Suzaku Follow-up of Heavily Obscured Active Galactic Nuclei Detected in Swift/BAT Survey: NGC 1106, UGC 03752, and NGC 2788A

3.2 MB·

by  Atsushi Tanimoto

Publ.Astron.Soc.Japan(2014)00(0),1–11 1
doi:10.1093/pasj/xxx000
Suzaku Follow-up of Heavily Obscured Active
Galactic Nuclei Detected in Swift/BAT Survey:
NGC 1106, UGC 03752, and NGC 2788A
6
1
0 Atsushi Tanimoto 1, Yoshihiro Ueda 1, Taiki Kawamuro 1, Claudio Ricci 1,2
2
1DepartmentofAstronomy,KyotoUniversity,Kyoto606-8502
n
a 2InstitutodeAstrofisica,PontificiaUniversidadCatolicadeChile,Casilla306,Santiago22,
J Chile

9
1 ∗E-mail:[email protected]

Received2015November11;Accepted2016January17
]
A
G Abstract
. We present the broadband (0.5–100 keV) spectra of three heavily obscured Active Galactic
h
p Nuclei (AGNs), NGC 1106, UGC 03752, and NGC 2788A, observed with Suzaku and
- Swift/Burst Alert Telescope (BAT). The targets are selected from the Swift/BAT 70-month cat-
o
r alog on the basis of high hardness ratio between above and below 10 keV, and their X-ray
t
s spectra are reported here for the first time. We apply three models, a conventional model
a
utilizingananalyticreflectioncodeandtwoMonte-Carlobasedtorusmodelswithadoughnut-
[
likegeometry(MYTorus;Murphy&Yaqoob2009)andwithanearlysphericalgeometry(Ikeda
1
torus; Ikeda etal. 2009). Thethree models cansuccessfullyreproducethe spectra,while the
v
6 Ikeda torus model gives better description than the MYTorus model in all targets. We iden-
5 tify that NGC 1106 and NGC 2788A as Compton-thick AGNs. We point out that the common
0
presenceofunabsorbedreflectioncomponentsbelow7.1keVinobscuredAGNs,asobserved
5
0 fromUGC03752,isevidenceforclumpytori. Thisimpliesthatdetailedstudiesutilizingclumpy
1. torusmodelsarerequiredtoreachcorrectinterpretationoftheX-rayspectraofAGNs.
0
Keywords:Galaxies:active—Galaxies: individual(NGC1106,UGC03752,NGC2788A)
6
1
:
v
i 1 Introduction instance, it is also proposed that a torus may be a remnant of
X
outflowlaunchedfromtheaccretiondisk(Elitzur&Shlosman
r
a Heavily obscured Active Galactic Nuclei (AGNs) are key ob- 2006),oratilteddisk(Lawrence&Elvis2010). Itisthusvery
jects to understand the origin of the cosmic X-ray back-
importanttoconstrainbasicproperties(e.g.,geometry)ofAGN
ground (e.g., Ueda et al. 2003; Gilli et al. 2007) and the “co-
torifromobservations.
evolution” of SupermassiveBlack Holes (SMBHs)and galax-
ies (Kormendy 2013), which may have rapidly grown dur- X-ray observations are a powerful tool to study the inner
ing the obscuration phase after major mergers (e.g., Sanders structureofanAGN,becauseX-rayscantraceallmaterialin-
et al. 1988; Hopkins et al. 2006). A generally accepted pic- cludinggasanddust. Atorussignificantlyaffectstheobserved
ture of AGN structure is that a central SMBH and an accre- X-rayspectrabyphotoelectricabsorptionofthedirectcompo-
tion disk aresurrounded by obscuring matter onparsecscales nent when they block the light-of-sight (i.e., in type 2 AGNs)
consistingofgasanddust,called“torus”(Antonucci&Miller (Awaki et al. 1991). In addition, reflected components from
1985;Antonucci1993). Whileatorusisbelievedtoplayacru- thetoruswithfluorescenceiron-Klinesareexpected,whichare
cialroleforgasfeedingontoaSMBH(e.g.,Kawakatu&Wada more easily observable when the direct emission is absorbed.
2008; Wada2015), itsoriginhasbeenpoorlyunderstood. For Hence, observations of heavily obscured AGNs, in particular
(cid:13)c 2014.AstronomicalSocietyofJapan.
2 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
Table1.ListofTarget
J0250.7+4142 J0714.2+3518 J0902.7-6816
(1) OpticalID NGC1106 UGC03752 NGC2788A
(1) Classification Seyfert2 Seyfert2 Galaxy
(1) Redshift 0.0144 0.0157 0.0133
(2) NGal[1022cm−2] 0.0735 0.0930 0.0152
H
(3) StartTime[UT] 2014August301:18 2015April523:51 2015May1501:08
(3) EndTime[UT] 2014August421:09 2015April621:42 2015May1605:21
(3) Exposure[ks] 67.0 67.4 23.6
Notes.
(1)TakenfromtheNASA/IPACExtragalacticDatabase.
(2)ThehydrogencolumndensityofGalacticabsorption(Kalberlaetal.2005).
(3)BasedonthegoodtimeintervalforXIS-0.
Compton-thickAGNs,whosehydrogencolumndensitiesalong scuredAGNs, NGC1106, UGC03752, andNGC2788A, ob-
theline-of-sightexceedlogNH ≥24cm−2,areveryusefulto servedwithSuzakuandSwift/BAT.Theyareselectedfromthe
investigatethetorusgeometry. Swift/BAT 70-month catalog on the basis of the large fluxra-
tio between above and below 10 keV. Their broadband X-ray
Surveys in hard X-rays above 10 keV provide one of the
dataarereportedherefor thefirsttime, and weidentify NGC
least biased sample of AGNs, thanks to their high penetrat-
1106andNGC2788AasnewCompton-thickAGNs,andUGC
ing power against obscuration as far as they are not heavily
Compton-thick(logNH≥24.5cm−2). X-rayfollow-upobser- 03752 as an Compton-thin heavily absorbed AGN. We apply
“torus”models based onMonte Carlosimulations by Murphy
vations covering lower energies of all-sky hard X-ray surveys
& Yaqoob (2009) and Ikeda et al. (2009) to the Suzaku and
withSwift/BurstAlertTelescope(BAT)andINTEGRALhave
Swift/BAT spectra, and discuss the degeneracy and limitation
producedmanyimportantresults,suchascolumndensitydistri-
ofthesemodelsandfutureprospects.
bution oflocalAGNs (e.g.,Markwardtetal.2005;Beckmann
etal.2006;Winteretal.2009a;Burlonetal.2011;Vasudevan The organization of this paper is as follows. Section 2 de-
et al. 2013; Ueda et al. 2014; Ricci et al. 2015) and simulta- scribestheobservationsofthetargets. Insection3,wepresent
neous broadband spectra (e.g., Ueda etal. 2007; Eguchi et al. the results of spectral analysis. In Section 4, implications
2009;Winteretal.2009b;Tazakietal.2013;Kawamuroetal. from our results are discussed. Section 5 gives the conclu-
2013). Because even hard X-ray fluxes are attenuated in the sion. The luminosities are calculated from the observed red-
case of very large absorption, however, the observed number shifts with the cosmological parameters (H0,Ωm,Ωλ) = (70
of Compton-thick AGNs was quite limited in shallow survey kms−1 Mpc−1,0.3, 0.7). Thesolarabundances byAnders&
data,liketheSwift/BAT 9-monthcatalog(Tuelleretal.2008). Grevesse(1989) areassumedinallcases. Inmodeling photo-
In fact, up to present, X-ray spectra of only a handful new electric absorption, we adopt the cross section by Balucinska-
Compton-thick AGNs selected inthehardX-raysurveyshave Church&McCammon(1992). Theerrorsattachedtospectral
beeninvestigatedindetail(e.g.,Gandhietal.2015). parameterscorrespondto90%confidencelimitsforasinglepa-
rameterofinterest.
C.Riccietal.(2016, inpreparation) areworking onasys-
tematicX-rayspectroscopicsurveyoftheSwift/BAT70-month
catalog,oneofthedeepestall-skyhardX-raysurveycurrently
2 Targetsand Observations
available (Baumgartner et al. 2013), using data mainly from
Swift/X-RayTelescope(XRT)below10keV.Oneofmaingoals
We selected Compton-thick AGN candidates from the
of this project is to establish the properties of Compton-thick
Swift/BAT 70-month catalog with the following criteria: (1)
AGNs in the local universe by significantly increasing their
very weak soft X-ray flux (2–10 keV) relative to the hard X-
sample size. Because typical exposures of the Swift/XRT ob-
ray flux (15–100 keV) by F2−10/F15−100 <0.03, which is a
servations are short, however, longer follow-up observations
goodindicatorforheavilyobscuredAGNs(Winteretal.2009a),
using large area X-ray telescopes such as Suzaku and XMM-
(2)indication ofastrongnarrowiron-KlineintheSwift/XRT
Newton are crucial to robustly identify new Compton-thick
spectrumofashortexposure(typically∼10ksec),and(3)op-
AGNs through accuratedetermination ofthe column densities
tical classification as Seyfert 2 or galaxy. Our targets, Swift
andtounderstandthenatureoftheirbroadbandspectra.
J0250.7+4142(NGC1106),SwiftJ0714.2+3518(UGC03752),
In this paper, we study the spectra of three heavily ob- andSwiftJ0902.7-6816(NGC2788A),correspondtobrightest
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 3
ones in the 15–100 keV band among those candidates whose
spectrabelow 10keV have not been reportedpreviously. The
basicinformationofthetargetsissummarizedinTable1.
We observed these targets in 2014 and 2015 with Suzaku
(Mitsuda et al. 2007), the fifth observatory in a series of
Bin time: 5760. s
Japanese X-ray astronomy satellites. Table 1 gives the obser-
vationlog. SuzakucarriesfourX-rayCCDcamerascalledthe (a) NGC 1106
X-rayImagingSpectrometers(XIS-0,XIS-1,XIS-2,andXIS-
3)coveringthe0.2–12keVasthefocalplanedetectorsoffour 2
0
0.
X-raytelescopes,andanon-imaginginstrumentcalledtheHard
X-ray Detector (HXD), consisting of Si PIN photodiodes and nt/sec
Gadolinium SiliconOxidescintillation counters, which covers ou
C
1
the 10–70 keV and 40-600 keV bands, respectively. XIS-0, 0
0.
XIS-2, and XIS-3 are front-side-illuminated CCDs, and XIS-
1istheback-side-illuminatedone.Inthispaper,weanalyzethe
dataoftheXIS-0,XIS-1,XIS-3becauseXIS-2becameinoper-
0
ablesince2007Novemberduetoadetector troubleandHXD 0 104 2×104 3×104 4×104 5×104 6×104
Time (s)
wasnotusableduetothepowershortageofthespacecraftdur-
Start Time 16872 2:23:22:184 Stop Time 16872 16:47:22:184
ingourobservationepochs.
Bin time: 5760. s
(b) UGC 03752
3 Analysis and Results
We analyze the Suzaku data in a standard manner, using 2
0
0.
HEAsoftversion6.17andcalibrationdatabasereleasedon2015
March12.FortheXISdata,weanalyzetheversion2.3cleaned nt/sec
u
events distributed bytheSuzakupipeline processingteam. To Co
1
extractthelightcurvesandspectra,wesetthesourceregionasa 0.0
circlearoundthedetectedpositionwitharadiusof100arcsec.
The background for the XIS data is taken from a source-free
region in the field of view with an approximately same offset
0
0 104 2×104 3×104 4×104 5×104 6×104
anglefromtheopticalaxisasthesource.
Time (s)
Start Time 17117 1:21:43:184 Stop Time 17117 15:45:43:184
Bin time: 5760. s
3.1 LightCurves
Figure 1 shows the background-subtracted light curves of our (c) NGC 2788A
targetsobtained withtheXISinthe2–10 keVband. Tomini-
mizeanysystematicuncertaintiescausedbytheorbitalchange 2
0
0.
ofthesatellite, wemergedatatakenduringoneorbit(96min-
utes)intoonebin.Then,tocheckifthereisanysignificanttime nt/sec
u
variabilityduringtheobservations,weperformasimpleχ2test Co
1
toeachlightcurveassuminganullhypothesisofaconstantflux. 0.0
Wedetect nosignificant timevariability onatimescalelonger
than96minutesforallthetargets. Thus,weanalyzethespec-
trumaveragedoverthewholeobservation.
0
0 104 2×104 3×104 4×104 5×104 6×104
Time (s)
Start Time 17157 2:31:41:184 Stop Time 17157 16:55:41:184
3.2 Spectra
Fig.1. Background-subtractedlightcurvesobtainedwiththeXIS(average
Forspectralanalysis,weusethedataoftheBI-XIS(XIS-1)and ofXIS-0,XIS-1,andXIS-3)inthe2–10keVband.Thebinsizeis96minutes.
FI-XISs(XIS-0+XIS-3)intheenergybandof0.5–8.0keVand
2.0–10keV,respectively; thedataoftheFI-XISsbelow2keV
are discarded because all targets show little counts owning to
thelowerquantumefficiencythanthatoftheBI-XIS.Toobtain
4 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
betterconstraintonthehardX-rayspectra,wealsousethe70-
month averaged Swift/BAT spectra in the 15–100 keV band.
The 1.7–1.9 keV band of the BI-XIS spectrum is excluded to
avoidsystematicuncertaintiesintheenergyresponsearoundthe
SiK-edgeregion. Thespectraofthethreetargets foldedwith 0.1
the energy responses are shown in Figure 2. The presence of
(a) NGC 1106
prominentiron-Kemissionlinearenoticeable.
1
We simultaneously fit the Suzaku and Swift/BAT spectra, 0.0
which totally cover the 0.5–100 keV band. The Galactic ab- V−1
ke
tshoerphtyiodnroigseanlwcoalyusminncdluednesidtyinNsHGpaelctersatlimmaoteddelfsrobmy pthheaHbsIwmiathp ounts s −1 10−3
(Kalberlaetal.2005).Totakeintoaccountthecross-calibration d c
uncertaintiesintheabsoluteeffectiveareabetweentheBI-XIS malize
andFI-XISs,aconstantfactorismultipliedtothemodel,which nor 10−4
is fixed at unity for FI-XISs and is left free for the BI-XIS.
Usually, to take into account possible time variability among
different epochs of observations, the cross normalization be- 10−5 1 10 100
Energy (keV)
tween Suzaku and BAT data must be introduced, at least for
1
the transmitted component (see e.g., Tazaki et al. 2013). In 0.
our case, however, we find that Suzaku data are consistent (b) UGC 03752
withnotimevariabilitycomparedwiththe70-monthaveraged
1
0
Swift/BAT fluxes for all targets within the statistical error. In 0.
V−1
fact, as we show later, a large fraction of the X-ray flux be- ke
ltoowth1e0rekfleeVctiinonNcGoCm1p1o0n6enatnfdroNmGtChe2t7o8ru8sA,wishliickheliyseaxttpriebcutteadbtloe ounts s −1 10−3
d c
oshfotwhelietmtleitvtianrgiarbeigliitoynoinstaimpeasrsceaclesscsahleo.rtHeretnhcaen,yweearfisxifththeecsoizne- normalize 10−4
stantfactorfortheSwift/BATspectrumatunityinourspectral
analysis.Weconfirmthatthemainresults(suchasline-of-sight
columndensities)arenotsignificantlyaffectedbypossibletime 10−5 1 10 100
variabilityinthetransmittedcomponent. Energy (keV)
1
0.
3.2.1 ConventionalModel
(c) NGC 2788A
Forconsistencywithpreviousworks,wefirstconsideranana-
lyticmodeloftenadoptedtorepresentthebroadbandspectraof 01
0.
obscuredAGNs(e.g.,Eguchietal.2009). InXSPECterminol- V−1
omgoyd,ethl1e=mozdpehlaisbesx∗przepssoewdearslw∗zhighect ounts s ke−1 10−3
d c
+const∗zpowerlw∗zhighect malize
+zphabs∗pexrav+zgauss+apec. (1) nor 10−4
Thismodeliscomposedoffivecomponents: (1)Thefirstterm
is the transmitted component absorbed by cold matter at the 10−5 1 10 100
source redshift (zphabs). It is known that the intrinsic spec- Energy (keV)
traofAGNsarewellapproximatedbyapowerlaw(zpowerlw) Fig.2. TheobservedXISspectrafoldedwiththeenergyresponseandthe
with an exponential cutoff (zhighect). Since we cannot con- Swift/BATspectra inunitsofphotoncm−2 ks−1 keV−1. (a)NGC1106.
(b)UGC03752. (c)NGC2788A.Theblackcrosses,redcrosses,andblue
strainthecutoffenergyforalltargets,wefixitat360keVfor
crossesrepresentthedataofFI-XIS,BI-XIS,andBAT,respectively.
consistencywiththetorusmodelbyIkedaetal.(2009).(2)The
secondtermisthescatteredcomponent. Wemultiplyascatter-
ingfractionfscat (const)tothesamecutoffpowerlawwithout
absorptionasthefirstterm. (3)Thethirdtermisthereflection
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 5
Table2.Best-fitparameterswithconventionalmodel
Parameter NGC1106 UGC03752 NGC2788A
(1) Xnorm 1.01+−00..1154 1.06+−00..1110 0.96+−00..2252
(2) NH[1024cm−2] 1.36+−00..2210 0.50+−00..0077 0.99+−00..2296
(3) Γ 1.79+0.23 1.56+0.17 1.40+0.28
−0.22 −0.15 −0.30
(4) Pnorm[10−2keV−1cm−2s−1] 0.13+−00..1078 0.05+−00..0032 0.03+−00..0072
(5) fscat[%] 2.93+−21..4322 3.85+−11..3043 3.11+−41..4709
(6) Ecent[keV] 6.40+−00..0023 6.38+−00..0043 6.40+−00..0022
(7) Fnorm[10−5cm−2s−1] 0.58+−00..1100 0.42+−00..1100 0.65+−00..1155
(8) kT [keV] 0.71+0.07 0.74+0.08
−0.09 −0.13
(9) EW[keV] 0.95 0.31 1.20
χ2/dof 23.6/29 28.3/29 18.1/17
Notes.
(1)ThenormalizationofBI-XISrelativetoFI-XISs.
(2)Thehydrogencolumndensity.
(3)Thephotonindexofthepowerlaw.
(4)Thenormalizationofthepowerlaw.
(5)ThescatteringfractionrelativetothefluxmeasuredwithSwift/BAT.
(6)Thecenterenergyoftheiron-Kemissionline.
(7)Thenormalizationoftheiron-Kemissionline.
(8)Thetemperatureofapecmodel.
(9)Theequivalentwidthoftheiron-Kemissionlinewithrespecttothewholecontinuum.
component. Weassume the sameabsorption (zphabs)as that isnoguaranteethatthereflectorinAGNscanberegardedtobe
forthetransmittedcomponent. Weadopttheanalyticmodelby opticallythickforComptonscattering.Inthissense,thismodel
Magdziarz&Zdziarski(1995)(pexrav),assumingComptonre- maynotbephysicallyself-consistent.
flectionfromopticallythick,coldmatter.Therelativereflection Monte Carlo simulations are very useful approach to in-
intensity is defined as R≡Ω/2π, where Ω is the solid angle vestigate the spectraof heavily obscured AGNs with complex
of the reflector. The inclination angle is fixed at 60 degrees. geometry of surrounding matter. Here we apply two major
Thephotonindexandnormalizationofthecutoffpowerlaware Monte-Carlo based numerical spectral models of AGN torus
linked to those of the transmitted component. (4) The fourth withdifferentgeometry,so-calledtheMYTorusmodel(Murphy
termistheiron-Kemissionlinearound6.4keV(zgauss). We &Yaqoob2009)andtheIkedatorusmodel(Ikedaetal.2009),
fixtheline width at10eVbut setthe centralenergy free. (5) toourbroadbandX-rayspectra.Bothmodelsassumecoldmat-
Thefifthtermrepresents emissionfromanoptically thinther- ter with a uniform density for the torus (“smooth torus”) and
malplasma(apec)inthehostgalaxy(Smithetal.2001),which isotropic irradiation from the central source with a power-law
isoftenobservedbelow1keV(e.g.,Terashimaetal.2002). spectrum(withanexponentialcutoffintheIkedatorusmodel).
Wefindthismodelcanwellreproducetheobservedbroad- TheMYTorusmodelconsidersatube-likesymmetricmatter
bandspectraintermsofχ2 values. Thebest-fitparametersare (see figure 1 in Murphy & Yaqoob 2009), corresponding to a
summarizedinTable2. Sincethereflectionstrengthcannotbe classicaldoughnut-type geometryfortheobscuringtorus. The
well constrained from our data, we fix R=1. In the case of halfopening angleofthetorusisfixedat60degrees. Besides
NGC2788A,thelastterm(thesoftcomponent)isnotrequired. the normalization and photon index of the incident spectrum,
We obtain the line-of-sight column densities of NH >∼ 1024 it has two parameters, the hydrogen column density along the
cm−2 in NGC 1106 and NGC 2788A. This is consistent with equatorial plane NHEq and the inclination angle θincl between
theirlargeequivalentwidthofthenarrowiron-Kemissionline theline-of-sightandthesymmetryaxisofthetorus(i.e.,θincl=
of∼1keV. 90correspondstotheedge-oncase). Theline-of-sightcolumn
densityNHLSinthecaseofθincl≥60isrelatedtoNHEqandθincl
as
3.2.2 TorusModels
Whiletheaboveconventionalmodelutilizingthepexravcode NHLS=(1−4cos2θincl)NHEq (θincl≥60). (2)
forcoldreflectionhasbeenoftenusedtofittheX-rayspectraof
obscuredAGNs,theactualgeometryofthereflectorisexpected From all targets, we obtained acceptable fits with the spec-
to be quite different from that assumed in the pexrav model tralmodelutilizingMYTorus(hereaftermodel2),whichisex-
(i.e.,asemi-infiniteslabofopticallythickmaterial).Also,there pressedinXSPECterminologyas:
6 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
model2=etable[mytorus Ezero v00.fits]∗zpowerlw
+const∗zpowerlw∗zhighect
+atable[mytorus scatteredH500 v00.fits]
+atable[mytl V000010nEp000H500 v00.fits]
+apec. (3)
0.01
(a) NGC 1106
The model is composed of fivecomponents: (1) the absorbed V)−1 10−3
tflreacntsimonitcteodmcpoomnepnotnefrnotm,(2th)ethteorsucsa,tt(e4r)etdhecoimropno-Kneenmt,i(s3s)iothnelirnee- m s ke−2−1 10−4
ffrroomm tthhee htoorsuts,gaalnadxy(5()onthlyefooprtiNcaGllCy-t1h1in06thaenrdmUalGcCom0p3o7n5e2n)t. Photons c 10−5
V (2
The best-fit parameters are summarized in Table 3. Figure 4 ke
10−6
e(an)e-Tr(gchy)epflIlkuoextsdaatthtteohrbeuesesntm-efirogtdymelEoad,seswluiimtnheussneaiptsanreoaaftreElymFsoEpd,hewelrhcicoearmlepgFoeEonmeinsettsthr.ye model)/error 02
− −2
w2iitnhItkweodahoetleasl.o2f0c0o9n)i,csailmshilaapretoaltohnagtathdeopptoeldarbyaxBisri(gshetemfiagnur&e (data 1 10 100
Energy (keV)
Nandra(2011a);Brightman&Nandra(2011b).Thismodelhas
0.01
three parameters on the torus structure, the hydrogen column (b) UGC 03752
densityalongtheequatorialplaneNEq,theinclinationangleof V)−1 10−3
fitThxheeedtoraartutiso0θ.b0ien1tc.wl,eAaennpdtohtwheeeirnhnlaaelwrfrowapdietinhuisnargHicnauntaognfldfeeoonufeterthgreyratodofiruu3ss6r0θooukpteenVis. Photons cm s ke−2−1 1100−−54
isassumedasthespectrumoftheprimaryemission. Although V (2
thehalfopeninganglecanbesetasafreeparameterunlike in ke 10−6
toθhouepreMdnaYatatT6oo0wrudnseinmggroetdeose,lt,hwweheliicmfihnietdendtahbpalhteoisttuocsnantsontaodttiisrbeteicctcslo.ynHcsotermnacipneae,rdwefewroifitmhx −model)/error −202
theMYTorusresults. Thehydrogen columndensity along the (data 1 10 100
line-of-sightNHLSinthecaseofθincl≥θopenisdeterminedas Energy (keV)
0.01
NHLS=0.010(.c9o9s(0θ.in0c1lc−oscθoisnθcolp+ens)in+(θsiinnc(lθ−inθclo−penθ)o)pen)NHEq.(4) V)−1 10−3 (c) NGC 2788A
WloweianlgsomoobdtealinutailciczeinpgtatbhleeIfiktesdfaotroraullstmheodseple(cmtraodwelit3h).the fol- m s ke−2−1 10−4
model3=torusabs∗zpowerlw∗zhighect Photons c 10−5
V (2
+const∗zpowerlw∗zhighect ke
10−6
++aattaabbllee[[rreeflfl12 ttoorruuss..fifittss]] model)/error 02
+atable[refl fe torus.fits]+apec. (5) (data− −2 1 10 100
Model3hassixcomponents:(1)theabsorbedtransmittedcom- Energy (keV)
ponent,(2)thescatteredcomponent,(3)theunabsorbedreflec- Fig.3. TheunfoldedspectrainunitsofEFEfittedwithmodel3(Ikedatorus
model).(a)NGC1106.(b)UGC03752.(c)NGC2788A.Theblackcrosses,
tioncomponentfromthefar-sidetorus,(4)thereflectioncom-
redcrosses, andbluecrossesrepresent thedataofFI-XISs,BI-XIS,and
ponents absorbed by the near-side torus, (5) the iron-K emis-
BAT,respectively.Thebest-fitmodelsareplottedwithsolidlines.
sionlinefromthetorus,and(6)theoptically-thinthermalcom-
ponent from the host galaxy (only for NGC 1106 and UGC
03752). Table4summarizesthebest-fitparameters. Thebest-
fitmodels areplotted inFigure 4 (d)-(f)for the 3targets, and
theunfoldedspectrainunitsofEFEareshowninFigure3.
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 7
Table3.Best-fitparameterswithMYTorusmodel
Parameter NGC1106 UGC03752 NGC2788A
(1) Xnorm 1.01+−00..1154 1.07+−00..1110 0.95+−00..2252
(2) NEq[1024cm−2] 2.33+0.43 4.97+0.47 2.11+0.59
H −0.40 −0.46 −0.55
(3) NLS[1024cm−2] 2.13 0.18 1.71
H
(4) θincl[degree] 81.6+−34..01 60.6+−90..73 77.5+−41.09.8
(5) Γ 2.24+0.26 1.75+0.13 1.94+0.36
−0.28 −0.09 −0.33
(6) Pnorm[10−2keV−1cm−2s−1] 2.65+−51..6861 0.15+−00..1016 0.81+−20..9692
(7) fscat[%] 0.17+−00..2160 1.24+−00..2286 0.12+−00..3160
(8) kT [keV] 0.75+0.08 0.75+0.08
−0.08 −0.12
(9) EW[keV] 1.12 0.27 1.21
χ2/dof 28.3/30 32.2/30 23.0/18
Notes.
(1)ThenormalizationofBI-XISrelativetoFI-XISs.
(2)Thehydrogencolumndensityalongtheequatorialplane.
(3)Thehydrogencolumndensityalongthelineofsight.
(4)Theinclinationangleofthetorus.
(5)Thephotonindexofthepowerlaw.
(6)Thenormalizationofthepowerlaw.
(7)ThescatteringfractionrelativetothefluxmeasuredwithSwift/BAT.
(8)Thetemperatureoftheapecmodel.
(9)Theequivalentwidthoftheiron-Kemissionlinewithrespecttothewholecontinuum.
Table4.Best-fitparameterswithIkedatorusmodel
Parameter NGC1106 UGC03752 NGC2788A
(1) Xnorm 1.00+−00..1164 1.06+−00..1110 0.95+−00..2252
(2) NEq[1024cm−2] 1.78+0.25 3.55+0.35 1.66+0.68
H −0.29 −0.34 −0.34
(3) NLS[1024cm−2] 1.75 0.53 1.58
H
(4) θincl[degree] 70.3+−94..43 60.1+−00..11 65.0+−62..89
(5) Γ 1.92+0.26 1.62+0.18 1.64+0.24
−0.22 −0.12 −0.14
(6) Pnorm[10−2keV−1cm−2s−1] 0.63+−00..6339 0.09+−00..0037 0.22+−00..3190
(7) fscat[%] 0.67+−00..7234 1.82+−00..4319 0.53+−00..4311
(8) kT [keV] 0.74+0.07 0.75+0.08
−0.08 −0.12
(9) EW[keV] 0.98 0.33 1.25
(10) F0.5−2[ergcm−2s−1] 1.35×10−13 8.07×10−14 2.29×10−14
(11) F2−10[ergcm−2s−1] 3.91×10−13 7.08×10−13 3.39×10−13
(12) F15−50[ergcm−2s−1] 7.20×10−12 6.10×10−12 7.60×10−12
(13) L2−10[ergs−1] 8.30×1042 2.33×1042 3.78×1042
χ2/dof 28.0/30 28.2/30 18.6/18
Notes.
(1)ThenormalizationofBI-XISrelativetoFI-XISs
(2)Thehydrogencolumndensityalongtheequatorialplane.
(3)Thehydrogencolumndensityalongthelineofsight.
(4)Theinclinationangleofthetorus.
(5)Thephotonindexofthepowerlaw.
(6)Thenormalizationofthepowerlaw.
(7)ThescatteringfractionrelativetothefluxmeasuredwithSwift/BAT.
(8)Thetemperatureoftheapecmodel.
(9)Theequivalentwidthoftheiron-Kemissionlinewithrespecttothewholecontinuum.
(10)TheobservedfluxwithBI-XISofSuzakuinthe0.5–2keVband.
(11)TheobservedfluxwithFI-XISsofSuzakuinthe2–10keVband.
(12)TheobservedfluxwithSwift/BATinthe15–50keVband.
(13)Theintrinsic(de-absorbed)luminosityinthe2–10keVbandband.
8 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
1 1
0. 0.
(a) NGC 1106 (d) NGC 1106
1 1
0 0
0. 0.
V)−1 V)−1
m s ke−2−1 10−3 m s ke−2−1 10−3
otons c 10−4 otons c 10−4
h h
P P
V (2 V (2
ke 0−5 ke 0−5
1 1
10−6 1 10 10−6 1 10
Energy (keV) Energy (keV)
1 1
0. 0.
(b) UGC 03752 (e) UGC 03752
1 1
0 0
0. 0.
V)−1 V)−1
m s ke−2−1 10−3 m s ke−2−1 10−3
otons c 10−4 otons c 10−4
h h
P P
V (2 V (2
ke 0−5 ke 0−5
1 1
10−6 1 10 10−6 1 10
Energy (keV) Energy (keV)
1 1
0. 0.
(c) NGC 2788A (f) NGC 2788A
1 1
0 0
0. 0.
V)−1 V)−1
m s ke−2−1 10−3 m s ke−2−1 10−3
otons c 10−4 otons c 10−4
h h
P P
V (2 V (2
ke 0−5 ke 0−5
1 1
10−6 1 10 10−6 1 10
Energy (keV) Energy (keV)
Fig.4. Thebest-fitspectralmodelswiththeMYTORUSmodel(left)andtheIkedatorusmodel(right). (a)and(d)NGC1106. (b)and(e)UGC03752. (c)
and(f)NGC2788A.Left:theblack,blue,orange,cyan,andredlinesrepresentthetotal,transmittedcomponent,scatteredcomponent,reflectioncomponent,
iron-Kemissionline,andemissionfromanopticallythinthermalplasma,respectively. Right:theblack,blue,orange,cyan,andredlinesrepresentthetotal,
absorbedreflectioncomponent, scatteredcomponent,unabsorbedreflectioncomponent,iron-Kemissionline,andemissionfromanopticallythinthermal
plasma,respectively.
PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0 9
4 Discussion andthephotonindex(Γ)arelargerintheMYToruscasethanin
theIkedatorusone,eventhoughthe90%confidenceregionsof
4.1 ResultSummaryonX-raySpectra
eachparameter overlapbetween thetwo models (seeTables 3
We are able to successfully fit the broadband spectra in the and 4). This can be qualitatively understood as follows. At a
0.5–100 keVbandofalltargets,NGC1106, UGC03752, and given inclination angle, a larger fraction of “unabsorbed” re-
NGC 2788A with three different models, (model 1) the con- flectioncomponentsfromthefar-sidetoruswillbeobservedin
ventional model utilizing the pexrav reflection code and two theMYTorusgeometry,whichisconstrainedfromthespectral
numericalmodelsbasedonMonteCarlosimulations,(model2) curvature below 7.1 keV. As aresult, alarger value of θincl is
theMYTorusmodeland(model3)theIkedatorusmodel.From obtainedintheMYTorusmodel.Sincetheline-of-sightcolumn
thetorusmodels,whicharemorephysicallyself-consistentthan density is a more rapidly decreasing function with decreasing
model 1, we find that the line-of-sight column densities are inclination intheMYTorusgeometry,alargervalueofNHEq is
larger than 1024 cm−2 for NGC 1106 and NGC 2788A, thus required. Owningtothelargercolumndensity,strongerreflec-
identifying them as Compton-thick AGNs, while UGC 03752 tion components are predicted, which make the overall spec-
is identified as a Compton-thin, heavily obscured AGN with trum harder. Tocancelthis effect, theincident spectrum must
NH ∼5.0×1023 cm−2. These identifications agree with the become softer in the MYTorus model than in the Ikeda torus
results by C. Ricci et al. (2016, in preperation) based on the model.
Swift/XRTdata.Thisfactdemonstratestheeffectivenessofse-
lectingCompton-thick objectsfromhardX-raysurveysbyus- The best-fit inclination angle, θincl = 60.6 (MYTorus) or
60.1 (Ikeda torus), is very close to the half opening angle
ingthefluxratiobetweenaboveandbelow10keVandstrong
iron-K line emission. Table 4 lists the observed fluxes in the θopen =60degreeforUGC03752. Thishappens becausethe
spectrum shape below the iron-K edge (7.1 keV) seems to be
0.5–2keV,2–10keV,and15–50keVbandandtheintrinsic2–
dominatedbyanunabsorbedreflectioncomponent,whichisap-
10keV luminosity based onmodel3. Theluminosities arein
the range of LX =1042−1043 erg s−1, which are of typical proximatedwithaveryhardpowerlawratherthananabsorbed
continuumwithastronglowenergycutoff.Similarresultshave
Seyfertgalaxies. All targetshave thebest-fitscatteredfracton
beenobtainedfromotherobscuredAGNswhenatorusmodelis
offscat>0.5%withmodels1and3,andhencewedonotiden-
applied(e.g.,NGC2273,Awakietal.2009;NGC3081,Eguchi
tifiyanyofthemasalowscattering-fractionAGN(so-calleda
etal.2011;3C403,Tazakietal.2011). Itwouldmeanthatwe
“newtype”AGN,Uedaetal.2007). Thisisnotsurprizingbe-
causeonly∼1/3ofobscuredAGNswithlogNH>23cm−2are indeedobservethenucleus alongtheline-of-sightintercepting
near the edge of the torus. However, considering thefact that
lowscattering-fractionAGNsintheSwift/BAT9-monthcatalog
asignificantfractionofobscuredAGNsshowsimilarfeatures,
(Uedaetal.2015).
suchinterpretationattributingonlytothegeometryeffectinthe
Aswehavementioned earlier,model1shouldberegarded
uniform-densitytoriwouldbeunrealistic.
asaphenomenological modelbecauseitdoesnottakeintoac-
countthecomplexgeometryandlimitedcolumndensityofthe
We interpret that these common features observed in ob-
X-rayreprocessingmatter.Hence,hereafterwecomparethere-
scured AGNs are evidence for clumpy tori (e.g., Krolik &
sultsbetweenmodels2and3. ThedifferenceintheMYTorus
Begelman 1988; Nenkova et al. 2008; Nenkova et al. 2008;
and Ikeda torus models is its geometry, in that there is more
Kawaguchi & Mori 2011), which naturally explain a large
materialbothinsideandoutsideofthe“doughnut”regioninthe
amountofunabsorbedreflectioncomponents emittedfromthe
Ikedatorusforagivenopeningangle.Accordingly,theline-of-
far-side torus. In fact, many observations imply that the tori
sightcolumndensityisgenerallysmallerintheMYTorusmodel
mustbeclumpy;forinstance,similarX-raytomid-infraredlu-
forthesameequatorial columndensityexceptfortheedge-on
minosity correlations are obtained between type 1 and type 2
viewingcase.
AGNs (Gandhi etal.2009; Ichikawa etal.2012; Asmus etal.
Model 3 gives smaller χ2 values for the same degree- 2015),whicharehardtobeexplainedbysmoothtorusmodels.
of-freedom in all targets, even though the difference is only In this context, application of clumpy torus models based on
marginal(∆χ2<4.4).Thisimpliesthattheactualgeometryof MonteCarlosimulationstothebroadbandX-rayspectraofob-
theAGNtorusmaybebetterapproximatedbyasphericalshape scuredAGNswouldberequiredtostudythedetailedstructure
rather than a doughnut shape, although we need much better oftori,includingthevolumefillingfactorofclumps.Following
quality broadband spectra to discriminate them. In fact, the the earlier works by Liu & Li (2014) and Furui et al. sub-
twobest-fitmodelsinFigure4lookquitesimilartoeachother, mitted (2015), we are working to produce numerical spectral
suggesting that model degeneracy is unavoidable. There are modelsfromclumpytori,usingtheMonteCarlosimulationfor
systematictrendsinthebest-fitresultsthatthecolumndensity Astrophysics and Cosmology (MONACO) framework (Odaka
along theequatorial plane(NHEq),theinclination angle(θincl), etal.2011).
10 PublicationsoftheAstronomicalSocietyofJapan,(2014),Vol.00,No.0
Table5.Basicpropertiesofthehostgalaxies
Parameter NGC1106 UGC03752 NGC2788A
(1) Galaxytype S0 S0a Sb
(1) Inclination[degree] 0 35.7 90.0
(2) W1-W2[mag] 0.35 0.90 0.31
(2) W2-W3[mag] 3.51 3.28 2.75
(2) logλLλ(22µm)[ergs−1] 43.37 43.59 42.79
Notes.
(1)BasedonHyperLeda.
(2)BasedonIRSA.
4.2 HostGalaxyProperties
Table5listsbasicpropertiesofthehostgalaxiesofourtargets
45
taken from the HyperLeda database. Their Hubble types (S0,
S0a, andSb) aretypical of normal Seyfertgalaxies. Thehost
44.5
inclinationofNGC2788Ais90degrees(i.e.,edge-ongalaxy).
This is consistent with the absence of an optically thin ther-
44
malcomponentinthespectraofNGC2788Abecauseemission
fromthegalactic disk maybe totally absorbed by thegalactic
1]43.5
interstellar medium along the line-of-sight. It has been sug- -s
g
gtheestenductlheautsthmeaflyuxalsooftbheesreodftuXce-draiynshcaigttheriendclcionmatipoonnegnatlafxroiems [erLX 43
g
(Ho¨nig et al. 2014). However, the two Compton-thick AGNs lo42.5
NGC1106andNGC2788Ainoursampleshowsimilarvalues
of fscat despite of the large difference in the host inclination. 42
Thissuggeststhatthescatteredcomponentisnotfullysubject
tointerstellarabsorptioneveninedge-ongalaxies,mostproba- 41.5
blyowningtoitslargerspatialextentthandiskemission.
41
41 41.5 42 42.5 43 43.5 44 44.5 45
log λ L (22µm) [erg s-1]
4.3 Mid-InfraredColorandLuminosity λ
Fig.5. Luminositycorrelationbetweentheinfrared(22µm)andhardX-ray
Weinvestigate themid-infraredfluxes ofour targets using the (14–195keV)bands. Theblacklineisthebestlinearregression lineob-
WISEcatalog(Wrightetal. 2010),whicharealsosummarized tainedfromallnon-blazarAGNsintheSwift/BAT9-monthcatalog(Ichikawa
etal.2012),whiletheredlineisthatobtainedfrom10Compton-thickAGNs
inTable5. WefindthatthereisalargevariationintheirWISE
includingNGC1106andNGC2788A(markedwithcircles).
colors (W1-W2 vs. W2-W3) and that only one target (UGC
03752) has the colors consistent with those of “AGN wedge”
defined by Mateos et al. (2013). This confirms the argument nary least-square bisector method. We also overplot the best-
byGandhi etal.(2015) thatthemid-infraredcolors cannot be fitregressionformobtainedbyIchikawaetal.(2012)for“all”
used to completely find obscured AGNs over a wide range of non-blazar AGNs in the Swift/BAT 9-month catalog by con-
luminositymostprobablybecauseofhostcontamination. verting the mid-infrared luminosities at 18 µm to those at 22
Figure5plotsthecorrelationbetweentheWISE22µmlu- µm with equation (4) in Ichikawa et al. (2012) and the X-
minosity(λLλ(22µm))andtheintrinsicluminosityinthe2–10 ray luminosities in the 14–195 keV ban d to those in the 2–
keVband(LX)forasampleofnineComptonthickAGNs in- 10 keV band by assuming aphoton index of 1.8. As noticed,
cluding NGC 1106, NGC 2788A, and those in the Swift/BAT there is a trend that Compton-thick AGNs show, on average,
9-month catalog except for NGC 1365, which shows abnor- higher mid-infrared luminosities than Compton-thin AGNs at
mallyrapidvariability inabsorption(Risalitietal.2005). The the same hard X-ray luminosity, even though the variation is
X-rayluminosities aretaken from Table 4 for NGC 1106 and large. This agrees with the finding by Matsuta et al. (2012),
NGC 2788A, and from Ricci et al. (2015) for the rest. From whousedtheAKARI9µmand18µmluminositiesandthe“ob-
this sample, weobtain the linear regression formof logLX = served”14–195keVluminosities(notcorrectedforextinction)
(1.14±0.26)logλLλ(22 µm)−6.96±11.4 with the ordi- forallAGNsincluding Compton-thickones. Theaveragedlu-