NGC454: unveiling a new "changing look" AGN

0.4 MB·English

by  Elena Marchese

Mon.Not.R.Astron.Soc.000,1–10(0000) Printed11January2012 (MNLATEXstylefilev2.2)
NGC454: unveiling a new “changing look” AGN
E. Marchese1, 2⋆, V. Braito1, 3†, R. Della Ceca1, A. Caccianiga1, P. Severgnini1
2 1INAF-OsservatorioAstronomicodiBrera,viaBrera28,20121Milano,Italy
1 2Universita`deglistudidiMilano-Bicocca,Piazzadell’AteneoNuovo,1-20126,Milano
0 3X-RayAstronomyObservationalGroup,DepartmentofPhysicsandAstronomy,LeicesterUniversity,LeicesterLE17RH,UK
2

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1 ABSTRACT

]
WepresentadetailedanalysisoftheX-rayspectrumoftheSeyfert2galaxyNGC454E,
O
belongingto the interactingsystem NGC454. Observationsperformedwith Suzaku,XMM-
C
Newton and Swift allowed us to detect a dramatic change in the curvature of the 2–10 keV
. spectrum, revealing a significant variation of the absorbing column density along the line
h
p of sight (from ∼ 1 × 1024cm−2 to ∼ 1 × 1023cm−2). Consequently, we propose this
- source as a new member of the class of “changing look” AGN, i.e. AGN that have been
o observedbothinCompton-thin(NH=1023cm−2)andreflectiondominatedstates(Compton-
tr thick,NH> 1024cm−2).Due tothequitelongtimelag(6 months)betweentheSuzakuand
s XMM-Newton observationswe cannotinfer the possible location of the obscuringmaterial
a
causing the observed variability. In the 6–7 keV range the XMM-Newton observation also
[
shows a clear signature of the presence of an ionized absorber. Since this feature is notde-

1 tectedduringtheSuzakuobservation(despiteitsdetectability),thesimplestinterpretationis
v that the ionized absorber is also variable; its location is estimated to be within ∼ 10−3 pc
9
fromthecentralblackhole,probablymuchcloserinthantheratherneutralabsorber.
6
1 Keywords: galaxies:active–galaxies:individual(NGC454)–X-rays:galaxies
2
.
1
0
2
1 INTRODUCTION andreflectiondominated states(N > 1024cm−2) (Risalitietal.
1 H
2002), implies that the absorbing material has to be clumpy and
v: There is now a general consensus that Active Galactic Nuclei atmuch smallerdistancethantheconventional obscuring “torus”
i (AGN) are powered by accretion of matter onto a supermassive
X with velocity, distance and size from the central X-ray source of
blackhole(SMBH),locatedatcenterofalmostallmassivegalax-
thesameorderofthoseoftheBroadLineRegion(BLR)clouds.
r ies. It is also clear that, according to the Unified Model of AGN
a Up to now, we can count only a few “changing look”
(Antonucci 1993),thedifferencebetweentype1andtype2AGN
AGN where such a variability has been discovered on time-
can be explained through orientation effects between our line of
scales from a few days down to a few hours: NGC 1365
sight tothe nucleus and “circum-nuclear material”. However, the
(Risalitietal. 2005, 2007, 2009), NGC 4388 (Elvisetal. 2004),
geometry,sizeandphysicalstateofthiscircum-nuclearmatterare
NGC 7674 (Bianchietal. 2005), NGC 4151 (Puccettietal.
still a matter of debate. In particular, the AGN X-ray spectra are
2007),NGC7582(Bianchietal. 2009),UGC4203(Risalitietal.
complexandconsistofmultiplecomponents(seeTurner&Miller
2010), NGC4051 (Uttleyetal. 2004; Lobbanetal. 2011) and 1H
2009, Done 2010 for a review), which are all intimately related
0419-577 (Poundsetal.2004).Among themwerecallNGC2992
tothestillpoorlyunderstood conditionofthematternearthenu-
(Weaveretal.1996),howeverforthissourceoneyearmonitoring
cleus.Thiscircum-nucleargasimprintsfeatures-lowenergycut-
with RXTE (Murphyetal. 2007) unveiled the presence of short-
offs,theComptonhumpandemissionandabsorptionlines-onto
term flaring activity rather than a change in the covering of the
theprimaryX-rayemission.TheX-rayspectraand,crucially,their
absorber.
variability observed in few nearby AGN showed that this matter
ishighlystructuredwitharangeofionisationstates,densities,ge-
Within a project investigating the occurrence of AGN in a
ometriesand locations(Turner&Miller 2009, Risaliti 2010).In
sampleofinteractinggalaxies,wecameacrossaninteractingsys-
thisrespect,thesignificantvariabilityoftheabsorbingcolumnden-
tem, NGC454, which was recently observed in the X-ray energy
sity(N )detectedinthesocalled“changinglook”AGN,i.e.AGN
that havHe been observed both in Compton-thin (N =1023cm−2) band withSuzaku, and ∼6 months later withXMM-Newton, and
H whosemainX-rayspectralcomponentspresentinterestingvariabil-
ityproperties.
⋆ E-mail:[email protected] Here we compare and discuss the X-ray observations from
† E-mail:[email protected] Suzaku, XMM-Newton and Swift that unveiled that NGC454 can
2 Marcheseet al.
beplacedamongthoseAGNwhoseabsorbingN isstronglyvari- 3.1.1 TheSuzakuXISanalysis
H
able(section4.2).Thepaperisstructuredasfollows.Theinteract-
TheXISdatawereselectedin3×3and5×5editmodesusingonly
ingsystemNGC454isdescribedin§2.TheX-rayobservationsand
goodeventswithgrades0,2,3,4,6andfilteringthehotandflicker-
datareductionaresummarizedin§3.In§4wepresentthespectral
ingpixelswiththescriptsisclean;thenetexposuretimesare103
analysisofbothdatasetsandthecomparisonbetweentheobserva-
ksec for each of the XIS. The XIS source spectra were extracted
tions,aimedtoassessthenatureoftheX-rayabsorber. Summary
fromacircularregionof2.2′radiuscenteredonthesource,andthe
andconclusionsfollowin§5.
backgroundspectrawereextractedfromtwocircularregionswith
Throughout this paper, a concordance cosmology with H = 71
km s−1 Mpc−1, ΩΛ=0.73, and Ωm=0.27 (Spergeletal. 20003) is tchaelibsaramtieornasdoiuusrcoefs.thTehseoXurIcSerreesgpioonns,eo(frfmseftsf)raonmdtahnecislolaurrycereasnpdonthsee
adopted.
(arfs) files were produced, using the latest calibration files avail-
able,withtheftoolstasksxisrmfgenandxissimarfgenrespectively.
ThespectrafromthetwoFICDDs(XIS0andXIS3)werecom-
2 NGC454 binedtocreateasinglesourcespectrum(hereafterXIS–FI),while
theBI(theXIS1)spectrumwaskeptseparateandfittedsimultane-
Optical studies (Arp&Madore 1987; Johansson 1988;
ously.Thenet0.5–10keVcountratesare:(0.0117±0.0005)cts/s,
Stiavellietal. 1998) of the interacting system NGC454 (see
(0.0142±0.0005)cts/s,(0.0132±0.0006)cts/sfortheXIS0,XIS3
Figure1,rightpanel)describeitasapairofemissionlinegalaxies
andXIS1respectively.Weconsidereddataintherange0.5–10keV
consistingofaredellipticalgalaxy(easterncomponent, hereafter
fortheXIS–FIandintherange0.6–7keVfortheXIS–BI(because
NGC454E) and a blue irregular galaxy (western component,
theXIS–BIisoptimizedforobservingbelow∼ 7keV).Forboth
hereafter NGC454W), at redshift z=0.0122. The distorted mor-
the XIS-FI and XIS-BI we ignored the band 1.6–1.9 keV, due to
phologyofboththesegalaxies,togetherwiththespectroscopicand
thepresenceofinstrumentalcalibrationuncertainties.ThenetXIS
photometricevidenceofayoungstellarpopulation,isaclearsign
sourcespectrawerethenbinnedtoaminimumof50countsperbin.
of the interacting nature of this system. Furthermore, three very
blueknots(discussedinsection3.3.1),probablyStrongrenspheres
surrounding clusters of very hot newly formed stars, are located 3.1.2 TheSuzakuHXD-PINanalysis
(andlikelyrelated) tothesouth of NGC454W.HST observations
of the system, performed with the Wide Field Planetary Camera For the HXD-PIN data reduction and analysis we followed the
2, confirmed that NGC454 is in the early stages of interaction latestSuzakudatareductionguide(theABCguideVersion22),and
(Stiavellietal. 1998). The above authors stated also that an usedtherev2data,whichincludeall4clusterunits.TheHXD-PIN
important fraction of gas has drifted to the center of the eastern instrument team provides the background (known as the “tuned”
component,butithasyetnotproducedanysignificantvisiblestar background) eventfile,whichaccountsfortheinstrumental“Non
formationactivity;apopulationofyoungstarclustershasformed X-rayBackground”(NXB;Kokubunetal. 2007).Thesystematic
aroundthewesterncomponent. uncertaintyofthis“tuned”backgroundmodelis±1.3%(atthe1σ
The optical spectrum of NGC454E is consistent with that of a levelforanet20ksecexposure3).
Seyfert 2 galaxy (although none of the high excitation lines, e.g. We extracted the source and background spectra using the same
HeII lines, can be seen) while no optical evidence of an AGN is commongoodtimeinterval,andcorrectedthesourcespectrumfor
present in the spectrum of NGC454W which is fully consistent thedetector deadtime.Thenetexposure timeafterthescreening
withthatofastar-forminggalaxy(Johansson 1988). was106ksec.WethensimulatedaspectrumforthecosmicX-ray
backgroundcounts(Boldt 1987;Gruberetal. 1999)andaddedit
totheinstrumentalone.
3 OBSERVATIONSANDDATAREDUCTION
NGC454isdetectedatalevelof3.4%abovethebackground
3.1 Suzakudata andthenetcountrateinthe15–30keVbandis0.01±0.002cts/s.
Forthespectralanalysisthesourcespectrumwasrebinnedinorder
NGC454 was observed on April 29, 2009 by the Japanese X-ray to have a signal-to-noise ratio >3 in each energy bin. We fit the
satelliteSuzaku (Mitsudaetal.2007)for atotal exposure timeof Suzaku-HXDspectrumwithasingleabsorbedpower-lawcompo-
about130ksec.SuzakucarriesonboardfourX-rayImagingSpec- nentwithaphotonindexΓ = 1.9andderivedanobserved15–30
trometers (XIS, Koyamaetal. 2007), with X-ray CCDs at their keVfluxof∼3.4×10−12ergcm−2s−1.
focal plane, and a non-imaging hard X-ray detector (HXD-PIN,
Takahashietal. 2007).Atthetimeofthisobservation onlythree
oftheXISwereworking:oneback-illuminated(BI)CCD(XIS1) 3.2 TheSwift-BATobservation
andtwofront-illuminated(FI)CCDs(XIS0andXIS3).Alltogether
NGC454 was also detected with the BAT detector on board of
theXISand theHXD-PINcover the0.5–10 keV and12–70 keV
Swift (Gehrelsetal. 2004). BAT is a coded aperture imaging
bands respectively. The spatial resolution of the XIS is ∼ 2 ar-
camera that operates in the 14–150 keV energy range; it has a
cmin(HEW),whilethefieldofview(FOV)oftheHXD-PINis34
large field of view (1.4 steradian half coded), and a point spread
arcmin radius. Data from the XIS and HXD-PIN were processed
function(PSF)of18arcmin(HEW).Swift-BATisdevotedmainly
using v2.1.6.14 of the Suzaku pipeline and applying the standard
screeningparameters1.
256softheSAAwereexcludedfromtheXISandwithin500softheSAA
fortheHXD.Cut-offrigidity(COR)criteriaof>8GVfortheHXDdata
1 ThescreeningfiltersalleventswithintheSouthAtlanticAnomaly(SAA) and>6GVfortheXISwereused.
aswellaswithanEarthelevation angle(ELV)< 5◦ andEarthday-time 2 http://heasarc.gsfc.nasa.gov/docs/suzaku/analysis/abc/
elevation angles(DYEELV)lessthan20◦.Furthermorealsodatawithin 3 ftp://legacy.gsfc.nasa.gov/suzaku/doc/hxd/suzakumemo-2008-03.pdf
NGC454:unveilinga new“changinglook”AGN 3
NGC454E
NGC454E
XE XE
XW NGC454W
XS
XSE XSE XS XW
1 arcmin
Figure1.Leftpanel:XMM-NewtonEPIC-pnimage(0.5–10keV)withsuperimposedtheSuzakuXISextractionregion.RightPanel:DigitalSkySurvey(DSS)
opticalimagewithoverlaidtheXMM-NewtonPN0.5–10keVcontours.WemarkedthemainX-raysourcesdiscussedinthetext.Itisevidentthatthemain
X-raysourceispositionallycoincidentwithNGC454E(classifiedasaSeyfert2)whilenoX-rayemissionisdetectedatthepositionofNGC454W.
tothemonitoringofalargefractionoftheskyfortheoccurrence ScienceAnalysisSoftware(SASver.6.5)andanalysedusingstan-
of gamma ray bursts (GRBs); while waiting for new GRBs, it dardsoftwarepackages(FTOOLSver.6.1andXSPECver.11.3).
continuously collects spectral and imaging information in survey Event fileshave beenfilteredfor high-background timeintervals,
mode,coveringafractionbetween50%and80%oftheskyevery and only events corresponding to patterns 0–12 (MOS1, MOS2)
day. andtopatterns0–4(pn)havebeenused.Thenetexposuretimesat
NGC454(BATname:SWIFTJ0114.4-5522)ispartofthePalermo thesourcepositionafterdatacleaningare∼23.9ksec(pn),∼29.1
Swift-BAT 54-Month hard X-ray catalogue (Cusumanoetal. ksec(MOS1)and∼29.2ksec(MOS2).
2010) and the Swift-BAT 58-Month Hard X-ray Survey IntherightpanelofFigure1wereporttheopticalDSSimage
(heasarc.gsfc.nasa.gov/docs/swift/results/bs58mon/). This of the system NGC454, together with the XMM-Newton 0.5–10
last survey detected 1092 sources in the 14–195 keV band keV contours (green, in the electronic version only) from EPIC-
down to a significance level of 4.8σ, reaching a flux level pn.ItisevidentthatthebulkoftheX-rayemissionispositionally
of 1.1 × 10−11erg cm−2 s−1 over 50% of the sky (and coincidentwithNGC454E(thegalaxyspectroscopicallyclassified
1.48 × 10−11erg cm−2 s−1 over 90% of the sky); as part of as a Seyfert 2) while no strong X-ray emission is detected at the
this new edition of the Swift-BAT catalogue, 8-channel spectra positionofNGC454W(thesourcespectroscopicallyclassifiedasa
and monthly-sampled light curves for each object detected inthe star-forminggalaxy).WealsodetectedaweakX-raysourcetothe
surveyweremadeavailable(Baumgartneretal. 2011). southofNGC454,whichispositionallycoincidentwithoneofthe
The 14–195 keV observed flux of NGC454 is 1.90+0.5 × threeveryblueknotsdiscussedabove,likelyastarformingregion
−0.5
10−11erg cm−2 s−1 , in agreement, when accounting for the belongingtoNGC454W.
differentbands,withthefluxquotedintheCusumanoetal. 2010 Thepn,MOS1andMOS2sourcespectrawereextractedfrom
catalogue(F14−195keV ∼ 1.7×10−11ergcm−2 s−1 ).Thisflux a circular region of 0.46 arcmin radius centered on the source
is also in good agreement with the expected 14–195 keV flux (NGC454E), while the background spectra were extracted from
(∼ 1.6×10−11ergcm−2 s−1 )extrapolatedfromthatmeasured twocircularregionswith0.5arcminradiusoffsetfromthesource.
withSuzakuinthe15–30keVrange. TheMOS1andMOS2spectrawerecombined,thenboththeEPIC-
pn and EPIC-MOS spectra were grouped with a minimum of 30
countsperchannel.
3.3 TheXMM-Newtonobservation
NGC454wasobservedwithXMM-NewtononNovember5,2009
3.3.1 ContaminationfromunresolvedsourcesintheSuzaku
foratotalexposuretimeofabout30ksec.TheXMM-NewtonOb-
(XIS,HXD)andSwift-BATextractionregion/fieldofview
servatory (Jansen et al. 2001) carries, among its onboard instru-
ments, three 1500 cm2 X-ray telescopes, each with EPIC (Euro- In the left panel of Figure 1 we show the XMM-Newton 0.5–10
peanPhotonImagingCamera)imagingspectrometersatthefocus. keVpnimagealongwiththeSuzakuextractionregion(circlewith
TwooftheEPICuseMOSCCDs(Turneretal. 2001)andoneuses 2.2′radius).AsdiscussedabovethemainX-raysourceiscentered
apnCCD(Stru¨deretal. 2001).TheseCCDsallowobservationsin on NGC454E but given the XMM-Newton better angular resolu-
the range ∼0.5–10 keV. The spatial resolution of the 2 MOSs is tion (14′′–15′′ HEW) as compared to Suzaku (120′′ HEW), we
∼14′′(HEW),and∼15′′(HEW)forthepn(Ehleetal. 2001). canclearlydistinguish4other X-raysources, besidesNGC454E,
During this observation the pn, MOS1, and MOS2 cameras entering in the Suzaku XIS extraction region. We extracted the
hadthemediumfilterappliedandtheywereoperatinginfullframe XMM-Newton spectra for the 3 brighter sources (XS, XE and
Windowmode.Thedatahavebeenprocessedandcleanedusingthe XSE,marked in Figure1 for clarity) and analysed them in order
4 Marcheseet al.
to estimate their possible contribution to the Suzaku spectrum; spatial resolution, we stressthat no emission was detected below
theremainingsource(XW)hasonly∼ 80countsdetectedinthe 10keVfromNGC454W,whileacontributionwouldbeexpected
∼0.5−10keVband(seebelow). eveninthecaseofadeeplyburiedAGN(seee.g.DellaCecaetal.
2002).Wethusconcludethatwedonotexpectsignificantcontam-
XS is well fitted with a power law, modified only by inationsfromthenearbysourcestotheSuzakuandSwiftspectra.
Galactic absorption, with a photon index Γ ∼ 1.8 and a 2–10
keV flux F ∼ 1.9 × 10−14erg cm−2 s−1 ; the ex-
[2−10]keV
trapolated flux in the 14–70 keV band (assuming Γ ∼ 1.8) is
F[14−70]keV ∼< 3 × 10−14erg cm−2 s−1 . As said above this 4 SPECTRALANALYSIS
source is likely associated with a star forming region related to
4.1 TheSuzakuandSwiftbroadbandX-rayemission
NGC454W;ifso,assumingz = 0.0122,its2–10keVluminosity
isL[2−10]keV ∼ 6.2×1039 ergs−1. Wecannot establish ifthis We first considered the X-ray spectrum of NGC454E in the
luminosity is due to one or more sources and thus speculate on 0.5–100 keV band by fitting simultaneously the Suzaku XIS,
its/their nature, because we lack both the spatial resolution and
Suzaku HXD and Swift-BATdata. Thecross-normalisation factor
a good enough sampling to assess its variability and spectral betweentheHXDandtheXIS-FIwassetto1.18,asrecommended
properties. The source to the east of NGC454E (hereafter XE) forHXDnominalobservationprocessedafter2008July(Manabu
canbefittedwithapower-lawandathermalcomponent,yielding et al. 2007; Maeda et al. 20086), while the cross-normalisation
Γ ∼1.6,kT ∼ 0.3andF[2−10]keV ∼ 8.8×10−15ergcm−2 s−1 betweenSwiftandXISwasallowedtovary.
(F[14−70]keV ∼< 5 × 10−14erg cm−2 s−1 ). The source to the In the subsequent sections the χ2 statistics was used for the fit,
south-east of NGC454E (hereafter XSE) can be fitted with an
the errors are quoted to 90% confidence level for 1 parameter of
absorbed power law (NH∼ 2.2 × 1022 cm−2) with photon interestandallthespectralparametersarequotedintherestframe
index set to 1.8 and F[2−10]keV ∼ 2.7 × 10−14erg cm−2 s−1 ofthesource.
(F[14−70]keV ∼< 3 × 10−14erg cm−2 s−1 ). Finally, the fourth
source located to the west of NGC454E (hereafter XW) has not
Wefittedthecontinuumwitharedshiftedunabsorbedpower-
enough counts for a meaningful spectral analysis (∼ 80 counts) lawmodel,modifiedonlybyGalactic(N = 2.73×1020 cm−2,
anditsestimatedfluxesareF[2−10]keV ∼2.0×10−14ergcm−2s−1 Dickey&Lockman 1990)absorption.ThHismodeldidnotprovide
andF[14−70]keV ∼< 2.5×10−14ergcm−2s−1(adoptingΓ∼1.9). an adequate description of the broadband spectrum of NGC454E
According to the extragalactic logN-logS distributions computed (χ2/dof=522.6/122). If we fit only the 2–5 keV continuum, ex-
by Mateosetal. (2008), at this flux level the number of random
cludingpossiblecomplexityinthesoftenergyrangeandnearthe
2–10 keV sources in the Suzaku extraction region is ∼ 2, thus Fe K emission line complex we found a very flat photon index
the sources XE, XSE and XW are probably those expected by
(Γ ∼ 0.15), strongly suggesting that we are dealing with an ab-
”chance”. There is no NED identification available for XE, XSE
sorbedAGN, inagreement withtheoptical spectral classification
andXW.
ofNGC454E.
Thecombined2–10keVfluxofallthese4possiblecontam-
Theresiduals withrespect tothissimpleunabsorbed power-
inating sources (F[2−10]keV ∼ 7.5×10−14erg cm−2 s−1 ) im- law model, which are shown in Figure 2, allowed us to infer the
ply that they will provide a negligible contribution to the Suzaku
main features of the observed spectrum. An excess at energies
XISspectrumofNGC454(F[2−10]keV ∼6×10−13ergcm−2s−1 below1keV,anemissionlinefeatureat∼6.4keV(likelyassoci-
). More important their estimated F arewell below the
[14−70]keV atedwithFeKα),togetherwithaline-likefeatureat∼7keV,and
Suzaku HXD-PIN or Swift-BAT sensitivity. On the other hand
anexcessatenergiesbetween10and20keV areclearlyevident.
this check is still not sufficient for these two latter instruments
TheresidualsinthesoftX-rayssuggestthepresenceofathermal
since their FOV is larger than that of the Suzaku XIS instru-
component probably relatedtothehostgalaxy. Thesimultaneous
ment. Assuming that the X-ray emission above 10 keV detected
occurrenceofastrongFeKαemissionlineat∼6.4keV(figure2
with the HXD-PIN or the Swift-BAT is associated to the same
upperandlowerpanel),averyflatobservedΓandanexcessinthe
source, as the good agreement of the measured fluxes strongly
hardX-rays(above10keV)isthedistinctivespectralsignatureof
suggests,wecanusetheinstrumentwiththesmallerFOV(Swift-
ahighlyabsorbedsource,withapossiblestrongComptonreflected
BAT) to perform further checks. In particular using known cata-
component.Theexcessobservedat∼7keV(Figure2lowerpanel)
loguesorarchives(NED4 andSIMBAD5)wesearchedforbright
islikelyduetothecombinationoftheFeKβ emissionline(7.06
X-ray/optical sources within 6 arcmin radius error circle (corre-
keV)andtheFeXXVI(∼6.97keV)andtheFeedge(∼7.11keV).
sponding to 99.7% confidence level for a source detection at 4.8
standarddeviations, Cusumanoetal. 2010)thatcouldberespon-
Giventhesefeatures,weperformedabroadbandfitincluding:
sibleoftheobserved X-rayemissionabove 10keV. Noplausible
contaminant source was found and, in the following, we will as- (i) a thermal component (modelled with the MEKAL model,
sumethattheemissionabove10keVcomesfromNGC454E.We Meweetal. 1986);
notethatwearealsoassuminganegligiblecontributiontotheemis- (ii) anabsorbedprimarypower-lawcomponent;
sion above 10 keV from the companion galaxy in the interacting (iii) anunabsorbedpower-lawcomponentwiththesamephoton
system,NGC454W.Whileaconfirmationofthisassumptionhasto indexΓ;
waitfordirectimagingobservations above10keVwithadequate
6 http://www.astro.isas.jaxa.jp/suzaku/doc/suzakumemo/suzakumemo-
4 http://ned.ipac.caltech.edu/ 2007-11.pdf;
5 http://simbad.u-strasbg.fr/simbad/ http://www.astro.isas.jaxa.jp/suzaku/doc/suzakumemo/suzakumemo-2008-06.pdf
NGC454:unveilinga new“changinglook”AGN 5
1
0
0.
6
0−3
V) 1
e
k
atio 4 cm s 2 10−4
R V/
e
V (k 0−5
e 1
2 k
0−6
1
1 10 100 1 10 100
Observed Energy (keV) Energy (keV)
6
Figure 3. Unfolded Suzaku spectrum, showing separately the different
components ofthebest-fitmodel:ingreen(intheelectronic version)the
scattered power-law and the primary absorbed power-law, in red (in the
electronicversion)thesoftthermalcomponent,inyellow(intheelectronic
version)theironKαandKβemissionlines,inlightblue(intheelectronic
4 version)thereflectioncomponent,andinblue(intheelectronicversion)the
o totalresultingspectrum.
ati
R
sincetheyrepresentthesamemediumproducingtwodifferentef-
2
fects(i.e.thenon-relativisticComptonscatteringandphotoelectric
absorptionoftheprimaryradiation,respectively).
Themodelsetupis:
5 6 7 8 WABS×[MEKAL+ZPOWERLW+ZGAUSS+ZGAUSS+PEXRAV
Rest Energy (keV)
+CABS×ZPHABS×(ZPOWERLW)]
Figure 2. Upper panel: ratio between the Suzaku and Swift data (XIS-
Wefoundthatthismodelprovidesagoodrepresentationofthe
FI:blackfilledsquares;XIS1:redopencircles;HXD:greenrhombs;and
BAT:blueopensquares,colorsintheelectronic versiononly)andtheun- X-ray emission of NGC454E (χ2/dof=104.5/103). The resulting
absorbedpower-lawmodelusedtofitSuzakudatainthe2–5keVenergy best-fitparametersarereportedintable1.Inparticular,thisbest-
range.Lowerpanel:zoomintothe5–8keVenergyrange(XIS-FI:black fitmodelyieldedΓ = 1.92+0.29,N = 2.05+4.25 ×1024 cm−2.
−0.36 H −1.38
filledsquares,XIS1:redopencircles).Wecanclearlyseeat6.4keVthe Therest-frameenergyoftheFeKαisEKα = 6.38±0.02 keV
excesscharacteristicoftheFeKαemissionlineandat∼7keV,thecom- anditsequivalentwidthwithrespecttotheobservedcontinuumis
binedcontributionoftheFeKβemissionline(7.06keV),FeXXVI(∼6.97 EW=340+60eV.AttheSuzakuspectralresolutionthisemissionline
keV),andthereflectoredge.ThecentralenergiesoftheFeKαandFeKβ −80
isunresolved;leavingthewidthσfreetovarywefoundσ.70eV
aremarkedwithdashedverticallines.
(atthe90%confidencelevel),thuswefixedittobe≃ 10eV.The
cross-normalisationfactorbetweentheSwift-BATandtheXIS-FI
is1.05+0.64.Westressthatadifferentchoiceofthecut-offenergy
−0.39
(iv) twoGaussianemissionlinesat∼6.4keV(FeKα)and7.06 intherangebetween100and300keVdoesnotaffectsignificantly
keV(FeKβ)respectively.WekepttheenergyoftheFeKβ fixed thebest-fitreflectionparametersobtained inthiswork. fTherel-
to7.06 keV, tieditsintrinsicwidth(σ) tothewidth of thecorre- ativeimportanceofthereflectioncomponent isgivenbytheratio
spondingFeKαlineandfixeditsfluxtobe13%oftheFeKαflux, betweenthenormalizationsoftheprimaryabsorbedpower-lawand
consistentwiththetheoreticalvalue(Kaastra&Mewe 1993); thereflection component; inour casethisratio is∼0.5, whichat
(v) aComptonreflectedcomponent,modelledwiththePEXRAV firstorderwouldcorrespondtoareprocessorcoveringasolidangle
model in XSPEC (Magdziarz&Zdziarski 1995). The parameters 2π.Thefractionofscatteredradiationis∼1%.Theobserved2–10
ofthereflectedcomponentare:aninclinationangleifixedto63◦, keVfluxis∼ 6.3×10−13ergcm−2 s−1 whiletheintrinsic2–10
abundanceZ=Z⊙,areflectionfraction(definedbythesubtending keVluminosityobtainedwiththismodelis7.2×1042ergs−1.
solid angle of the reflector R = Ω/4π) fixed to be -1 (i.e. pure
reflection)7, the cut-off energy (fixed at 200 keV, Dadina 2008)
andthenormalisation. 4.2 ComparisonwithXMM-Newtondata
The absorber was modelled by a combination of the CABS and In Figure 4 we report the Suzaku XIS (black, lower spectrum),
ZPHABS models in XSPEC,assuming the same column density, HXD(black) andSwift-BATspectra(green,intheelectronicver-
siononly).Inred(upperspectrum)wealsoshowtheXMM-Newton
pnandMOSdata,revealingadramaticchangeinthespectralcur-
7 Sinceinthe“purereflection”PEXRAVmodelthereisadegeneracybe- vaturebetween3and6keV.Thisvariationismostlikelyduetoa
tweenRandthenormalisation,wesetthereflectionscalingfactorto-1and changeintheamountofabsorptionoftheprimaryradiation.Totest
allowedthenormalisationtovary. thishypothesisweappliedtheSuzakubest-fitmodeltotheXMM-
6 Marcheseet al.
Table1.SummaryoftheSuzakuandXMM-Newtonparametersforthebest-fitmodelsdescribedinsection4.1,and4.2.1.
ModelComponent Parameter Suzaku XMM-Newton
Powerlaw Γ 1.92+0.29 1.99+0.11
−0.36 −0.07
Normalisationa 7.39+30.00,b 2.77+0.71
−4.39 −0.65
ScatteredComponent Normalisationa 8.55+5.48×10−3 1.62+0.29×10−2
−4.52 −0.22
Absorber N 2.05+4.25×1024cm−2 1.0+0.1×1023cm−2
H −1.38 −0.2
Thermalemission kT 0.62+0.10keV 0.62+0.11keV
−0.17 −0.11
Normalisationc 6.94+2.40×10−6 3.49+1.52×10−6
−2.22 −1.50
Neutralreflection Normalisationa 3.46+2.14 3.55+1.52
−1.61 −1.81
FeKαd Energy 6.38+0.02keV 6.36+0.03keV
−0.02 0.03
EW 340+60eV 120+40eV
−80 −40
Normalisatione 3.62+0.79×10−3 4.75+1.35×10−3
−0.78 −1.40
IonisedAbsorber N .. 6.05+8.95×1023cm−2
H −4.10
logξ .. 3.55+0.49ergcms−1
−0.25
vturb .. 300kms−1
χ2/dof 104.5/103 190.7/197
F(0.5−2)keV ∼4.9×10−14ergcm−2s−1 ∼5.8×10−14ergcm−2s−1
F(2−10)keV ∼6.3×10−13ergcm−2s−1 ∼1.9×10−12ergcm−2s−1
F(14−150)keV ∼1.4×10−11ergcm−2s−1 ∼1.3×10−11ergcm−2s−1
L(0.5−2)keV ∼4.7×1042ergs−1 ∼2×1042ergs−1
L(2−10)keV ∼7.2×1042ergs−1 ∼2.5×1042ergs−1
L(14−150)keV ∼1.4×1042ergs−1 ∼4.8×1042ergs−1
aunitsof10−3photonskeV−1cm−2s−1.
bDuetoadegeneracybetweenthenormalisationsoftheprimarypowerlawandPEXRAV,theerrorswerecomputedfixingthereflectionnormalisationtoits
best-fitvalue.
cThenormalisationofthethermalcomponentisdefinedasK= 4π(D1A0(114+z))2 RnenHdV whereDAistheangulardiameterdistance,zistheredshift,
neandnHaretheelectronandhydrogendensity(cm−3)respectively,anddVisthevolumefromwhichthedeprojectedemissionoriginates.
dThelineisunresolved;theintrinsicwidthhasbeenfixedtobe≃10eV.
eunitsof10−3photonscm−2s−1.
to ∼ 2.6×1023cm−2); this change in the amount of absorption
0−4 issufficienttoexplainthebulkofthedifferencesbetweentheob-
1
m−2 servedXMM-NewtonandSuzakuspectra.
eV c−1 Fthoerncoomrmpalelitseanteiosns,owfebaoltshotahlleowpoewdetor lvaawryctohmeppohnoetonnts,intdheexth(Γer)-,
s k−1 10−5 mal component and the Kα energy and normalisation. The fit
s yielded χ2/dof=213.9/199 and the only parameter changing well
nt
ou beyond the Suzaku errors is, as expected, the NH, decreasing to
alized c 10−6 2be.7tw8+−ee00n..1167X×MM10-2N3ewcmto−n2a.nTdhSisuzcaoknufiirsmdsuethtaot athcehsatnrogneginvacroilautmionn
m densityof∆N ∼1.8×1024cm−2.
or H
n Promptedfromthisresultwealsoinspectedthe3Swift-X-rayTele-
scope(XRT)observationstakenin2006withatimelagoftheorder
0−7 of1–2daysfromeachother,andwefoundthatthesourcewasin
1 1 10 100
Observed Energy (keV) a state similar to that observed by XMM-Newton. The exposure
timeofeachof theobservations islessthan10ksec(8713, 8661
Figure4.Comparison between the SuzakuXIS(black, lower spectrum),
and 3667 sec respectively), thus the relatively low statistics does
HXD(black),Swift-BAT(green,colorsintheelectronicversiononly)and
notallowustoestablishifthereisavariabilitybetweenthesingle
theXMM-Newton(red,upperspectrum)datashowingthedramaticchange
inthecurvatureinthe3–6keVenergyrange.Theunderlyingmodel(black observations.
andgreenline)istheoneobtainedfittingonlytheSuzakuXIS(black)and A closer inspection of the residuals in the 5–8 keV energy
Swift-BATdata(green). range to this best-fit model showed some residual curvature
between 5and 6keV, together witha possibleabsorption feature
centeredat∼6.7keV(seeFigure5),whichispresentinboththe
pn and MOS spectra and is suggestive of a more complex and
Newtonspectra, leavingonly theabsorbing column density (N ) likely ionized absorber. After checking the significance of this
H
freetovary. Wealsoleftboththecross-normalisation factorsbe- absorption line, we included in the model an additional ionized
tweenthepnandtheMOSspectraandbetweenSwift-BATandpn absorber (see §4.2.1). We note that, after accounting for the
datafreetovary;theywerefoundtobe1.02±0.04and1.06+0.14 absorptionfeatureat∼6.7keV,theexcessofcurvatureintherange
−0.16
respectively. During the XMM-Newton observation the N de- 5–6keVisnotpresentanymore.
H
creasedbyaboutoneorderofmagnitude(from∼2.1×1024cm−2
NGC454:unveilinga new“changinglook”AGN 7
(Tombesietal. 2010b).Wenotethatred-andblue-shiftedabsorp-
tionlinesarepredictedinseveraltheoreticalmodelsoffaileddisk
2 winds (Proga&Kallman 2004; Simetal. 2010) or of aborted jet
(Ghisellinietal.2004).However,beforeproceedingwithanyfur-
thermodelingoftheabsorptionfeaturewecheckeditssignificance.
5
o 1. To assess the significance of the absorption feature we
Rati performedextensiveMontecarlosimulationsasdetailedbelow.We
assumedasournullhypothesismodelthebest-fitmodeldiscussed
1 attheendof section4.2,andwesimulatedS=3000spectra(with
thefakeit command in XSPEC),withthesameexposure timeas
thereal data. Eachone of thesesimulatedspectra wasthen fitted
withthenull hypothesis model toobtainaχ2 value, andwesys-
5
0. tematicallysearchedforanabsorptionlineinthe2–10keVenergy
4 5 6 7 8 range,steppingtheenergycentroidoftheGaussianinincrements
Rest Energy (keV)
of 0.1 keV and refitting at each step. We then obtained for each
Figure5.ResidualsoftheXMM-Newtondata(pndataaretheblackfilled simulatedspectrumaminimumχ2andcreatedadistribution3000
squaresandMOSdataaretheredopencircles)intherange4–8keVwith simulated values of the ∆χ2 (compared to the null hypothesis
respecttothespectralmodeldiscussedinsection4.2.Anabsorptionfeature
model).Thisindicatesthefractionofrandomgeneratedabsorption
atabout6.7keVandaspectralcurvatureinthe5–6keVrangeareclearly features in the 2–10 keV band that are expected to have a ∆χ2
present.
greater than athreshold value. If N of thesesimulated values are
greaterthantherealvalue,thentheestimateddetectionconfidence
level is1-N/S. Using thisanalysis we can then conclude that the
The parameters of the XMM-Newton best-fit model are re- linedetectionissignificantat>99.97%level.
portedintable1andthefinalmodelsetupisdescribedinsection
4.2.1.Wenotethatthedifferenceinthebest-fitnormalisationsof In order to obtain a physical description of the absorber we
thethermalandscatteringcomponentbetweenSuzakuandXMM- replaced the Gaussian absorption line with a model representing
Newtonarelikelyduetoadegeneracybetweenthesetwoparame- aphotoionized absorber, whichhasbeenproduced usingamulti-
ters.Indeedthe0.5–2keVfluxdidnotstronglyvarybetweenthe plicativegridofabsorptionmodelgeneratedwiththeXSTARv2.1
twoobservations. code (Kallman&Bautista 2004). This grid describes an ionized
The fraction of scattered radiation is ∼ 5%. The 2–10 keV flux absorberparametrisedbyitscolumndensity(N ),anditsionisa-
H
is∼ 1.9×10−12ergcm−2 s−1 ,whilethe2–10keVluminosity, tionparameter,definedas:
L ,is∼ 2.5×1042ergs−1,aboutafactor2.8belowthe
lu(m2−in1o0s)kiteyVcomputedusingonlytheSuzakudata.Althoughsucha ξ= Lion (1)
nR2
variationoftheintrinsicluminosityisnotunusualinAGN,partof
thisdifferencecouldbeduetothegeometryassumedbythemodels where Lion is theionising luminosity between 1–1000 Rydbergs
(13.6eVto13.6keV),nisthehydrogengasdensityincm−3 and
adoptedforthehighcolumndensityabsorber.Indeedwewillshow
Risthe radial distance of the absorber fromthe ionising source.
insection4.3thatthisdifferenceissmaller(afactor1.7)whenwe
Since there is no apparent broadening of the absorption line we
adopttheMytoruscodefortheabsorber.
assumedaturbulencevelocityofv =300kms−1.
turb
The inclusion of this ionized absorber significantly improved
4.2.1 The∼6.7keVabsorptionfeatureintheXMM-Newton the fit (χ2/dof=190.7/197, ∆χ2 = 25 for 2 dof), with a column
density of N = 6.05+8.95 × 1023 cm−2and an ionisation of
observation H −4.10
log(ξ/erg cm s−1) = 3.55+0.49. The improvement in the χ2 is
−0.25
Asafirststeptomodeltheabsorptionfeatureinthe6–7keVband determinedsolelybyfittingtheabsorptionfeatureinthe6–7keV
we added a Gaussian absorbing component; setting σ = 0.05 band, since an ionised absorber withsuch a high level of ioniza-
keV we found that the centroid of the line is at E=6.75+−00..0064 tiondoesnotproduceanyfeatureinthesoftbandofthecontinuum.
keV, the normalisation is −3.75+1.12 × 10−6 and ∆χ2=24 for
−1.13
2 dof. The absorption lines appears to be marginally resolved; The parameters of the XMM-Newton best-fit model are re-
however leaving its width free to vary we can set only an upper portedintable1andthesetupisthefollowing:
limit σ < 0.3 keV, while the energy centroid is found to be
consistent within the errors E=6.77+0.08keV. The energy of this WABS×[MEKAL+ZPOWERLW+ZGAUSS+ZGAUSS+PEXRAV
absorption line suggests an assoc−ia0ti.o06n with absorption from +XSTAR*CABS*ZPHABS×(ZPOWERLW)]
highly ionized Fe (i.e. FeXXV at E∼6.7 keV) and thus a clear We can now estimate what is the maximum distance of this
signature of the presence of an ionized absorber. The presence ionisedabsorberfromthecentralblackhole,usingequation1,re-
of an ionized absorber is not exceptional since recent sensitive latingtheionisationparameter,thedensityoftheabsorberandthe
observations with Chandra, XMM-Newton, and Suzaku unveiled continuumluminosityLion.InthiscaseLion(intheenergyrange
thepresenceofred-andblue-shiftedphotoionizedabsorptionlines between 13.6 eV and 13.6 keV) is 7.3× 1042 erg s−1. Assum-
both in type 1 and type 2 AGN as well as in Radio Quiet and ing that the thickness of the absorber ∆R=N /n is smaller than
H
RadioLoud AGN(Tombesietal. 2010b,2011).Thus, itappears thedistanceRion(∆R/Rion<1),wecansetanupperlimittothe
that there is a substantial amount of ionized gas in the nuclei of distance:
AwGithNvs,elwochiitciehsmfraoymbheunlidnrkeeddstoofkgmas/souuptfltoowvionugto∼n0p.a0r4se−c s0c.a1l5ecs Rion= LNioHnξ∆RR < NLiHonξ =2.3×1015cm (2)
8 Marcheseet al.
This maximum distance of ∼ 10−3pc is consistent with a intoaccountthefactthattheFeKαisratherconstant(seetable1).
location of the ionised absorber within the Broad Line Re-
gion of the AGN. Indeed an estimate of the BLR size RBLR Wehavedonethisbydecouplingtheline-of-sightcontinuum
for NGC454E can be inferred by using the relation between passing through the reprocessor (or zeroth order continuum, see
RBLR and the monochromatic luminosity at 5100 A˚, L ˚ http://www.mytorus.com/manual/index.html)andthereflected(or
5100A
(Kaspietal. 2005, RBLR = 2.45 × (λL (5100A˚))0.608). scattered continuum, see mytorus model) continuum from repro-
10lt−days λ
Sincetheluminosityoftheopticalcontinuumcannotbemeasured cessor.Inpracticeweallowedthecolumndensitiesoftheline-of-
directly from the spectrum, because of the strong absorption, sightcontinuumandscattered-reflectedcontinuatobeindependent
we estimate L ˚ from the intensity of the [OIII]5007A˚ of each other. The reflected continuum, and the fluorescent line
5100A emission which is consistently produced in the same location, is
line flux, assuming a mean F[OIII]5007A˚/F(5100A˚) ratio. This
not extinguished by another column of intervening matter. Since
ratio has been inferred from the AGN template presented in
theXMM-Newtonspectrumunveiledthepresenceofanadditional
Francisetal. (1991) (F(5100A˚) = 0.059F([OIII])). Using
ionizedabsorber,whichaffectstheline-of-sightcontinuumwealso
the [OIII]5007A˚ flux published in Johansson (1988) we obtain
L5100A˚ ∼1×1041ergs−1A˚−1and,thus,anapproximatesizeof iXnSclTuAdeRdgarnidioansizdeedscarbibsoerdbeinr,swechtiicohnis4.m2.o1d.eIlnleodraddeorptotindgotthheast,amwee
theBLRof0.05pc,i.e.about50timesRion.
disentangledtheabsorbingcolumndensityoftheline-of-sight(los)
component fromthatforthescatteredcontinuumplusfluorescent
SincewedonotobservethisabsorptionfeatureintheSuzaku
emissionlines.Theinclinationangleoftheloscomponenthasbeen
spectrumweaddedtotheSuzakubest-fitmodelagaussianabsorp-
fixed at 90 degrees; the inclination angle for the reflected/ scat-
tionlinewiththesameparametersobtainedwiththeXMM-Newton
teredcontinuumpluslinecomponentcomponentwas,forsimplic-
data. The lower limit for the detection of anabsorption line with
centralenergyof6.75keVandwidthof0.05keV,is-1.18×10−6 ity,fixedat0degreessincetheeffectoftheinclinationangleonthe
shapeofthescatteredcontinuumisnotsufficientlylargewhenthe
for Suzaku data. Thus, being the normalisation of this line -
3.75×10−6 inthe XMM-Newton spectrum, wecan infer that the scatteredcontinuumisobservedinreflectiononly.Physically,the
situationwearemodellingbymeansofthisdecouplingcouldcor-
ionisedabsorbershouldbedetectablebySuzaku.Thesimplestin-
respondtoapatchyreprocessorinwhichthescatteredcontinuum
terpretationisthatalsotheionizedabsorberisvariable;whichisnot
isobserved fromreflection inmatter on the far-sideof the X-ray
surprisingsincethereareseveralreportedcasesofvariableabsorp-
source,withoutinterceptinganyother“clouds,”,whiletheintrinsic
tion feautures (Tombesietal. 2010b, 2011) (Braitoetal. 2007;
continuumisfilteredbyclouds”passing”throughourline-of-sight
Cappietal. 2009;Dadinaetal. 2005;Risalitietal. 2005).More-
tothecentralengine.
over instability of the outflowing ionized absorbers is predicted
WeappliedthismodeltothetheXMM-EPICandSwift-BAT
both in disk winds models (Proga&Kallman 2004; Simetal.
spectra and we found a god fit with the same absorbing column
2010)orofabortedjet(Ghisellinietal.2004).Thiswillcausethe
density(N = 2.75+0.05 ×1023cm−2;χ2/dof=201/192) filter-
presenceoftransientabsorptionfeaturesandvariabilityofthede- H −0.04
ing the line-of-sight intrinsic continuum and producing the scat-
rived outflowing velocities and their EW as observed in several
teredcomponent(includingtheproductionofthefluorescentemis-
sources(seee.g.Tombesietal. 2010b)
sion lines). The parameters of the ionized absorber are: N =
H
6.46+5.04×1023cm−2andlogξ =3.26+0.20ergcms−1;theseval-
−1.96 −0.21
uesareingoodagreementwiththosefoundwiththebest-fitmodel
4.3 AphysicalinterpretationwithMytorusmodel
described in table1. Thephoton index of theprimary power-law
Themodelsdiscussedsofar,whicharebasedonspectralcompo- component is now Γ = 1.86−0.11 and the intrinsic emitted lu-
+0.17
nentslargelyusedfromtheastronomical community,donottreat minosity is L ∼ 1.4×1042erg s−1. Using the Suzaku
[2−10]keV
bothfluorescentlinesandcontinuumcomponentsself-consistently. and Swift data, we found that a good fit can be obtained with an
Furthermoreallthesespectralcomponentsmaybedeficientinone absorberproducingthereflectedcomponentshavinganN statis-
H
ormoreaspectofmodellingthecomplextransmissionandreflected ticallyconsistentwiththatobtainedusingtheXMM-Newtondata
spectrumofAGNoverabroadenergyrangeandforalargerangeof (thussuggesting thatthiscomponent islikelyassociated withthe
absorbingcolumndensities(seesection2ofMurphyandYaqoob, distantreflectorortorus),whileourlineofsighttothecentralen-
2009foracriticaldiscussionofthesepoints). gineinterceptsacolumndensityN =(0.88±0.09)×1024cm−2.
H
Inordertoalleviatetheseproblemsand,thus,tofurtherassess In summary thisanalysis, which is based on amodel which
the possible geometry and/or nature of the variable absorber, takesintoaccountconsistentlythephysicalprocessinplacewithin
we tested the most recent model for the toroidal reprocessor 8 the X-ray absorber, shows that the change of state of NGC454E
(Murphy&Yaqoob 2009). This model, recently included in the canbeunderstood simplybyachancechangeintheline-of-sight
XSPEC software package, is valid for column densities in the obscuration(∆N ∼6×1023cm−2)whiletheglobalobscurerre-
H
range 1022 to 1025 cm−2 and for energies up to 500 keV (the mainsunchanged.TheintrinsicluminosityderivedfromtheSuzaku
relativisticeffectsbeingtakenintoaccount);moreimportantlythe dataisL ∼ 2.4×1042ergs−1.WenotethatusingMy-
[2−10]keV
reprocessed continuum and fluorescent line emission are treated torusthederivedchangeoftheintrinsicluminositybetweenthetwo
self-consistently for the first time. This model assumes that the datasetsareinbetteragreement(afactor1.7)withrespecttothose
absorbergeometryistoroidalwithanopeningangleof60◦.Since found in section 4.2. However, with the present statistic and the
weareclearlyseeingavariationoftheabsorbingcolumndensity complexityoftheobservedspectra,wecannotruleoutorconfirm
along the line of sight we have used a spectral configuration of apossiblevariationinluminosityofaboutafactor2,frequentlyob-
MyTorus that can mimicaclumpy absorber and which also takes servedinAGN;indeedbycomparingthe54-monthsand9-months
BAT high energy (14–195 keV) spectra of this source, we found
thattheintensityishigherinthe9-monthsspectrum.However,fit-
8 http://www.mytorus.com/ tingthespectrawithasingleabsorbedpower-lawcomponent, we
NGC454:unveilinga new“changinglook”AGN 9
foundthataconstantfluxisalsowellwithintheerrorsonthebest- of its distance from the central black hole (i.e. within 10−3 pc),
fitnormalizationsofthisprimarypower-law. themostlikelylocationforthisabsorberismuchcloserinthanthe
stableandratherneutralone.
5 SUMMARYANDCONCLUSION
WehavepresentedtheresultsofSuzaku,XMM-NewtonandSwift
ACKNOWLEDGEMENTS
observations of the interacting system NGC454 (z=0.0122). The
bulk of the measured 2–10 keV emission comes from the active WewarmlythankT.Yaqoobfortheusefuldiscussionandforhelp-
galaxyNGC454E(L ∼ 2×1042ergs−1);noemission ing us while fitting the Mytorus model to mimic the variable ab-
[2−10]keV
from the center of the companion galaxy (NGC454W) in the sorber. Wethank theanonymous refereefor many useful sugges-
interacting system is detected. The nuclear X-ray emission of tions and comments that significantly improved the paper. This
NGC454Eisfilteredbyanabsorbingcolumndensitytypicalofa research has made use of data obtained from the Suzaku satel-
Seyfert2galaxy,inagreementwiththeopticalclassification. liteanddataobtainedfromtheHighEnergyAstrophysicsScience
ArchiveResearchCenter(HEASARC),providedbyNASA’sGod-
A comparison between Suzaku and XMM-Newton observa- dardSpaceFlightCenter.Theauthorsacknowledgefinancialsup-
tions (taken 6 months later) revealed a significant change in the port from ASI (grant n. I/088/06/0, COFIS contract and grant n.
spectraofNGC454Eintheenergyrangebetween3and6keV.This I/009/10/0).VBacknowledgesupportfromtheUKSTFCresearch
variationcanbewellexplainedbyavariabilityof about anorder council.
of magnitude in the absorbing column density along the line of
sight:from∼1×1024cm−2(Suzaku)to∼1×1023cm−2(XMM-
Newton). This study also adopted the most recent model for
REFERENCES
the toroidal reprocessor (Murphy&Yaqoob 2009), which takes
into account consistently the physical processes in place within Antonucci,R.1993,ARA&A,31,473
the X-ray absorber. Furthermore, regarding the XMM-Newton Arp,H.C.,&Madore,B.F.1987,ACatalogueofSouthernPe-
spectrum,wedetectedastatisticallysignificantabsorptionfeature culiarGalaxiesandAssociations(Cambridge:CambridgeUniv.
a∼6.7keV,aclearsignatureofthepresenceofaionisedabsorber, Press)
withionisationparameterlog(ξ/ergcms−1) = 3.55andcolumn Baumgartneretal,2011ApJS,submitted
densityN = 6.05×1023 cm−2.Theabsenceofthisfeaturein Bianchi,S.,Guainazzi,M.,Mattm,G.,etal.2005,A&A,442,185
H
the Suzaku spectrum, despite its detectability, implies that it has Bianchi,S.,Piconcelli,E.,Chiaberge,M.,Bailo`n,E.J.,Matt,G.,
varied between the two observations. Absorption linesassociated &Fiore,F.2009,ApJ,695,781
withionized ironhavebeennow observed inseveral sources and Boldt,E.1987,Phys.Rep.,146,215
there is also a clear evidence that these lines are variable as in Braito,V.,Reeves,J.N.,Dewangan,G.C.,George,I.,Griffiths,
the case of NGC454. Furthermore, in some cases the measured R.E.,Markowitz,A.,Nandra,K.,Porquet,D.,Ptak,A.,Turner,
blue-shiftsoftheenergycentroidsimplyalargevelocityofthese T.J.,Yaqoob,T.,&Weaver,K.,2007,ApJ,670,978
absorbersandalikelyassociationwithpowerfuldiskwinds(King Cappi,M.,Tombesi,F.,Bianchi,S.,etal.2009,A&A,504,401
&Pound2003),whileinothercasesthereisnomeasurablemotion CusumanoG.,LaParolaV.,SegretoA.,FerrignoC.,MaselliA.,
asinourcase. SbarufattiB.,RomanoP.,ChincariniG.,GiommiP.,MasettiN.,
MorettiA.,ParisiP.,TagliaferriG.,2010,A&A,524,A64
In summary, with respect to the absorbing column density Dadina, M., Cappi, M., Malaguti, G., Ponti, G., & de Rosa, A.
variability, NGC454E is a new member of the class of “chang- 2005,A&A,442,461
ing look” AGN, i.e. AGN that have been observed in both Dadina,M.2008,A&A,485,417
Compton-thin (N =1023cm−2) and reflection dominated states DellaCeca,R.,Ballo,L.,Tavecchio,F.,etal.2002,ApJ,581,L9
H
(N > 1024cm−2).Apossiblescenarioisthatastableandlikely denHerder,J.W.,etal.2001,A&A,365,L7
H
distant absorber responsible for the iron emission line is present. Dickey,J.M.,&Lockman,F.J.1990,ARA&A,28,215
However, there is also a clear variation of the N of the line DoneC.,2010,arXiv,arXiv:1008.2287
H
of sight absorber, probably indicative of the clumpy nature of Ehle,M.,etal.2001,XMM-NewtonUsersHandbook
the rather neutral absorber itself. Unfortunately the comparison Elvis,M.,Risaliti,G.,Nicastro,F.,Miller,J.M.,Fiore,F.,&Puc-
between different observations, typically performed at intervals cetti,S.2004,ApJ,615,L25
ofmonthstoyears(asthosediscussedhere),providesonlyupper Fabian,A.C.2010,IAU
limits to the intrinsic time scales of N variations and thus on FrancisP.J.,HewettP.C.,FoltzC.B.,ChaffeeF.H.,Weymann
H
thepossiblelocationof thethicker obscuring material(obscuring R.J.,MorrisS.L.,1991,ApJ,373,465Symposium,267,341
“torus” vs. Broad Line Region clouds). The low exposure of the Gruber,D.E.,Matteson,J.L.,Peterson,L.E.,&Jung,G.V.1999,
Swift XRT 2006 observations, when the source was in a state ApJ,520,124
similar to the XMM-Newton one, did not allow us to establish Gehrels, N., Chincarini, G., Giommi, P., et al. 2004, ApJ, 611,
the N variability on smaller time scales (i.e. intra-day) of the 1005
H
singleobservations.Animprovementoftheestimatesofvelocity, Ghisellini,G.,Haardt,F.,&Matt,G.2004,A&A,413,535
distance and size from the central X-ray source of the obscuring Jansen,F.,etal.2001,A&A,365,L1
materialcouldbeobtainedonlythroughmonitoringobservational Johansson,L.1988,A&A,191,29
campaigns withinafew daysor weeks and/or through thesearch Kaastra,J.S.&Mewe,R.1993,A&AS,97,443
for N variations within single long observation. For what con- Kallman,T.R.,Palmeri,P.,Bautista,M.A.,Mendoza,C.,&Kro-
H
cernstheionisedabsorber,asderivedfromourfirstorderestimate lik,J.H.2004,ApJS,155,675
10 Marcheseetal.
Kaspi,S.,Maoz,D.,Netzer,H.,etal.2005,ApJ,629,61
King,A.R.,&Pounds,K.A.2003,MNRAS,345,657
Kokubun,M.,etal.2007,PASJ,59,53
Koyama,K.,Tsunemi,H.,Dotani,T.,etal.2007,PASJ,59,23
Lobban,A.P.,Reeves,J.N.,Miller,L.,etal.2011,MNRAS,414,
1965
Magdziarz,P.,&Zdziarski,A.A.1995,MNRAS,273,837
MateosS.etal.,2008,A&A,492,51
Mewe, R., Gronenschild, E.H.B.M., and van den Oord, G.H.J.
1985,A&AS,62,197
Murphy,K.D.,Yaqoob,T.,&Terashima,Y.2007,ApJ,666,96
Murphy,K.D.,&Yaqoob,T.2009,MNRAS,397,1549
Mitsuda,K.,etal.2007,PASJ,59,1
Pounds,K.A.,Reeves,J.N.,Page,K.L.,&O’Brien,P.T.2004,
ApJ,616,696
Proga,D.,&Kallman,T.R.2004,ApJ,616,688
Puccetti,S.,Fiore,F.,Risaliti,G.,Capalbi,M.,Elvis,M.,&Nicas-
tro,F.2007,MNRAS,377,607
Ross&Fabian(2005),MNRAS,358,211
Risaliti,G.,Elvis,M.,&Nicastro,F.2002,ApJ,571,234
Risaliti,G.,Elvis,M.,Fabbiano,G.,Baldi,A.,&Zezas,A.2005,
ApJ,623,L93
Risaliti,G.,Elvis,M.,Fabbiano,G.,Baldi,A.,Zezas,A.,&Sal-
vati,M.2007,ApJ,659,L111
Risaliti,G.,etal.2009,ApJ,696,160
Risaliti,G.2010,inAmericanInstituteofPhysicsConferenceSe-
ries,Vol.1248,AmericanInstituteofPhysicsConferenceSeries,
ed.A.Comastri,L.Angelini,&M.Cappi,351354
Risaliti G., Elvis M., Bianchi S., Matt G., 2010, MNRAS, 406,
L20
Shu, X. W., Yaqoob, T., Murphy, K. D., et al. 2010, ApJ, 713,
1256
Sim,S.A.,Proga,D.,Miller,L.,Long,K.S.,&Turner,T.J.2010,
MNRAS,408,1396
Spergel,D.N.,etal.2003,ApJS,148,175
Stiavelli,M.,Panagia,N.,Carollo,M.C.,Romaniello,M.,Heyer,
I.,Gonzaga,S.1998,ApJ492,L135
Stru¨der,L.,etal.2001,A&A,365,L5
Takahashi,T.,etal.2007,PASJ,59,35
Tombesi F.,Cappi M.,ReevesJ.N.,PalumboG.G.C.,Yaqoob
T.,BraitoV.,DadinaM.,2010,A&A,521,57
Turner,T.J.,Miller,L.,Kraemer,S.B.,Reeves,J.N.,&Pounds,
K.A.2009,ApJ,698,99
TombesiF.,CappiM.,ReevesJ.N.,PalumboG.G.C.,BraitoV.,
DadinaM.,2011,ApJaccepted
Turner,M.,etal.2001,A&A,365,L27
TurnerT.J.,MillerL.,2009,A&ARv,17,47
Uttley, P., Taylor, R. D., McHardy, I. M., et al. 2004, MNRAS,
347,1345
Weaver,K.A.,Nousek,J.,Yaqoob,T.,etal.1996,ApJ,458,160
Wilms,J.,Allen,A.,&McCray,R.2000,ApJ,542,914
Yaqoob,T.1997,ApJ,479,184