Table Of ContentAstronomy & Astrophysics manuscript no. absAGN˙last (cid:13)c ESO 2008
February 4, 2008
INTEGRAL
An X-ray view of absorbed AGN
1 2 1 1 2 3 4
A. De Rosa , L. Bassani , P. Ubertini , F. Panessa , A. Malizia , A. J. Dean , and R. Walter
1 INAF/IASF,via del Fosso delCavaliere 100, I-00133 Rome, Italy
2 INAF/IASF,via Gobetti 101,I-40129 Bologna, Italy
3 School of Physics and Astronomy,University of Southampton Highfield, Southampton,United Kingdom,
8 4 INTEGRAL Science Data Centre, Chemin d’Ecogia 16, CH-1290 VERSOIX,Switzerland
0
0 Received /Accepted
2
ABSTRACT
n
a Aims.Wepresenta0.2–200keVbroad-bandstudyofabsorbedAGNobservedwithINTEGRAL,XMM-Newton,Chandra
J
and ASCA to investigate thecontinuum shape and the absorbing/reflecting medium properties.
0 Methods. The sources are selected in the INTEGRAL AGN sample to have a 20–100 keV flux below 8×10−11
3 erg cm−2 s−1 (5 mCrab), and are characterized by a 2–10 keV flux in the range (0.8–10)×10−11 erg cm−2 s−1. The
goodstatisticsallowusadetailedstudyoftheintrinsicandreflectedcontinuumcomponents.Inparticular,theanalysis
] performedonthecombinedbroad-bandspectraallowustoinvestigatethepresenceofComptonreflectionfeaturesand
h
high energy cut-off in theseobjects.
p Results. The column density of the absorbing gas establishes the Compton thin nature for three sources in which a
-
o measure of the absorption was still missing. The Compton thin nature of all the sources in this small sample is also
r confirmed by the diagnostic ratios Fx/F[OIII]. The Compton reflection components we measure, reflection continuum
t andironline,arenotimmediately compatiblewith ascenario inwhichtheabsorbingandreflectingmediaareoneand
s
a thesame,i.e.theobscuringtorus.Apossiblesolutionisthattheabsorptionismoreeffectivethanreflection,e.g.under
[ thehypothesisthattheabsorbing/reflectingmediumisnotuniform,likeaclumpytorus,orthatthesourceisobserved
through a torus with a very shallow opening angle. The high energy cut-off (a lower limit in two cases) is found in
1 all sources of our sample and the range of values is in good agreement with that found in type 1 Seyfert galaxies. At
v lower energies thereisclear evidenceof asoft component (reproducedwith athermal and/or scattering model), in six
5 objects.
7
6 Key words.X-rays:galaxies - galaxies: Seyfert - galaxies: nuclei
4
.
1
0 1. Introduction ing/absorbing medium are one and the same (the putative
8 ”molecular torus”). A cold iron line at 6.4 keV is often as-
The broad-band properties of Seyfert 2 AGN have been
0 sociated to this continuum Compton reflection hump.
extensively studied in the past years mainly using Beppo-
: The IBIS gamma-ray imager on board INTEGRAL
v SAX data (Risaliti 2002). The continuum shape in these
is surveying the whole sky above 20 keV with a mCrab
i
X objects can be reproduced by a steep power-law with (∼10−11 erg cm−2 s−1) sensitivity in well exposed re-
photon index Γ ∼ 2. This model matches well a
r gions. With its angular resolution of 12 arcmin and the
a Comptonization scenario in which seed photons from a point source location accuracy of 1-2 arcmin for moder-
cold gas (the accretion disc) are up-scattered in a hot
ately bright sources(Ubertini et al.2003)INTEGRAL has
corona of relativistic electrons (Haardt & Maraschi 1991,
already detected a number of AGN, a large fraction of
Haardt, Maraschi & Ghisellini 1997). A photoelectric cut-
which were previously unknown as high energy emitters
off is present at X-ray energies and its value depends on
(Bird et al. 2006, Bassani et al. 2006a, Bird et al. 2007).
the absorbing column density of the source. For N in the
H In this paper we present a deep broad-band spectral
1022−1023cm−2rangethecut-offisbelow2keV,forN in
H analysis of seven absorbed AGN detected by INTEGRAL.
the 1023−1024 cm−2 range the cut-off is in 3–10keVband
ThesampleisextractedfromtheBassaniet.al(2006a)sur-
and for N > σ−1 = 1.5×1024 cm−2 (i.e. the Compton
H T vey, updated to include a number of optical classifications
thick regime with σT−1 being the inverse of the Thomson obtainedafterwards(seeBassani et al. 2006b).Thesample
crosssection),thecontinuumcanbeobservedonlythrough includes Seyfert 2 galaxies having a 20–100 keV flux be-
its reflected component above 10 keV. low 5 mCrab and with X-ray data available at the time
The presence of a high energy cut-off is still an open that this work started; from the sample we have excluded
issue. Risaliti (2002) found that the high energy cut-off is sources already studied by Beppo-SAX except for one ob-
present only in ∼ 30 per cent of a sample of 20 bright Sy ject(ESO103-G35)whichwasretainedinorderto provide
2observedwithBeppo-SAX.Above 10keVthe presenceof a direct comparison between this work and previous stud-
a Compton reflection component has often been observed, ies using Beppo-SAX data and therefore an a posteriori
however is its origin still unclear as well as if the reflect- check of our analysis procedure which is affected by the
limitation of using non simultaneous X and soft gamma-
Send offprint requests to: [email protected] rays data. Overall we can conclude that the sample used
2 DeRosa et al.: An X-rayview of absorbed INTEGRALAGN
in this study is representative of the population of type 2 between 15 keV and 1 MeV, and PICsIT (Di Cocco et al.
AGNdetectedbyINTEGRALabove10keV;furtherbroad 2003)a bottom CsI layer sensitive in the range 200 keV to
band X/gamma-rayanalysis on a complete sample of type 8 MeV. In the present paper, we refer to data collected by
2 AGN selected in the 20-40 keV band is on going and the first layeronly, since the sources are too weak above ∼
will be the objective of a future paper1. Four sources (IGR 100 keV for detection by PICsIT.
J12391-1610=LEDA170194 hereafter, IGR J07565-4139, For each source, INTEGRAL data from several point-
IGRJ12026-5349andIGRJ10404-4625)havebeen discov- ings performed between revolution 12 and 429 were added
eredfirstinsoftgamma-raybandandimmediatelyafterob- together in the following way to provide sufficient statis-
servedwith Chandra (LEDA170194,IGR J07565-4139and tics. First ISGRI images for each available pointing were
IGR J12026-5349) and XMM-Newton (IGR J10404-4625). generated in 13 energy bands using the ISDC offline scien-
Twoobjects(ESO103-G35andIC4518A)areknownAGN tificanalysissoftwareOSAversion5.1.Then,countratesat
(although IC 4518A was never reported at X-ray ener- eachsourcepositionwereextractedfromindividualimages
giesbefore) andfollowup observationswithXMM-Newton inordertoprovidelightcurvesinthevariousenergybands
have been used in our broad-band analysis. For one ob- sampled; since the light curves did not show any sign of
ject (NGC 788) archival ASCA data have been used in variability or flaring activity, average fluxes were then ex-
this work. Using X-ray measurements in conjunction with tracted in each band and combined to produce the source
INTEGRAL data allow us to build a spectrum in three spectrum. A detailed description of the source extraction
decades of energy, and to distinguish between the various criteria can be found in Bird et al. (2006, 2007). We have
spectralcomponentscharacterizingthebroad-bandspectra testedthatthis methodofspectralextractionisreliableby
of absorbed AGN. This in turn will allow us, in particular, comparing the Crab spectrum obtained in this way with
tostudytheabsorption/reflectionmediumpropertiesofthe the one extracted using the standard spectral analysis.
sample. These good quality spectra also permit a detailed In Table 1 we report the details of the IBIS/ISGRI
investigationofthe intrinsiccontinuumslope including the observations; best fit positions, exposures and count rates
cut-off if present. The broad–band coverage available is a in the range 20–100 keV. The positional uncertainty for
powerfulanduniquetooltoaddressalltheseissues,making sources of this intensity is around ∼3 arcmin (Bird et al.
possible a simultaneous measurement of the intrinsic con- 2006).
tinuum(bothphotonindexandhighenergycut-off)andof
the reflected continuum together with the Fe line.
Note that a number of sources in the sample have data
2.2. Chandra data
already published in the literature both in the X-ray band
(Sazonov et al. 2005, Shinozaki et al. 2006) and in soft Chandra observed LEDA170194 on July 25, 2005, and
gamma-ray range (Beckmann et al. 2006, Molina et al. IGR J12026-5349and IGR J07565-4139 on June 16, 2005,
2006). However, most of these works did not provide a de- withACIS-I.DetailsontheobservationsaregiveninTable
tailed analysis of the broad band spectra of the sources in 1. Data were reduced using CIAO v3.2, following stan-
the sample nor attempted to put constraints on the high dardprocedures.Itisimportanttonoteherethatalthough
energy cut-off and reflection bump. The only exception is Sazonov et al. (2005) used Chandra spectra in conjunction
ESO 103-G35analysedby our team using Beppo-SAX and with INTEGRAL data, they did not attempt to estimate
INTEGRAL data; here we use a recent XMM-Newton ob- cut-off energy and reflection bump in their analysis proba-
servation of the source in conjunction with an updated blyduetothelimitedamountofspectralinformationavail-
INTEGRAL spectrum in order to have in the sample a ableabove10keV(asinglebininthe17-60keVband).All
well studied object to use as a source of reference. Chandra sources suffer from pile-up. Using WebPIMMS2
InSect.2wepresentthedataofoursmallsamplewhile we estimated the fraction of pile-up to be around 36 per
the models used to fit the spectra are described in Sect. 3. centforIGRJ07565-4139,30percentforIGRJ12026-5349
ResultsanddiscussionarepresentedinSect.4anddrawour and >20 per cent for LEDA170194.The effect of pile-up is
conclusions. The single cases are discussed in more details clearlyvisiblethroughanhardeningofthe spectrumabove
in the appendix. 4.5-5keV.Regardingthepile-upproblem,wepointoutthat
wehaveuseda differentapproachthanthe oneadoptedby
Sazonov et al. (2005): these authors removed the central
2. Observations and data reduction
pixel, i.e. excluding 80% of the instrument PSF and conse-
2.1. INTEGRAL data quentlyuseddataupto5keV;wehaveinsteadmaintained
the central pixel and dealed with the spectra affected by
The AGN discussed here have been observed with IBIS pile-up through the PILE-UP model available in XSPEC
(Ubertini et al. 2003), the imager on board INTEGRAL whichcancorrectthetotalspectrumforthepile-upeffects,
(Winkleretal.2003).This codedmaskinstrumentismade provided that this is less than 70% (Davis 2001).
by the combination of two detector layers: ISGRI (Lebrun
et al. 2003), an upper CdTe layer sensitive in the range
2.3. XMM-Newton data
1 6 other type 2 AGN from Bassani et al. survey are be-
The XMM-Newton EPIC observation of ESO 103-G35,
low the assumed threshold flux: 3 (IGR J12415+5750, IGR
IC 4518Aand IGR J10404-4625have been also.Details on
J20286+2544 and IGR J17513-2011) had no X-ray available
datawhenthisanalysis startedwhile 3(NGC1068, Mkn3and the observations are given in Table 1. Data reduction was
NGC6300) had BeppoSAXobservational results publishedbut performedwith version7.0 ofthe SAS software,employing
allarepeculiar objectsthefirst twobeingCompton thickAGN the most updated calibration files available at the time of
and the third one the prototype of ”changing look” Seyfert 2 (
i.e. a source that goes from thick to thin and viceversa). 2 http://heasarc.gsfc.nasa.gov/Tools/w3pimms.html
DeRosa et al.: An X-rayview of absorbed INTEGRALAGN 3
the datareduction(2006March).Patterns0to 12(4)were an e-folded power-law (CUTOFFPL model in XSPEC).
employedintheextractionoftheMOS(pn)scientificprod- In the third model (Model C), we added to the previ-
ucts. In the case of IC 4518A pn and MOS spectra for the ous continuum a Compton reflection component; in this
sourcewereextractedfromacircularregionof30arcsecra- model the primary emission is modelled with an e-folded
dius while backgroundspectra were taken from source-free power-law plus reflection from an infinite cold slab. In
circularregionsof10arcsecradius.InthecaseofESO103- the Compton reflection component (PEXRAV model in
G35 MOS (pn) spectra were extracted from a circular re- XSPEC, Magdziarz & Zdziarski 1995), the cosine of incli-
gionof35 (60)arcsecradiusaroundthe sourcewhile back- nationangleofthereflectorwasfixedto0.95.Weassumeel-
groundspectraweretakenfromsource-freecircularregions ementalabundancesfromAnders&Grevesse(1989).Inthe
of10(40)arcsecradius.Finally inthe caseofIGRJ10404- case of LEDA170194, IGR J07565-4139 and IGR J12026-
4625 MOS (pn) spectra were extracted from a circular re- 5349wechoosetofixinallmodelsthephotonindexΓ=1.7,
gionof40 (90)arcsecradiusaroundthe sourcewhile back- i. e. the average value observed in AGN, the energy of the
groundspectra were extractedfromsource-freecircularre- Fe line E =6.4 keV and the intrinsic width of the line
Fe
gionsof20(40)arcsecradius.Spectraarerebinnedtohave σ =10eV.Theseassumptionsareneededbecausethelow
Fe
at least 20 counts in each spectral channel. Spectral fit statistics of Chandra data did not allow us to constrain si-
were performed simultaneously with MOS1, MOS2 and pn multaneouslyallthe parametersofthe models,Γ,E and
cut
in 0.5–10 keV. Cross-calibration constant pn/MOS1 and reflection.
MOS2/MOS1wereleftfreetovaryandalwaysfoundinthe TheresultsofthefittingprocedurewithmodelA,Band
range 0.97-1.04. XMM-Newton data of ESO 103-G35 have CarepresentedinTable2,Table3andTable4respectively.
been already published in Shinozaki et al. (2006). Their
analysis was restricted to 0.5–10 keV energy range and is
consequently not conclusive about Compton reflection and 4. Results and Discussion
high energy cut-off. In Appendix A we will discuss also
ourresultsaboutabsorptionincomparisonwith thosepre- Allourmodelsprovideagoodfittoallspectra,withχ2/dof
sented in Shinozaki et al. (2006). around1.ModelA,BandChaveinfactanaveragereduced
χ2 valueof0.97,0.92and0.90respectively.Herewepresent
the main results of our analysis.
2.4. ASCA data
We finally analysed an archival ASCA spectrum of 4.1. The soft X-ray spectra
NGC 788. The relevant spectra and associated files were
downloaded from the TARTARUS archive3. We fitted the AllsourceswiththeexceptionofIGRJ07565-4139showev-
spectraofallinstrumentsSIS0,SIS1,GIS2andGIS3simul- idence for an unabsorbed component at energies less than
taneously in the 0.6–10 keV band. The cross-calibration 2 keV. Recent high resolution spectroscopic data available
constant between the different instruments were left free with XMM-Newton on bright absorbed AGN (Massaro et
duringthefitandfoundintherangeSIS1/SIS0=1.06+0.11, al.2006,Guainazzi & Bianchi 2007)showevidencethatthe
−0.11
GIS2/SIS0=0.96+0.10, GIS3/SIS0=1.09+0.10. soft excess component is dominated by emission lines and
−0.09 −0.09
originatesin a gas photoionizedby the primary continuum
in which the density is decreasing with radius like r−2, as
3. Spectral fitting observed in the Narrow Line Region (NLR). On the other
hand high resolution images available with Chandra show
The spectral analysis has been performed with XSPEC
that in some cases (e.g. NGC 4549 Schurch et al. 2002)
v11.3. Errors correspond to the 90 per cent confidence
level for one interesting parameter (∆χ2=2.7). We fit- the soft X-ray emission has a thermal origin and is domi-
nated by a starburst component. We have therefore taken
ted simultaneously the soft and hard X-ray spectra avail-
two approaches, in the first we modelled the soft excess
able with INTEGRAL and either Chandra, or XMM-
with a thermal component, i.e. a black-body model. With
Newton or ASCA. The cross-calibration constant be-
only one exception, the temperatures we found are in the
tween INTEGRAL and the low energies instruments
range0.2–0.9keV(seeTable2);inthecaseofIGRJ12026-
(C ) was fixed to 1. Possible miscalibration be-
INTEGRAL 5349the valueofkT=1.7+0.1 keVismuchhigherthantyp-
tween INTEGRAL and XMM-Newton, Chandra or ASCA −0.2
ically observed in type 2 AGN. In the second, we investi-
(i.e. C 6= 1) can mimic or hide the presence of
INTEGRAL
gatedthealternativescenarioinwhichthesoftX-rayexcess
a Compton reflection component above10 keV. We choose
is reproduced by a scattered component, i.e. a power-law
toputthisconstantequaltooneforallspectrabecausethe
fit we found with this value is good(all reduced χ2 around having the same photon index of the nuclear continuum
but a different normalization. In this case we still obtain a
1). However, in Sect. 4 we discuss the implications on our
good fit in the case of LEDA170194, NGC 788, ESO 103-
fit if C is left free to vary.
INTEGRAL G35, IC 4518A and IGR J10404-4625 (χ2/dof are 25/25,
To reproduce the broad-band spectra of our sample,
291/298,1081/1102,102/126and1136/1071respectivelyto
we employed three different models. The first one (Model
be compared with those in Table 2). The only exception is
A) consists of an intrinsic absorbed power-law, a ther-
again IGR J12026-5349.The value of the ratio A /A
mal soft X-ray component and a gaussian emission (Fe) IC soft
(relative to the case where the soft and hard photon in-
line. The data and data/model A ratio for all source
dices are frozen to the same value) for all sources is listed
is shown in Figure 1. In the second model (Model B),
in Table 2. This value provides an estimate of the rela-
we added to the primary emission a high energy cut-
tive strengthof the soft excess with respect to the primary
off; in this case the primary emission was modelled with
emission. The ratios in Table 2 are of the order of few per
3 http://tartarus.gsfc.nasa.gov cent for those sources that are well reproduced by a soft
4 DeRosa et al.: An X-rayview of absorbed INTEGRALAGN
Table 1. Journal of the observations
Name Mission Date Exp Time RA, DEC (J2000) Counts
(s) (h m s, ◦’”) (s−1)
LEDA170194 Chandra 2005/07/25 3213 12 39 06.24,-16 11 56.4 ⋆0.063±0.005
LEDA170194 INTEGRAL - 200000 12 39 11.04, -16 10 55.2 ◦0.76±0.09
IGR J07565-4139 Chandra 2005/06/16 3216 07 56 15.36, -41 38 09.6 ⋆0.215±0.008
IGR J07565-4139 INTEGRAL - 968000 07 56 29.52, -41 38 31.2 ◦0.16±0.03
IGR J12026-5349 Chandra 2005/06/16 3220 12 02 36.72, -53 49 12.0 ⋆0.209±0.008
IGR J12026-5349 INTEGRAL - 728000 12 02 50.16, -53 49 12.0 ◦0.47±0.05
NGC 788 ASCA 1999/01/16-18 39160 02 01 04.30, -06 52 48.0 †0.024±0.001
NGC 788 INTEGRAL - 594000 02 01 03.36, -06 48 50.4 ◦0.85±0.07
ESO 103-G35 XMM-Newton 2002/03/15 11987 18 38 08.30, -65 24 28.8 ‡0.507±0.007(1.71±0.01)
ESO 103-G35 INTEGRAL - 44000 18 38 46.80, -65 24 28.8 ◦0.84±0.13
IC 4518A XMM-Newton 2006/08/07 11460 14 57 44.32, -43 09 33.8 ‡0.061±0.03(0.218±0.007)
IC 4518A INTEGRAL - 898000 14 57 38.40, -43 07 44.4 ◦0.293±0.03
IGR J10404-4625 XMM-Newton 2006/11/29 13530 10 40 22.32, -46 25 25.6 ‡ 0434±0.003(1.40±0.01)
IGR J10404-4625 INTEGRAL - 626000 10 40 25.68, -46 24 36.0 ◦0.283±0.04
⋆ In1-7 keV. † In 2-10 keV for SIS0.‡ in 0.5-10 keV for MOS(pn).◦ In20-100 keVfor IBIS/ISGRI.
power-lawcomponent; this value is also in goodagreement IGR J07565-4139). The energy of the line suggests that it
withthatmeasuredinasampleofabsorbedAGNobserved could be due to K fluorescence of highly ionized elements
α
withXMM-NewtonandChandra(Bianchietal.2006).This like S or Si (SXV with the theoretical value of energy of
soft component is likely produced by electron scattering of 2.430keVandSiXIVwiththetheoreticalvalueofenergyof
the intrinsic continuum and is observed in most Seyfert 2 2.461keV)andcouldbeproducedinascatteringwarmgas
galaxies (Antonucci 1993, Matt et al. 1997). The evidence photoionized by the primary emission as already observed
thattheΓ andΓ areconsistentwitheachotherand in other obscured AGN such as NGC 1068 and Circinus
soft hard
thatA /A isinthe expectedrangeofvalues,strongly galaxy (Guainazzi et al. 1999, Massaro et al. 2006). The
IC soft
support this scenario. lack of any other emission feature due to high ionized ele-
mentspreventsthecreationofaionizationstructureforthis
InthecaseofIGRJ12026-5349ascatteredcomponentis
notabletoreproducethesoftX-rayexcess(χ2/dof=119/51 gas. If we assume that the warm medium has NH ∼ 1022
cm−2 and ionization parameter U (defined as the ratio
when we substitute the thermal component with a power- X
between the density of ionizing photons and the gas den-
law). Fitting with the scattered model Chandra data only,
sity) in the range in 0.1–5, we can produce the observed
we get a good fit, however the ratio between the normal-
ization of the two power-laws is ∼ 0.12, making this sce- line. This gas could be associated to the NLR and/or the
[OIII] ionization cones as demonstrated to be the case for
nario unlikely. The high value of kT and A /A in
IC soft
other obscured AGN (Bianchi et al. 2006).
IGR J12026-5349 give indication that the contribution of
the softexcesscomponentto the emissionishigherthanin
the other sources. This could be due to a starburst contri-
4.2. Absorption
bution. We will come back to this source in the appendix.
If left free to vary, the photon index of the soft compo- A column density in excess to the Galactic value was mea-
nent remains consistent with that of the primary emission sured in all sources,as expected in type 2 Seyfert galaxies.
in the case of LEDA170194, ESO 103-G35 and NGC 788 The minimum and maximum values of N in our sam-
H
(Γsoft = 1.0+−00..97, Γsoft = 1.8+−00..42 and Γsoft = 2.1+−00..55 re- ple are 0.1 and 40×1022cm−2 respectively, with an average
spectivelytobecomparedwiththevaluesinTable2).This valueN¯ =(10.6+0.2)×1022 cm−2 (seeTable4).Forallour
H −0.2
supports the ”scattered” scenario for these three sources. sourceswehavealsoestimatedtheF /F[OIII]λ5007ratios
X
On the contrary for IC 4518A if left free to vary the pho- (see Table 5). The flux of [OIII] λ5007 has been corrected
tonindexofthesoftunabsorbedcomponentissteeperwith forintrinsicgalacticreddeningfollowingtheprescriptionof
respect to that of the absorbed one (Γsoft = 2.7+−00..44) sug- Bassani et al. (1999) and is used here as an isotropic in-
gestingthatinthiscasethesoftX-raycomponentcouldbe dicator of the intrinsic brightness of each source. We find
of thermal origin. In the case of IGR J10404-4625the soft that all F /[OIII] ratios are indeed well above those mea-
X
photon index gets instead harder than that of the nuclear suredinComptonthicksourcesandcomparablewiththose
component(Γsoft =0.9+−00..21);suchahardvalueisprobably observedin type 1 AGN (Bassaniet al.1999).The column
relatedtothepresenceoftheComptonhumpabove10keV. densities measured as well as the F /F[OIII]λ5007 ratios,
X
Thisclearlyshowstheimportanceoffittingsimultaneously shown in Table 5, allow us to confirm the already estab-
the data from 0.5 to 200 keV. lished Compton thin nature in the case of LEDA170194,
IGR J07565-4139 is a peculiar case, as in this source IGRJ07565-4139,IGRJ12026-5349andESO103-G35and
we do not find evidence for ”continuum” soft X-ray emis- toestablishthatalsoNGC788,IC4518AandIGRJ10404-
sion.Anarrowemissionline at2.46+0.08 keVisrequiredin 4625 are Compton thin AGN.
−0.05
the model for this source; the line has an equivalent width Maliziaetal.(2007)recentlyanalysedasampleofAGN
of 184+87 eV with respect to the unabsorbed continuum observed with INTEGRAL and Swift/BAT providing a
−76
(see Table 4; model C represents the best fit in the case of diagnostic tool (Flux (2-10 keV)/F(20-100 keV) softness
Table 2. Model A composed by an absorbed power-law component, a thermal soft emission and a gaussian iron line.
D
1NgHal 1NH Γ Asoft/AIC 2KTsoft 2Eline 2σline 3EWline 4Fu2−na10bskeV 4F20−100keV χ2/dof eR
o
s
a
LEDA170194 0.0369 4.4+−11..25 1.7⋆ 0.01 0.9+−00..11 6.4⋆ 0.01⋆ < 1000 2.7 4.9 16/24 et
a
l.:
A
IGR J07565-4139 0.458 0.4+0.2 1.7⋆ - - 6.4⋆ 0.01⋆ < 548 0.8 1.5 68/51 n
−0.2
2.4±0.5 <0.2 109+88 X
−78 -
r
a
y
IGR J12026-5349 0.164 31+−12270 1.7⋆ 0.12 1.7+−00..12 6.4⋆ 0.01⋆ < 3000 10 3.9 57/50 vie
w
o
NGC 788 0.0217 40+−62 1.6+−00..11 0.01 0.30+−00..1007 6.3+−00..11 0.27+−00..2135 243+−18707 1.7 3.8 304/297 fab
s
o
r
b
ESO 103-G35 0.0764 18.8+−00..77 1.88+−00..0067 0.002 0.28+−00..0054 6.3+−00..11 0.47+−00..1111 197+−3333 6.1 7.9 1078/1101 ed
I
N
T
IC 4518A 0.09 15.0+−12..77 1.3+−00..11 0.03 0.22+−00..0033 6.26+−00..0174 <0.3 335+−119676 0.58 2.9 76/125 EG
R
A
L
IGR J10404-4625 0.137 4.1+0.3 1.83+0.06 0.009 0.39+0.08 6.21+0.05 0.19+0.11 209+67 1.7 2.5 1132/1070 A
−0.1 −0.05 −0.08 −0.07 −0.06 −50
G
N
(1) In 1022 cm−2. (2) InkeV. (3) In eV.(4) In 10−11 erg cm−2 s−1. ⋆ Frozen during thefit.
5
Table 3.ModelB composedbyanabsorbedpower-law componentwithhighenergyexponentialcut-off,athermal softemissionandagaussian
iron line.
N
AG 1NH Γ 2Ec 2KTsoft 2Eline 3EWline 2σline χ2/dof
L
A
R
G LEDA170194 4.2+1.5 1.7⋆ >160 1.1+2.9 6.4⋆ <1600 0.01⋆ 18/23
−1.5 −0.3
E
T
N
I IGR J07565-4139 0.5+0.2 1.7⋆ >80 - 6.4⋆ <540 0.01⋆ 58/50
d −0.3
rbe 2.4+−00..21 108+−9721 <0.2
o
s
ab IGR J12026-5349 15+−3193 1.7⋆ 83+−11637 1.6+−00..31 6.4⋆ <2000 0.01⋆ 43/49
of
w
vie NGC 788 33.8+−23..86 1.3+−00..31 60+−5200 0.29+−00..1008 6.3+−00..11 334+−81403 0.4+−00..21 297/296
y
a
r
-
X ESO 103-G35 18.6+0.8 1.82+0.09 >70 0.28+0.04 6.28+0.11 189+63 0.47+0.13 1076/1100
n −0.8 −0.15 −0.04 −0.10 −50 −0.11
A
al.: IC 4518A 15+3 1.2+0.1 133+280 0.22+0.03 6.27+0.07 297+184 <1.0 74/124
t −2 −0.2 −75 −0.03 −0.13 −141
e
a
s
o
R IGR J10404-4625 4.10+0.03 1.82+0.04 >170 0.39+0.10 6.21+0.05 210+65 0.19+0.11 1132/1069
e −0.07 −0.06 −0.07 −0.07 −48 −0.04
D
(1) In 1022 cm−2. (2) InkeV. (3) In eV.(4) In 10−11 erg cm−2 s−1. ⋆ Frozen during thefit.
6
Table 4. Model C composed by an absorbed e-folded power-law component, a reflection from a cold slab, a thermal soft emission and a
gaussian iron line.
D
1NH Γ 2EC †R 2KTsoft 2ELine 3EWline 2σline χ2/dof e
R
o
s
a
LEDA170194 2.9+−10..33 1.7⋆ >210 <2.4(<2.4) 1.3+−10..73 6.4⋆ <1800 0.01⋆ 18/22 et
a
l.:
A
IGR J07565-4139 0.4+0.2 1.7⋆ 40+160 <7(<7) - 6.4⋆ <621 0.01⋆ 55/49 n
−0.3 −10
2.46+0.08 184+87 <0.2 X
−0.05 −76 -
r
a
y
IGR J12026-5349 3.3+−421.7 1.7⋆ 170+−15300 1.3+−50..74 (1.3+−61..72) 1.8+−00..33 6.4⋆ <3000 0.01⋆ 40/48 vie
w
o
NGC 788 30+−63 1.25+−00..0157 62+−3284 0.9+−10..37 (<2.5) 0.29+−00..1016 6.31+−00..1113 539+−213019 0.44+−00..2289 296/295 fab
s
o
r
b
ESO 103-G35 18.9+−01..61 1.9+−00..22 50+−22550 1.7+−00..65 (2.1+−11..51) 0.28+−00..0043 6.27+−00..1112 325+−110000 <0.06 1062/1099 ed
I
N
T
IC 4518A 14+−31 1.5+−00..23 70+−6300 2.6+−31..55 (1.9+−11..33) 0.23+−00..0032 6.28+−00..0055 360+−110960 < 0.35 67/123 EG
R
A
L
IGR J10404-4625 4.6+0.3 1.95+0.28 >120 1.1+2.4 (2.2+3.2) 0.39+0.20 6.22+0.03 215+70 0.13+0.12 1120/1068 A
−0.4 −0.07 −0.7 −1.5 −0.05 −0.06 −80 −0.06
G
N
(1) In1022 cm−2. (2) InkeV.(3) IneV.(4) In10−11 ergcm−2 s−1.⋆ Frozen duringthefit.† Reflection fraction obtained with CINTEGRAL=1andCINTEGRAL free tovary(in
theparentheses).
7
8 DeRosa et al.: An X-rayview of absorbed INTEGRALAGN
1
LEDA170194 0.
IGR J07565−4139
nts/sec/keV 0.01 nts/sec/keV 0.01
u u
malized co 10−3 malized co 10−3
nor nor 10−4
4 2
atio 2 atio 0
r r
2
−
0
10 100 10 100
channel energy (keV) channel energy (keV)
1
0.
IGR J12026−5346
NGC 788
alized counts/sec/keV 100.01−3 malized counts/sec/keV 100.01−3
norm 10−4 nor
2 6
5 4
ratio 11. ratio 2
5 0
0.
10 100 1 10 100
channel energy (keV) channel energy (keV)
0.1
IC 4518A
V
malized counts/sec/ke 0.01
nor 10−3
4
ratio 2
0
1 10
channel energy (keV)
Fig.1. Spectra and data/model ratio for all sources of our sample. The ratios are plotted using our model A: absorbed
power-law+thermalemission+iron line.
ratio) to isolate peculiar objects and to investigate their able to confirm again the Compton thin nature of all our
Compton thin/thick nature. Using this new tool they were sources, but identified IGR J07565-4139 as a peculiar ob-
DeRosa et al.: An X-rayview of absorbed INTEGRALAGN 9
Table 5. Fux of [OIII] λ5007 correctedfor absorption and
its ratio with the unabsorbed flux in 2–10 keV.
source ⋆F([OIII]) FX/F[OIII] ref
LEDA170194 0.55 4.9 (1)
IGR J07565-4139 < 0.02 > 40 (2)
IGR J12026-5349 0.2454 40.7 (2)
NGC 788 0.027 62.9 (3)
ESO 103-G35 0.112 54.5 (4)
IC 4518A - - -
IGR J10404-4625 0.004 425 (5)
⋆ In 10−11 erg cm−2 s−1. (1) Masetti et al. 2006a. (2)
Masetti et al. 2006c. (3) Vaceli et al. 1997. (4) Bassani et al.
1999. (5) Masetti et al. 2006b
ject as its location in this new diagnostic plot was outside
the expected trend.
Fig.2.Reflectionfractionversusphotonindex inoursam-
ple. Dot-dash box show the average values found in the
4.3. The reflecting medium, the origin of the Compton bump
Beppo-SAX sample of type 2 AGN (Risaliti 2002).
and of the iron line
We measure the reflection fraction through the pa-
rameter R that represents the ratio between the re- case of ASCA data of NGC 788,CINTEGRAL is 0.8+−00..66. In
flected and primary components. The model (PEXRAV, the case of Chandra data, CINTEGRAL was much less con-
Magdziarz & Zdziarski 1995) assumes a perfect reflection strained with respect to XMM-Newton and ASCA, mainly
froman infinite slab covering2π angle.This means that in becausethelargergapbetweenIBISandACIS(7–20keV).
the case of a face-on line of sight, R has to be in the range In fact for Chandra data, if left free to vary in model C,
0–2. The importance of the reflection component is given the cross-calibration constant is found in the range 0.5-4;
byacomparisonbetweenTable3andTable4.Theaddition however the photon index fixed to 1.7 in the fit, still al-
of one interesting parameter, R in model C, with respect low us to measure the reflectionfraction R in IGR J12026-
to model B is statisticallyrequiredin the case ofESO 103- 5349,whileonlyupperlimitsarefoundinLEDA170194and
G35, IC 4518A and IGR J10404-4625, with a significance IGR J07565-4139. In table 4 we report the value of R we
of 99.98, 99.95 and 99.93 per cent respectively, according findleavingCINTEGRAL freetovary,anditisevidentthat,
tothestandardF-test.InthecaseofIGRJ12026-5349and even if the value of R are less constrained, we are able to
NGC 788, R is constrained but with a F-test probability measure the Compton Reflection in four objects, and their
of only 93.6 and 99.1 per cent respectively. In the case of best fit value are not very different from those obtained
LEDA170194 and IGR J07565-4139 we are only able to with CINTEGRAL=1.We stress also that the errorsassoci-
find an upper limit to R even if with an unphysical value ated to R are higher than those of Γ or kT and this is due
greaterthan2.Wewillreturnto this pointlater.InFigure to the degeneracy between the different spectral parame-
2 we plot the reflection fraction R versus the photon in- ters, in particular between Γ, Ec and R. If we keep fixed Γ
dex Γ. The values of R we obtain are in good agreement and/orEc,thevaluesofRaremuchmoreconstrainedwith
with those found in the Beppo-SAX sample of type 2 AGN respect to those reported in Table 4 and Fig. 2.
(Risaliti 2002), Beppo-SAX average values are also shown The simplest scenario to explain the presence of the
in Figure 2. IC 4518A and NGC 788 show evidence of a Compton hump above 10 keV in type 2 AGN, is one in
flatter photon index than the other sources,this effect will which the reflecting and absorbing medium are the same,
be discussed in Sect. 4.4 and Appendix A. No evidence for probablytheputativemoleculartorus.Withinthisscenario
the correlation claimed by Zdziarski et al (1999) between the question to address is if the reflection fraction we find
R and Γ is found (correlation coefficient r = 0.0582, sug- is consistent with the absorbing column densities we mea-
gesting no real evidences against the null hypothesis). It sured.ForNH =1023−1024−1025 cm−2, the contribution
is important to stress here that the value of the Compton of the torus to the 30 keV flux is 8, 29 and 55 per cent
reflection fraction R is strongly dependent on the value of respectively (Ghisellini et al. 1994). This means that the
thecross-calibrationconstantC betweenthesoft valueofRmeasuredinoursourcesistoohightobe associ-
INTEGRAL
gamma (INTEGRAL) and X-ray data (Chandra, XMM- atedonlytotheabsorbinggasofthe torus(seeTable4).A
NewtonandASCA),thatweassumedtobe equaltoonein possiblesolutiontothisproblemisthattheabsorberisnot
all spectra. It is important to note that Kirsch and col- homogeneous and that a Compton thick medium covers a
laborators (2005) analysed the Crab spectrum observed large fraction of the solid angle but not the line of sight
with different instruments, and they concluded that at as proposed by Risaliti (2002). This absorber can be iden-
20 keV the cross-calibration XMM-Newton/INTEGRAL tified with the clumpy torus recently proposed to explain
Chandra/INTEGRALandASCA/INTEGRALwascloseto infraredobservationsofAGNandtheirclassificationinthe
1 (within fews per cent). This strengthens the assumption unification theory scenario (Elitzur & Shlosman 2006).
we made onC . Howeverwe alsocheckedouras- In the case of NGC 788, ESO 103-G35, IC 4518A and
INTEGRAL
sumption a posteriori. In the case of XMM-Newton data, IGR J10404-4625,it has been possible to measure the Kα
if left free to vary in the model C, the cross-calibration iron line. For the threee Chandra sources, LEDA170194,
constant MOS1/ISGRI ranges between 0.5 and 2. In the IGR J07565-4139 and IGR J12026-5349, we have freozen
10 DeRosa et al.: An X-rayview of absorbed INTEGRALAGN
IC 4518A 3 NGC 788
4
2
ratio ratio
2
1
0 0
5 5
channel energy (keV) channel energy (keV)
Fig.3. The iron line profile when the continuum is reproduced with model C. For NGC 788 stars, triangles, circles and
squares representsSIS0, SIS1, GIS2 and GIS3 respectively For IC 4518A,ESO 103-G35and IGR J10404-4625triangles,
circles and squares represent MOS1, MOS2 and pn data respectively.
1
NGC 788
0.6 IC 4518A
0.8
ma (keV) 0.4 ma (keV) 0.6
sig sig 0.4
0.2
0.2
0 0
5.8 6 6.2 6.4 6 6.2 6.4 6.6
Energy (keV) Energy (keV)
Fig.4. Contour plots 99, 90, 68 per cent of the intrinsic width σ vs Energy for the iron lines.
