(L20−100)+11.91±2.52 (1)
∼30-40keV contributes to the observedflux. If the origin
of the FeKα line is due to the same matter responsible of Theslopeoftherelationfoundinthisworkisinagree-
the reflection, one should expect that the EW of the Iron mentwithwhatpreviouslyobtainedbyPageetal.(2004)
lineshouldincreasesasthereflectioncomponentaugments (EW∝(L2−10)−0.17±0.08) using a sample containing both
the20-100keVflux(i.e.theIronlineEWshouldcorrelate radio-quiet and radio-loud objects. Present result is also
with the 20-100 keV flux). Thus, the net contribution of consistentwithwhatfoundbyZhou&Wang(2005)(who
the reflectioncomponentshouldactinthe opposite direc- used a sample containing both radio-quietand radio-loud
tion to that observed (i.e. larger iron line EW at smaller objects founding EW∝(L2−10)−0.15±0.05) and Bianchi et
al. (2007) (who used only radio-quiet objects obtaining
8 Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX
Fig.5. Left panel: Photon index Γ plotted versus the measured NH (in units of cm2). No significance trend relating these two
quantitiesis found; center panel: R vs.Ec (in unitskeV),nocorrelation is found between these quantities; right panel R vs. Γ.
The two quantitiesare correlated with a high significance level (Pnull≤1%). The line is thelinear regression best fit.
EW∝(L2−10)0.17±0.03). On the contrary, Jiang, Wang & ativistic broadening due to the contribution of the inner
Wang (2006) found that, excluding the radio-loud AGN orbits of the accretion disk.
from a sample similar to the one used by Page et al.
(2004), found that EW∝(L2−10)0.10±0.05. It is worth re-
4. The Γ-R relation.
calling here, that the the present sample is composed by
bothradio-loudandradioquietsources(Dadina2007).In In the fitting procedure some parameters may degenerate
particular, it contains 7 radio-loud Seyfert 1, and only given the interdependence among them. This is the case,
for 5 of them the 20-100 and iron line data are avail- for example of the photon index with the column density
able. Nonetheless, the presence of these sources in the forlowstatisticsobservations.Thesamecouldhappenfor
sample does not affect the FeKα EW vs. L20−100 rela- the determination of R and Ec, since R introduces in the
tion(EW∝(L2−10)0.21±0.05excludingradio-loudtypeIob- spectrumabumppeakedbetween20-40keVanddeclining
jects). at higher energies where the Ec may appear. To check if
the results presentedhere are affectedby sucheffects, the
The originofthe X-ray“Baldwineffect” isunclear.In correlations between these parameters have been studied
the light bending scenario (Miniutti & Fabian 2004) the and the results are presented in Figure 5.
height of the source above the accretion disk determines LeftpanelofFigure5showshow,onaverage,theesti-
the degree of beaming along the equatorial plane of the mate of Γ is not affected by the simultaneous determina-
high energy emission. Because of relativistic effects, the tionoftheabsorbingcolumn.Infact,notrendisobserved
closerthesourceistothedisk,thegreaterwillbethefrac- between Γ and NH. Obviously, this does not imply that
tionofX-raysbeamedintheequatorialplane(i.e.towards this is true for each single source included in the origi-
the disk) and correspondingly lower will be the observed nal sample. On the other hand, this is an expected result
flux. Thus, the EW of the relativistically blurred FeKα since the broad band of BeppoSAX should reduce this
line producedinthe inner regionsofthe diskandthe EW spuriouseffect. Similarresults areobtainedalsowhen the
of the narrowIronline produced in the outer parts of the Γ vs. Ec, and R vs. Ec (center panel of Figure 5) rela-
disk would appear enhanced in low-state sources. tions are investigated. In these cases the Spearman ρ and
Kendall’sτ testsdonotsustainthe existenceofarelation
Onthecontrary,Pageetal.(2004)speculatedthatthis between these quantities (Pnull∼15%). All these indica-
effectcouldindicatethatluminoussourcesaresurrounded tions suggest that, if any, the possible degeneracy in the
by dusty tori with lower covering factor thus pointing to- fitting procedure did not introduce strong spurious rela-
wards a torus origin of most of the narrow FeKα line. tions between the spectral parameters.
The present work supports this view. The FeKα line EW However a strong correlation (Pnull≤0.01%) is
of the Iron line correlates with the observed NH as pre- recordedbetweenΓandRthatarelinkedbythefollowing
dicted by theory (Makishima 1988; Lehay & Creighton relation:
1993;Ghisellini,Haardt&Matt1994).Moreover,alsothe
case of the extremely low state of NGC 4051 (Guainazzi R=(4.54±1.15)×Γ-(7.41±4.51) (2)
etal.1998)includedin this datasetseems to pointinthis
direction.ThehugeEWofthenarrowFeKαlinerecorded It is hard to define if this correlation is the result of
in this observation, in fact, is typical of Compton-thick a systematic effect or not since it is possible that these
typeIIobjects,butthelinedoesnotshowevidenceofrel- two quantities degenerate in the fitting procedure. Flat
Mauro Dadina: Average X-rayproperties of Seyferts observed by BeppoSAX 9
power-lawwithsmallreflectioncouldbedescribedalsoby – theaverageslopeofthepower-lawis1.84±0.03forthe
steep power-law plus strong reflection. The total absence entire sample of objects. Considering the two families
of similar correlations between Γ and Ec and R vs. Ec of AGN separately, it turns-out that Γ=1.89±0.03 for
seems to suggest that this correlationis indeed real. type I objects (including Seyfert 1, 1.2 and 1.5) while
A similar relation was previously found using Ginga Γ=1.80±0.05fortypeIIobjects(includingSeyfert1.8,
and RXTE data (Zdziarski, Lubinski, & Smith 1999; 1.9 and 2);
Gilfanov, Churazov & Revnivtsev 1999). Zdziarski, – theaveragevalueoftherelativereflected-to-directnor-
Lubinski, & Smith (1999) interpreted it as evidence of malizationparameterR is 1.01±0.09with a slight dif-
thermalComptonizationasoriginofX-raysprovidingthat ference between the two classes of Seyfert galaxies
theoptical-UVseedphotonsweremainlyproducedbythe (R=1.23±0.11fortype I objectsandR=0.87±0.14for
same material responsible for the reflected component. In the type II ones);
this case, in fact, the cooling rate of the hot corona is di- – thehighenergycut-offwasmeasuredtobeEc=287±24
rectly linked to the power-law slope. But the cooling rate (Ec=230±22keVforSeyfert1andEc=376±42keVfor
isalsorelatedtotheanglesubtendedbythereflector.This Seyferts 2);
resultisalsoinagreementwithpredictionsofmodelsthat – as expected and as known from previous works, the
consider mildly relativistic outflows driven by magnetic absorbing column is very different in the two classes
flares(Beloborodov1999).More in general,Merloniet al. of objects. On average NH∼3.66×1022 cm−2 type I
(2006)demonstratedthatanygeometryinwhichthe hot, and NH∼6.13×1023 cm−2 for type II AGN. The high
X-ray emitting plasma, is photon starved (i.e. geometries mean value obtained for Seyfert 1 is caused by a se-
of the accretting systems in which the accretion disk is lection effect induced by the energy coverage of the
only partially covered by the Comptonizing plasma such MECS+PDS instruments (2-100 keV).
as patchy coronae,inner ADAF plus outer disks etc.) will – evidence of a X-ray Baldwin effect is found in Seyfert
produce hard X-ray spectra, little soft thermal emission 1 galaxies when the EW of the FeKα line is plotted
and weak reflection component. On the other hand, ge- against the 20-100 keV luminosity.
ometry corresponding to a very large covering fraction of – asignificantcorrelationhasbeenfoundbetweenRand
the cold phase, have strong soft emission, softer spectra Γ.
andstrongreflectionfraction(Collin-Souffrinetal.1996).
Thus,movingalongtheΓvs.Rrelationitimpliesmoving These results are well in agreement with the basic as-
from lower to higher accretting systems. sumptions of the UM for AGN (Antonucci 1993).In fact,
no differences are measured in the observables that are
supposedtobeisotropicwhiletheabsorbingcolumnseems
tobetheonlydiscriminatorbetweenthedifferenttypesof
5. Summary and conclusions Seyfert galaxies.This reflects also in the properties of the
FeKα line. No difference is measured in the line centroid
The average properties of Seyfert galaxies in the local
(see table 3) between the two classes of Seyfert galaxies.
Universe (z≤0.1) as seen by BeppoSAX has been in-
Type II objects, however, display more intense features
vestigated, analyzing the sample of objects presented in (EW=222±33 eV for Seyfert 1 and EW=693±195eV for
Dadina (2007). Multiple observations of single objects
Seyfert 2). The physical origin of the X-ray “Baldwin ef-
were treated independently, i.e. the multiple measure-
fect” here measured for Seyfert 1 using the 20-100 lumi-
mentsofparameterswerenotaveragedforstatisticalpur-
nosity is unclear. Both light bending (Miniutti & Fabian
poses. This method has been chosen since, in the frame-
2004) and torus models (Page et al. 2004) are consistent
work of the simplest version of UM for AGN (Antonucci,
with present data even if the strong relation of the FeKα
1993) the AGN are thought to be very similar to each-
lineEWintypeIIobjectswiththeabsorptioncolumnin-
other and only orientation/absorption effects and the
dicatesthatthemostofthenarrowlinecomponentshould
activity-level of the targets could introduce observational
beduetothetorus.Finally,themeasuredΓ-Rrelationship
differences between different objects. In this scenario,the
is consistent with thermal Comptonization models.
monitoring of a single source could be reproduced by the
observations of many sources in different states and “vice
Acknowledgements. I thank G.G.C. Palumbo and M. Cappi
versa”.
for helpful discussion and for careful reading of the previous
BeppoSAX offered the advantage of a useful X-ray versions of the manuscript. I also thank the ASDC people for
broad energy band. Data studied here fall in the 2-100 theirwonderful work in mantaining the BeppoSAX database.
keV band for a majority of objects. This advantage has I really thank the referee for her/his helpful comments and
been used to investigate the properties ofthe high-energy suggestions that contributed to improve the quality of the
manuscript. Financial support from ASIis aknowledged.
continuum of Seyfert galaxies. As stated in the previous
paper (Dadina, 2007), the basic template was a power-
lawwithahighenergycut-offplusareflectioncomponent
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