The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick AGN in NGC 7674

19 Pages·2017·0.49 MB·English

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed7February2017 (MNLATEXstylefilev2.2)
The weak Fe fluorescence line and long-term X-ray evolution of the
Compton-thick AGN in NGC7674
1,2 2 2,3 4 2 5,6,7
P. Gandhi, A. Annuar, G.B. Lansbury, D. Stern, D.M. Alexander, F.E. Bauer,
8 9 1 10,11,12 13
S. Bianchi, S.E. Boggs, P.G. Boorman, W.N. Brandt, M. Brightman,
14 15 14,16 17 18
F.E. Christensen, A. Comastri, W.W. Craig, A. Del Moro, M. Elvis,
19,20 21 13 22 22 23
M. Guainazzi, C.J. Hailey, F.A. Harrison, M. Koss, I. Lamperti, G. Malaguti,
15,24 8 25,26 5 13 3,4,13
A. Masini, G. Matt, S. Puccetti, C. Ricci, E. Rivers, D.J. Walton ,
27
W.W. Zhang
Authoraffiliationsappearattheendofthepaper.
7February2017
ABSTRACT
WepresentNuSTARX-rayobservations oftheactivegalacticnucleus(AGN)inNGC7674.
ThesourceshowsaflatX-rayspectrum,suggestingthatitisobscuredbyCompton-thickgas
columns.Baseduponlong-termfluxdimming,previousworksuggestedthealternatepossibil-
itythatthesourceisarecentlyswitched-offAGNwiththeobservedX-raysbeingthelagged
echofromthetorus.Ourhigh-qualitydatashowthesourcetobereflection-dominatedinhard
X-rays, but with a relatively weak neutral FeKα emission line (equivalent width [EW] of
≈0.4keV)andastrongFeXXVIionisedline(EW≈0.2keV).Weconstructanupdatedlong-
termX-raylightcurveofNGC7674andfindthattheobserved2–10keVfluxhasremained
constantforthepast≈20years,followingahighfluxstateprobedbyGinga.Lighttraveltime
arguments constrain the minimum radius of the reflector to be ∼3.2pc under the switched-
offAGNscenario,≈30timeslargerthantheexpecteddustsublimationradius,renderingthis
possibilityunlikely.ApatchyCompton-thickAGN(CTAGN)solutionisplausible,requiring
aminimumline-of-sightcolumndensity(NH)of3×1024cm−2 atpresent,andyieldsanin-
trinsic2–10keVluminosityof(3–5)×1043ergs−1.Realisticuncertaintiesspantherangeof
≈(1–13)×1043ergs−1.Thesourcehasoneoftheweakestfluorescencelinesamongstbona
fideCTAGN,andispotentiallyalocalanalogueofbolometricallyluminoussystemsshowing
complex neutral and ionised Fe emission. It exemplifies the difficulty of identification and
propercharacterisationofdistantCTAGNbasedonthestrengthoftheneutralFeKαline.
Keywords: Seyfert–X-rays:individual(NGC7674)
1 INTRODUCTION veryfewrobust(bonafide)CTAGNareknown(DellaCecaetal.
2008;Gouldingetal.2012;Gandhietal.2014).
Obscuredactivegalacticnuclei(AGN)dominatetheoverallpopu- One candidate of a nearby, luminous CTAGN is NGC7674,
lation of AGN in the cosmos, especially the efficiently-accreting thebrightestmemberoftheHickson96interactinggalaxygroup.
sources which power the cosmic hard X-ray background radia- NGC7674 is a known Seyfert 2, showing strong narrow optical
tion (e.g. Setti & Woltjer 1989; Comastri et al. 1995; Gandhi emissionlineswithfullwidthsathalfmaximum(FWHM)ofless
& Fabian 2003; Gilli et al. 2007; Treister et al. 2009; Ballantyne than 500kms−1 in spectral observations carried out over three
etal.2011;Uedaetal.2014).Yet,findingandcharacterisingthese decadesago(Feldmanetal.1982).Thesourcealsoshowsapro-
objects is made difficult by the strong absorption they can suf- lificoutflow,whichmanifestsasaprominentshoulderontheblue
fer across the electromagnetic spectrum. In particular, the census wings of all the prominent narrow optical lines. The outflow has
ofthesourceshiddenbehindextremeobscuringcolumndensities been studied in detail using HST spectroscopy by Fischer et al.
ofgas–inparticularCompton-thickAGNwithcolumndensities (2013),whofindblueshiftsofupto∼1700kms−1(andevenlarger
NH∼>1.5×1024cm−2; hereafter, CTAGN – remains highly in- FWHM)alongthenarrowlineregionalignedwiththejetaxisofthe
complete (see Ricci et al. 2015 and Koss et al. 2016 for recent source.Intheradio,NGC7674showsatleastthreeseparatecom-
updatesonthehardX-rayselectedCTAGNcensus).Evenlocally, pact components in VLBI observations on scales of ≈0.7arcsec
(cid:2)c 0000RAS
2 P.Gandhietal.
corresponding to ≈0.4kpc, and there have been suggestions that
Table1.ObservationLog
the collimated ejecta associated with the radio source could also
drive the optical-emitting line outflow (Unger et al. 1988). Even
higherresolutionVLBIobservationsbyMomjianetal.(2003)re- Mission Instrument(s) Observationdate Exposure
vealacomplex‘S’–shapedstructure,whichcouldresultfromin- ks
teractionsofthejetwiththeinterstellarmedium.Usingthe[OIII]
NuSTAR FPMA/B 2014-09-30 52.0/51.9
emissionlineasabolometricluminosityindicator,Xuetal.(1999)
Suzaku XIS0/1/3 2013-12-08 52.2/52.2/52.2
classifyNGC7674asaradioquietAGN. Swift XRT 2011-01-28...2014-10-08† 48.8†
The source was first reported to be a refection-dominated
AGN by Malaguti et al. (1998) from BeppoSAX X-ray observa- †ForSwift,17observationsarecombinedhere.SeeAppendixfordetails.
tionscarriedoutin1996,withthedirect(intrinsic)continuumbe-
ingfullyabsorbedbyaCompton-thickgascolumn.BeppoSAXde-
tectedNGC7674overtheenergyrangeof∼0.1–60keV.Malaguti tanceof126Mpccorrectedtothereferenceframedefinedbythe
etal.reportedacomplexstructuretotheneutralFeKαline,andes- cosmic microwave background. The source systemic redshift is
timatedahighintrinsicAGNluminosityassumingascatteringge- z=0.0289.AllX-rayspectralfittingiscarriedoutwiththeXSPEC
ometrysimilartoNGC1068. ButtheBeppoSAXobservationwas package v12.9.0 (Arnaud 1996) and uncertainties are quoted at
notthefirstX-rayobservationofthesource.Asdetailedinahistor- 90%confidence,unlessstatedotherwise.
icalX-rayanalysisbyBianchietal.(2005),thesourcewaslikely
detectedbytheHEAOmissioninthelate1970sata2–10keVflux
level almost 30times brighter (Grossan 1992) and subsequently
2 OBSERVATIONS
with an intermediate flux by Ginga (Awaki et al. 1991). Bianchi
et al. (2005) discuss several potential caveats to these detections, AlogoftheNuSTAR,SuzakuandSwiftX-rayobservationsanal-
includingHEAOcontaminationbynearbysources,andsystematic ysedhereinispresentedinTable1,andtheindividualdatasetsare
uncertaintiesrelatedtothebackgroundlevelmeasuredbyGinga, describedinthissection.
andconcludethatthecombinedweightofevidencefavoursthehis-
toricalsourcedetectionsbeingreal,althoughtheGingafluxmea-
surement is considered to be more reliable of the two, due to a 2.1 NuSTAR
more robust background determination. The Ginga spectrum is a
lightly absorbed power law (with NH<2×1022cm−2) with an NGC7674 was observed by NuSTAR (Harrison et al. 2013) for
about52ksofexposureon2014Sep30(ObsID708023010).NuS-
upper limitof80eV tothe equivalent width(EW)of any neutral
FeKαemissionline.Thesourcethendeclinedbyanorderofmag- TARisthefirstorbitingtelescopecapableoffocusingX-raysabove
∼10keV, operating over the energy range of 3–79keV. The data
nitudeincontinuumfluxbythetimeitwasobservedbyBeppoSAX
wereprocessedusingstandardstepsandtheNuSTARDataAnalysis
to be Compton-thick. Such behaviour might argue for the source
beingamemberoftheclassof‘changing-look’AGN(e.g., Matt Software(NUSTARDAS)v.1.3.0whichisprovidedaspartofHEA-
et al. 2003), associated with clumps of obscuring clouds transit- SOFT27andassociatedFTOOLS(Blackburn1995).NuSTARCALDB
calibrationfileswereusedtogeneratecleanedeventfilesafterfil-
ingacrosstheline-of-sight(l.o.s)resultinginapparentchangesin
NH(l.o.s). The most famous example of this class is NGC1365 teringforSouthAtlanticAnomalypassagesandthestandarddepth
whichshowsdramaticNHvariabilityontimescalesofdays(Risal- cut,inSoourrdceerstopercetdraucweeirnesterxutmraecntetadlubsaicnkggarocuirncdu.laraperture45(cid:2)(cid:2)in
itietal.2005).
radiuscentredonthesourcepositioninbothfocalplanemodules
However, an XMM-Newton observation carried out 6 years
(FPMs). Background spectra were extracted from source free re-
later(in2004)alsoshowedareflection-dominatedspectrumcom-
gionsonthedetector.Thenuproductstaskwasusedtoextract
pletelyconsistentinshapeaswellasfluxwithBeppoSAX,unlike
these calibrated spectra and to generate corresponding response
what may be expected in a changing-look AGN. Bianchi et al.
files. All spectra were grouped to a minimum signal-to-noise of
(2005)interpretthesourceaspotentiallyhavingswitched-off,with
atleast4pergroupedenergybinafterbackgroundsubtractionfor
thespectrumobservedbyBeppoSAXandXMM-Newtonbeingthe
fittingpurposes.
reflection component which is delayed with respect to the direct,
ThesourceiswelldetectedinbothFPMswith3–78keVcount
illuminating power law (PL). Another source that has been dis-
ratespersecondof2.92±0.08(FPMA)and2.53±0.08(FPMB).
cussedfromthesetwoopposingperspectivesrecentlyisNGC7582
(Rivers et al. 2015). Bianchi et al. (2005) also found a relatively
weakFeKαemissionlineinNGC7674,possiblyblendedwithan
2.2 Suzaku
ionisedFeXXVIlineat6.97keV.
Approximately 10 years after the last XMM-Newton obser- About 52ks of exposure were obtained on 2013 Dec 08 with
vation, NGC7674 was observed by Suzaku in 2013 and then by Suzaku. The X-ray Imaging Spectrometer (XIS; Koyama et al.
NuSTAR in 2014, in addition to several snapshot observations by 2007) is sensitive over ≈0.5–10keV. Standard FTOOLS software
theSwiftsatellitebetween2011to2014.Here,wepresentaspec- forSuzakuwasusedfordatareductionandcleanedeventfilegen-
tral analysis of these unpublished observations, and combine this erationwithrecommendedfiltering.Sourcecountswereextracted
with historical data to study the long-term source evolution. We fromwithina3.4arcminradiusapertureforintegratingXISsource
discuss and place constraints on the switched-off AGN as well counts,andbackgroundcountsfromalargersource-freepolygon.
as the CTAGN scenarios. Finally, we touch upon the relevance The generated spectra and responses of the two front-illuminated
of the complex Fe lines for the identification and characterisa-
tionofdistantCTAGN.WeassumeH0=67.3kms−1Mpc−1 and
ΩΛ=0.685 (Planck Collaboration 2014), corresponding to a dis- 27 https://heasarc.gsfc.nasa.gov
(cid:2)c 0000RAS,MNRAS000,000–000
NuSTARobservationsofNGC7674 3
(FI) detectors were combined together, and this was analysed si- ofNGC7674withafluxF14−195=9.9+−52..14×10−12ergs−1cm−2,
multaneouslywiththeback-illuminated(BI)detectordata.Forthe consistentwiththatinferredfromourNuSTARanalysisdescribed
fitting,weignoretheenergyrangesof1.7–1.9keVand2.1–2.3keV later.
becauseofcalibrationuncertainties.
The source was also observed by the Hard X-ray Detector
(HXD;Takahashietal.2007;Kokubunetal.2007).TheHXD/PIN 2.4 OpticalSpectroscopy
data (the PIN array is sensitive between ≈15–60 keV) were re-
In preparation for, and in support of, the NuSTAR observations,
duced using standard tasks. The FTOOLS routine hxdpinxbpi we also obtained optical spectroscopy of NGC7674 using the
is a pipeline task that first extracts spectral counts and corrects
Low ResolutionImaging Spectrometer (LRIS)on the Keck Tele-
these for deadtime. Using the ‘tuned’ background model for the
scope (Oke et al. 1995). The observations were carried out on
targetobservationprovidedbytheSuzakuteamasastartingpoint 2014June25 through a 1(cid:2).(cid:2)0 wide slit, using both the blue (600
(Fukazawaetal.2009),hxdpinxbpialsooutputsabackground linesmm−1)grismandthered(400linesmm−1)grating,withthe
spectrumincluding thecosmicX-raybackground (CXB)compo-
D560 dichroic. The night was photometric, with seeing close to
nent. However, after background subtraction, the residual source 0(cid:2).(cid:2)7.
count rate was found to be 0.013ctss−1 (15–60 keV), which is
Thebluespectralregionisdominatedbystrongemissionlines
≈5.0% of the gross count rate and similar to the level of back-
withblueshiftedcomponents,asreportedinmanypreviousworks
groundreproducibilityforobservationsafter2012.28Weconserva-
(e.g.Feldmanetal.1982).Theredspectralregioncontainstheiso-
tivelyconsiderthesourceasanon-detectionbutnotethatfittinga
latedCaIIabsorptiontriplet,whichcanbeusedtoestimatethecen-
PLtothedetectednetcountsbetween15and60keVreturnsanob-
tral black hole mass. This estimate is presented in the Appendix.
servedfluxF15−60≈5×10−12ergs−1cm−2,whichissimilarto Theinstrumentalresolutionintheredspectralregionwasmeasured
theobservedNuSTARfluxinthesamebandwiththebestfitmodels to be 7.4A˚ (FWHM) using arc lamp spectra, corresponding to a
thatwewilldiscussintheResultssection. velocity resolution of σinstr.=107kms−1 close to the observed
NGC7674isalsotoofainttobedetectableintheHXD/GSO
wavelengthoftheCatripletfeature.
arraysensitivetomuchhigherenergies,andthosedataarenotcon-
sideredhere.
2.5 X-raySpectralAnalysisMethodology
2.3 Swift In this section, we start by checking for consistency of the data
betweenthevariousmissions,andthendescribethedetailsofthe
ThesourcehasbeenobservedbySwift(Gehrelsetal.2004)on17
spectralanalysismodels.
occasions2011onwards,withexposuretimesrangingover≈470–
5100susingtheX-RayTelescope(XRT;Burrowsetal.2005)sen-
sitive to photons between ∼0.3–10keV. The individual observa- 2.5.1 Basiccharacterisation
tions are listed in the Appendix. We extracted source and back-
ground spectra, together with response files, using the standard Fig.1showstheNuSTAR,Suzaku,andSwiftdatasetsoverplotted
XRT Data Analysis Software tools with HEASOFT. The version incountrateunits,stretchingovertwodecadesinobservedenergy
of XRTPIPELINE used was 0.13.0. Source counts were extracted from 0.5–78keV. The spectral shape approximately matches be-
withina20(cid:2)(cid:2) radiusaperture,andbackgroundwasextractedfrom tweenthemissionsandinstrumentsovercommonenergyranges.
an off-source sky region. Upper limits (for detection significance In particular, there appears to be a broad hump dominating the
lessthan10−3)wereestimatedusingthesostacommandinthe NuSTARbandabove10keVandasharpemissionfeaturearound
XIMAGEpackage. 6keV.Thesearestronglyreminiscentofreflectionresultingfrom
Wefirstanalysedthespectraindividually,butthesourceliesat ComptonscatteringandneutralFefluorescence,acommonchar-
thelimitofdetectabilityintheseobservations,yieldingonlyweak acteristicofobscuredAGNX-rayspectra.Thehumpextendsdown
detectionsforobservationslongerthan2ks,andnon-detectionsin to∼3keVinalldatasets,belowwhichadifferentcomponentap-
other cases. Nevertheless, the individual observations (detections pearstodominateinboththeSuzaku/XISandSwift/XRTdatawith
andlimits)allowafirstcheckforanystrongvariationswithtime. thespectrumrisingandpeakingaround1keV(thisisrelatedtothe
These fits are also described in the Appendix, and no significant peakintheeffectivearea,inthespectralunitsofFig.1.).
variationsarefound. Fitting a PL to the continuum over the common energy
We then extracted a coadded XRT spectrum by combin- range of 3–10keV (after ignoring the range of 5.5–7keV around
ing the event files of the individual observations. This yields a the neutral FeKα line) simultaneously to all missions returns a
dataset with a total exposure time of 48.8ks. The source is well photon index Γ=0.73±0.11 with an acceptable fit statistic of
detected in this combined exposure, with a net count rate of χ2/dof=99.9/92.Thisismuchharderthanthetypicalintrinsicpho-
7.9(±0.4)×10−3cts−1overtheenergyrangeof0.5–10keV. tonindices((cid:4)Γ(cid:5) ∼ 1.9)seeninAGNX-rayspectra(cf. Nandra
Thesourceisclassifiedasanon-detectionbytheBurstAlert etal.1997;Mateosetal.2005;Piconcellietal.2005)andissugges-
Telescope(BAT;Barthelmyetal.2005)sensitiveover14–195keV tiveofheavyobscuration.Fittingthesamemodeltothedatafrom
using standard analysis adopted for the 70-month all-sky survey each mission separately, the observed 2–10 keV fluxes span the
(Baumgartner et al. 2013), and we do not consider the BAT data range of F2−10≈(7–14)×10−13ergs−1cm−2 between the mis-
further in this work. We do note, however, that a custom analy- sions(withtheharderSwiftphotonindexgivingthehighestflux),
sis by Koss et al. (2013) finds a 4.2σ detection at the position andarefullyconsistentwitheachotherat90%confidence.
We note a mild discrepancy in the Swift XRT data with re-
specttotheothermissions.Whenexaminingtheindividualphoton
28 www.astro.isas.jaxa.jp/suzaku/analysis/hxd/pinnxb/ indicesintheabovefittoeachmissionseparately,wefindΓXRT=–
tuned/140530bgdd.pdf 0.68±1.26.ThisisharderthanthemedianΓvaluefromtheother
(cid:2)c 0000RAS,MNRAS000,000–000
4 P.Gandhietal.
missionsat90%confidence.Butat95%confidence,wefindthat in NGC7674 will require high spatial and spectral resolution
allmissionsdoagree.Thecauseofthisslightmismatchisnotclear observations with Chandra and Athena, respectively. Our main
butisunlikelytoberelatedtodifferingaperturesizesusedforex- focusistheoriginofthehigherenergyX-rays,soweusetheAPEC
tracting spectra for the different missions, because we expect the andPLsoft componentssimplytoensurethatthespectralfitsover
unresolvedAGNalonetobethedominantcontributorattheseen- the soft regime are statistically acceptable. In addition, we may
ergies.Instead,atsoftenergies,onemayexpectspatiallyextended expect spatially extended soft emission to contribute in differing
emissionandlargerdifferences,whichwewilldiscusslater.Inany amounts to the Suzaku and Swift spectra because of the differing
case, since this discrepancy is relatively mild, and since all other spectral extraction apertures tuned to the sizes of the respective
observations are fully consistent with each other, we consider a telescope point spread functions. We account for this simply by
jointanalysisoftheNuSTAR,SuzakuXIS,andcoaddedSwiftXRT allowing the APEC components to vary independently between
datatobejustified,andthisistheapproachwefollowintherestof thesetwomissions.Theonlycross-checkrequiredinthisregardis
thispaper. thatthesoftX-rayfluxmeasuredbySwiftXRT(withthesmaller
aperture) should not exceed that observed in Suzaku XIS. This
cross-checkwasappliedpost-fittingandfoundtoholdforthebest
2.5.2 Reflectionmodels fitspresentedinthefollowingsections.
Withregardtoothercomponents,somemodelspreferredsev-
For the detailed spectral fits, we will be fitting X-ray reflection
erallayersofabsorptioninadditiontoGalactic,asfollows:
modelsovermuchofthehardX-rayenergyrange.Therearesev-
eralcanonical modelsavailable forfittingheavilyobscuredAGN (i) Nuclearobscuration:Allmodelswithatransmissioncompo-
spectrainXSPEC.Traditionally,PEXRAV(Magdziarz&Zdziarski nentrequiredathicknuclearabsorber,whichweassociatewiththe
1995) and its successor PEXMON including fluorescence (Nandra classical compact torus having a l.o.s. column density ‘NH(nuc)’
etal.2007)havebeenusedtocharacterisereflectionfeatures.These wellabove1024cm−2.
assume a slab obscurer/reflector with an infinite optical depth, as (ii) Hostgalaxyabsorption:Most models preferred the inclu-
may be expected in a standard geometrically-thin accretion disc. sionofaweakabsorberscreeningthesoftthermalandpowerlaw
TheincidentsourceinX-raysisassociatedwithpowerlawradia- components (denoted by ‘NH(host)’ with values of a few times
tion(PLAGN)fromahotelectronaccretiondisccorona.Morere- 1021cm−2).Thiscorrespondstoweakhostgalaxyreddeningand
cently,therearemodelswhichsimulateX-rayprocessinginfinite isconsistentwithopticalreddeningoftheNarrowLineRegion,as
opticaldepthtoroidalmediawhicharemorephysicallyappropriate wediscusslater.
forobscuredAGN.Murphy&Yaqoob(2009)providetabulatedre- (iii) Scatteringscreen:Onealternativescenariothatwewillin-
sultsofMonteCarlosimulationsofanAGNilluminatingadough- vestigate requires an additional absorbed power law continuum
nutshapedtoruswithafixedopeningangleandcoveringfactorof component, which can potentially be attributed to scattering of
0.5 (the MYTORUS model). Brightman & Nandra (2011) assume, theintrinsicPLAGN intothel.o.s.Thescatteredfractionisfscatt,
instead,atorusdefinedasaconicalsectionofaspherewithvari- i.e.PLscatt=fscatt×PLAGN.Absorptionisrequiredforthiscom-
ableopeningangleandhencevariablecoveringfactor(theTORUS ponent, but as we will discuss later, the data cannot distinguish
model). Both models assume Solar abundances and treat absorp- between the two possibilities of (a) a screen that absorbs only
tion,reflectionandFeKαfluorescenceself-consistently.MYTORUS the scattered power law, and (b) a screen that affects all the
additionallyallowsthefreedomtovarytheparametersofthescat- compact nuclear components (direct, reflected, as well as scat-
terer,thel.o.sobscurer,andthefluorescer,decoupledfromonean- tered), with both possibilities allowed for in our analysis. We
other.Bothtorusmodelsassumethatthereisnoe-foldingcut-off follow the more generic case (b) above, and term this compo-
energytoPLAGN.Forconsistency,wemakethesameassumption nent‘NH(scatt).’Itshowsintermediatecolumndensityvalueswith
inPEXMON.Allmodelsassumeauniformgasdensityspatialdis- NH(scatt)∼1023cm−2.
tribution.
Finally, we found that inclusion of a Hydrogen-like FeXXVI line
Sincethegeometryoftheobscuring/reflectingmediumisun-
at 6.97keV, probably related to the ionised scattered continuum,
known,wewilluseallthreegeometriesaboveandinvestigatethe
providedasignificantimprovementinallmodels.
rangeinintrinsicpropertiesthatcansatisfytheobservations.
2.5.3 Additionalmodelcomponents Our model values at any energy E can most generically be de-
scribedasfollows.
Fixed Galactic absorption (PHABSGal) with
NH=4.27×1020cm−2 (Dickey & Lockman 1990) was in-
cluded in all models. We also included constants to account for F(E) = Ce−τ(Gal)e−τ(host)[APEC(s)+PLsoft
cross-calibration uncertainties between the various detectors and +e−τ(scatt)[Torus(Γ, NH(nuc)) + PLscatt(Γ) + Line]],
missions. We found that one or two APEC (Smith et al. 2001)
components and a soft power law (PLsoft) were required to where‘Torus’representsoneofourthreeprimarymodelsincluding
represent the energy range below ∼2keV in some models. We either(i)PEXMON(ModelP),or(ii)Brightman&NandraTORUS
emphasisethatthesecomponentsaremeanttoserveasparametri- (Model T), or (iii) Murphy & Yaqoob MYTORUS (Model M), in
sationsonly.Previousworkshaveshownthattheoriginofthesoft ordertomodeltheprimarynuclearobscurerandreflectorwithcol-
X-ray photons in obscured AGN is a complex mixture of AGN umndensityNH(nuc).‘C’representscross-calibrationconstants,
photoionisation, starburst emission, and power law contributions τ istheopticaldepthNH×σ(Ez)attherest-frameenergyEz of
from X-ray binaries, among various possible origins (e.g. Sako the absorber, and ‘Line’ refers to the FeXXVI emission line. We
etal.2000;Kinkhabwalaetal.2002;Cappietal.2006;Guainazzi emphasise that not all models require the complexity implied by
& Bianchi 2007). Separating these various possible components theabovegenericdescriptionofcomponents.
(cid:2)c 0000RAS,MNRAS000,000–000
NuSTARobservationsofNGC7674 5
1
0
0.
V−1 0−3
e 1
k
1
−
s
s
Ct 4
0− NuSTAR FPMA
1
NuSTAR FPMB
Suzaku XIS/FI
5 Suzaku XIS/BI
−
0
1 Swift XRT
1 10
Energy (keV)
Figure1.TheNuSTAR,SuzakuandSwiftdataplottedinobservedcountrateunits.
3 RESULTS end,theSuzakuXISdatarequiretwoAPECcomponentswithtem-
peratureskT≈0.1and0.6keV,whereastheSwiftXRTdataprob-
3.1 ModelP:PEXMON
ingsmalleraperturesrequireonlyasinglelowertemperaturecom-
We began with fitting Model P (PEXMON). This slab reflection ponent. The APEC abundances are ∼0.01–0.2. Multiple thermal
model can fit the Compton hump successfully but strongly over- models with widely differing temperatures and abundances are
predictstheFeKαlinestrengthwhenusingfixedabundancesatthe quite common in luminous infrared galaxies (e.g. Ranalli et al.
Solarvalue.Lettingtheelementalabundance(A=AFe)varyfreely 2008) and have also been seen in other CTAGN (e.g. Konami
yieldsanacceptablesolution(χ2/dof=324.7/305)withΓ=2.1+0.2, et al. 2012). There is, however, strong degeneracy between the
−0.1
A=0.5±0.1andaslabinclinationofatleast77deg.Theinclina- abundancesandtemperaturesindatawithlowspatialandspectral
tionaffectsthe‘peakiness’oftheComptonhump,withlowerincli- resolution, and tieing all the Suzaku and Swift model abundances
nationsbeingtoo‘peaky’ascomparedtothedata.29ThismodelP toeachotheralsoyieldsanacceptablesolution(χ2/dof=332/307)
solutionisplottedinFig.2andfitparametersarelistedinTable2. withaglobalabundanceA=0.27+−00..5164,aSwiftAPEC3component
ThebottompanelinthesamefiguredemonstratesthestrongFeKα temperatureofkT3=0.19+−00..0038keV,andnosignificantchangesto
residualswithAfixedtoSolarandΓ=2.1(fixedtothesamevalue thecorrespondingtemperaturesforthe APEC1 and APEC2 Suzaku
asthecanonicalmodelP);lettingΓvaryfreelydidnotprovidea components.However,weemphasisethatwearenotattemptingto
betterfitthanthatstatedinTable2. constraintheoriginofthesoftX-rayemissionindetailhere.
A transmitted component of the direct AGN power law The best fit model needs only one additional layer of ab-
(PLAGN)isincluded inthis model.Comptonscatteringandpho- sorbing column density NH(host)=4.0+−21..16×1021cm−2. This is
toelectricabsorptionbythetorusaresimulatedwithstandardmul- a rather thin screen and corresponds to an optical extinction
tiplicative models CABS and ZPHABS, respectively, with the gas EB−V=0.7+−00..43mag for a standard Galactic gas-to-dust ratio
columndensityNH(nuc)tiedbetweenthetwomodels.Thebest-fit (Bohlinetal.1978).Thisisconsistentwiththereddeningexpected
NH(nuc)=3.4−+00..86×1024cm−2.Thereflectioncomponentdomi- fromtheBalmerdecrementof4.80(Bassanietal.1999).Removing
natestheabsorbedtransmittedcomponentovertheentirespectral this layer significantly worsens the fit to χ2/dof=342.1/306. But
rangeprobed,i.e.thesourceisfullyreflection-dominated.Theex- mostofthechangeisconcentratedatthesoftestenergies(resulting
cessofthereflectioncomponentisafactorof∼3around30keV. inanincreasedAPECtemperature)whichwearenotmodellingin
We note that excluding the transmission component results in a detail;theprimaryAGNreflectioncomponentisunaffected.
moderateincreaseinthefitstatistictoχ2/dof=334.5/306,which
is only marginally significant at the 3σ level. In other words, a
3.2 ModelT:TORUS
transmissioncomponentisnotstronglyrequired.
Wefindthatthecross-calibrationconstantsbetweenthevar- PEXMON assumes a slab geometry with infinite density, which is
ious missions are consistent with 1 within the uncertainties. The unlikelytoberepresentativeofthetoroidalobscurerenvisagedby
strengthoftheFeXXVIlineisEW(FeXXVI)≈200eV.Atthesoft AGNunificationschemes.SowenextturnedtotheBrightman&
Nandra TORUS model.Thisalsoallowsustoexplorealternatives
to our low AFe solution, because abundances are fixed at Solar
29 Foranillustrationofthiseffect,seeFig.5ofMagdziarz&Zdziarski values in this model. In this case, an acceptable solution may be
(1995). found if we effectively dilute the continuum emission from the
(cid:2)c 0000RAS,MNRAS000,000–000
6 P.Gandhietal.
Fe K(cid:31)
Fe XXVI
−1
V
e 10−3 PEXMON
k APEC
−1
s
−2
m
c
V 2
e 10−4 Transmitted
k
PL
AGN
or
err 2
el)/
d 0
o
m
− −2
a
at
d
(
1 10
Observed frame Energy (keV)
error 2
el)/ 0
d
o
m 2
− −
ata 4 A=1 (fixed)
d −
(
1 10
Figure2.PEXMONmodelfittotheSuzakuXISFI(black),XISBI(red),NuSTARFPMA(green),FPMB(blue)andSwiftXRT(cyan)data.Thecentralpanel
showsthefitresiduals.TheFeabundanceforthisfitiscloseto0.5(Table2).ThebottompanelshowstheresidualstoafitwithfixedabundanceA=1.Notice
thestrongover-predictionattheneutralFeKαenergy.
torusattheFeKαlineenergywithastrongerscatteredcomponent affectsPL(scatt)alonealsoyieldsafullyacceptablesolutionwith
(PLscatt).30 Such a solution is plotted in Fig.3, where NH(nuc) χ2/dof=323.3/300. This consistency results from the fact that
is above 1025cm−2, well within the Compton-thick regime. The PLscatt completely dominates over the TORUS reflection contin-
scattered fraction is fscatt≈4%, which raises the continuum at uumatenergiesofaroundafewkeV–energiesbelowwhichgas
the FeKα line energy sufficiently to produce a fully acceptable withanintermediatecolumndensityof∼1023cm−2isaneffective
fit(χ2/dof=320.2/300).Forthisfit,wefrozetheinclinationangle absorber(cf.Fig.3).
pθilnocraotifotnheoftotrhuesftuollbreacnlgoeseoftopoedssgieb-loenc,oavseirsinrgecfoamctmoresn(dBerdigfhotrmeaxn- This above NH(scatt) column is ≈20times higher than the
etal.2015).Thereareuncertaintiesinthe TORUS modelatedge- hostgalaxy absorbinglayer (NH(host))discussedintheprevious
sub-section,andwouldimplycorrespondinglyhigheropticalred-
oninclinations,asdiscussedbyLiu&Li(2015).Wecheckedthat
varyingθincdidnotaffectourfinalinferences.Thebestfitopening dening.Sohowviableisthis?NGC7674isaknownluminousin-
angleisθtor=62+−1151deg.Smalleropeninganglesproduceaworse fraredgalaxywithongoingstarformationandphysicalinteractions
withitsneighbouringgalaxies–allofwhichcouldresultinobscur-
fitbecausetheyresultinComptonhumpspeakierthanrequiredby
thedata.31Largevaluesofθtorarealloweduptoθtor=77deg,be- ingmatterbeingstrewnaroundthehostgalaxyonmultiplescales.
Otherexamplesofnearbygalaxieswithcomplexmultiplelayersof
yondwhichthetoruseffectivelybecomestoogeometricallythinto
absorptionidentifiedbyNuSTARincludeNGC7582(Riversetal.
produceasufficientlystrongComptonhump.
2015)andIC751(Riccietal.2016).Therefore,thepresenceofan
In this model, however, an additional intermediate column
obscurer NH(scatt)∼1023cm−2 is required, so as not to over- additionalabsorberintheinnermostpartsofthehostgalaxywhich
obscurestheemergentfluxfromthenuclearregionsisnotimplau-
produce the soft X-ray flux. We note that in our model configu-
ration, NH(scatt) affects PLscatt as well as the compact nuclear sible.
TORUS component.ButmodifyingthemodelsuchthatNH(scatt) Detailedspatiallyresolvedstudiesofthenuclearregionsofar
neitherrequire,norruleout,suchascreen.Forexample,thestudy
30 See the presentation by T. Yaboob at ofFischeretal.(2013)doesnotshowanobvious[OIII]fluxdecre-
mentatthenucleus,butthespatialresolutionoftheirspectralsam-
http://cxc.cfa.harvard.edu/ChandraDecade/ for more
plingis≈30pc,muchlargerthanthetypicalsizesofcompacttori,
discussionsofsuchawarmscattererscenario.
31 seeFig.1ofBrightmanetal.(2015)foranillustrationoftheeffectof andmaypotentiallyallowforadditionalabsorbersbelowtheres-
changingθtor. olutionlimit.Alternatively,theoutflowinggasassociatedwiththe
(cid:2)c 0000RAS,MNRAS000,000–000
NuSTARobservationsofNGC7674 7
powerfulnuclearoutflowknowninNGC7674mayalsoserveasa We first confirmed that simply decoupling the normalisation
screeningmediumforX-rays. constantofthefluorescencelineALfromthenormalisationofthe
Insummary,theintroductionofNH(scatt)isclearlysomewhat ComptonscatteringcomponentAS allowedaverygoodfit(with
adhocandismotivatedbytheadoptedscatteringscenarioinafixed afittedsub-unityvalueofAL=0.26±0.06)withnoextraPLscatt
Solarmetallicitytorusmodel.Nevertheless,wecannotruleitout component required, equivalent to model P. The problem is that
baseduponthepresentevidence,andwealsonotethatthepresence suchadecouplingcannotbeself-consistentlyinterpretedashaving
ofaPLscattcomponent,itself,isnotsurprising,withthemeasured sub-Solar abundances, because of the assumption of Solar abun-
fscatt valueofafewpercentbeingfullyconsistentwiththatseen dances for the Compton scattering component that produces the
inothernearbyAGN(e.g. Cappietal.2006).Wehaveintroduced overallcontinuumshapeoftheComptonhump.
this only as one possible scenario, and as we will discuss in the Wealsoattemptedmorecomplexscenarios,includingthepos-
followingsection,morecomplexmodelscouldalleviatetheneed sibilityofhavingmultiplescatterers.Suchascenarioincludestwo
forthisscreenaltogether. components inclined at orthogonal angles of 0 [face-on] and at
The soft energies can be fitted in a very similar fashion to 90deg [edge-on] with AS=AL for each component, and is dis-
Model P, with one difference being the presence of a significant, cussed at length by Yaqoob (2012). But these models appeared
butfaint,PLsoft component.PLsoft dominatesonlyinaverynar- torequireextremedecouplingbetweenthetwoscatterers.Forin-
rowenergyrangearound2keV,asaresultofwhichwefoundthat stance, allowing freely varying cross-normalisation constants be-
itsphotonindexneededtobefixed,otherwisethefitattempteda tween these scatterers, the face-on reflection component (AS0 in
very hard slope with contribution to the highest energy NuSTAR Yaboob’sterminology)prefersanormalisation∼>5timesstronger
range.ThiscomponentisnotrequiredinmodelPbecausethere- than the edge-on component (AS90) responsible forthe l.o.s. ob-
flectedcomponentismuchmoreprominentinthatcase,leavingno scuration. This implies a strong departure from the default time-
deficitaround2keV(cf.Fig.2).Sincethepurposeofthiscompo- steadyilluminatedMYTORUSgeometrywithcoveringfactorof0.5.
nentistoaccountforthesoftenergies,wefixedΓsoft=2.0,simi- Suchascenariowithstrongvariabilityoncharacteristictimescales
lartothespectralslopeoftheemissionfromX-raybinaries(e.g. relevanttotheinnertorusisdiscussedandarguedagainstinSec-
Ranallietal.2003).Wealsocheckedthatallowingsmallvariations tion4.2.3.
of ΔΓsoft=±0.3 do not affect the modelling of the main AGN Alternatively, the column densities between the two scatter-
component. ersmayalsobeuntied.Thisisequivalenttosimulatingaclumpy
medium with the line-of-sight obscuration differing from global.
Such a solution is shown in Fig.4. The spectrum can be crudely
3.3 ModelM:MYTORUS fit(χ2/dof=371/301)withalowl.o.scolumnassociatedwiththe
edge on obscurer NH(nuc)=1.3(±0.3)×1023cm−2, This com-
Wefirstfittedastandard‘coupled’MYTORUSmodelherewithall
ponent is denoted by ‘MYT(Z90)’ in the figure, as in Yaqoob
parameters between the l.o.s. and toroidal scattering components
(2012). However, this also requires a very hard photon index for
tTi,edantdoethaechfiottshtaetri.sTtihcisofsoχlu2/tdioonf=is3v2e5ry/3s0i0miislacroimnpesastiebnlceewtoithmboodtehl PLAGN, with Γ=1.40+−0u.08 pegged at the lower limit allowed by
themodel.Theout-of-sightglobalcolumnfromtheface-onscat-
pthreevdioeruisvemdoedqeulast.oQriaulancotiltuamtivneldye,nmsiotydethlsroTugahndtheMtodruifsfe(rNiHn(ethqa))t tererisNH(S0)=2.4+−10..78×1024cm−2(withcorrespondingComp-
is pegged at the allowed upper model threshold of 1025cm−2 in tonscatteredandfluorescencecomponentsdenotedby‘MYT(S0)’
and‘MYT(L0)’;Ibid.).Thereismorethananorderofmagnitude
model M. Similarly, θinc is just above (and very close to) the al-
lowedlowermodelthresholdofθinc=60◦.Suchparameterpegging differencebetweenthetwocolumndensities.Inordertopreventthe
strong normalisation departure mentioned in the preceding para-
hasbeenseeninotherobjectsalso,andreflectstheconstraintofthe
graph,welimitedAS0toamaximumvalueof1.20,andthefitdoes,
av2sa0sl1uu4em;oeBdfaNldookHuo(gvnhiunc´cu)ettigsaeldo.im2re0ec1ttr4lyy;iLcnoatnuhspibsleumdryotodeetθlaianl.sc2a(0er1.eg5s.)u,.ltSGomafanwldlhehriiciehntctahlile-. Tinhdeeesdca,twtearnedt tforaecxticoenedisthhiigshl,imwiitt,hwfsicthattA=S00.1p2e+−gg00.e.00d46abtu1t.w20e+−stu0r.e0s7s.
thatthereisnoadditionaladhocscreenassociatedwiththeafore-
nationanglesarenotobscuredbythetorusinthismodel,whereas
higher θinc values produce a Compton hump which is too peaky mentionedNH(scatt)componentinthisdecoupledmodel(orinany
of the others discussed in this section). The overall fit statistic is
relative to the data. Additional model components are also very much worse (Δχ2=+46 for a single extra dof) than the default
similartomodelT.Thefittedparametersofthismodelarelistedin
modelMsolutionshowninTable2.
Table2andthesolutionisshowninAppendixfigureA1.
Scenarioswithstrongclumpinesshavealsobeendiscussedfor
Mrk3(Yaqoobetal.2015;Guainazzietal.2016).Butsuchasce-
narioisnotobviouslyapplicabletoNGC7674.Mrk3isknownto
3.3.1 DecoupledMYTORUSmodel
becontinuouslyvariableinfluxandline-of-sightcolumndensity,
We next tried several versions of the more complex ‘decoupled’ whereasNGC7674hasbeenstablesincetheBeppoSAXobserva-
mode in MYTORUS.Inthis mode, the direct l.o.s.absorption, the tions in 1996. Furthermore, it is clear from the discussion of the
toroidalComptonscatteringemission,andthefluorescenceemis- peggedphotonindexandnormalisationvaluesabovethatthefitis
sioncomponentsarenotnecessarilycoupledtoeachother.Byvary- attemptingtoconvergeonaharder,morereflection-dominatedcon-
ing the NH, the inclination angles or the relative normalisations tinuumshape,suggestingthatitdoespreferahigherline-of-sight
associated with these components, one can effectively simulate a columndensity.
variety of scenarios including a clumpy obscurer or varying ele- UnlessthedirectPLAGNcomponentcanberobustlydetected,
mentalabundances(forexample).Thismayalsopotentiallyallow suchsolutionsarehighlycomplexanddegenerate,anddonotyield
ustoremovetheneedfortheadditional‘NH(scatt)’layerthatwas anyobviousphysicallyusefulinsightsonthenuclearmediumbe-
introducedinSection3.2forabsorbingthescatteredpowerlawthat yondourthreecanonicalmodelsdiscussedinTable2.Thisispar-
dilutesEW(FeKα). ticularlytrueforreflection-dominatedAGNwithnodirectdetec-
(cid:2)c 0000RAS,MNRAS000,000–000
8 P.Gandhietal.
tion of PLAGN (e.g., see detailed discussion in Yaqoob 2012 on Gandhi et al. 2009) predicts an intrinsic L2−10 range of (6–
thispoint).Therefore,althoughwecannotruleoutmorecomplex 9)×1043ergs−1 at 68% confidence, which is marginally higher
modelsatthisstage,wedonotinvestigatethedecoupledmodesin than the mean best fit luminosities from our spectral analysis,
greaterdetailherein.Futurehighsignal-to-noisebroadbandspec- but entirely consistent with the full confidence regions for both
tra,androbustdetectionsofreflectionfeaturessuchasthefluores- models shown in Fig.5. Assuming a 6μm luminosity lower by
cenceComptonshoulderwithX-raycalorimeters,couldyieldmore a factor of 2 as compared to L12 (cf. Goulding et al. 2012) to-
insightonsuchmodels. gether with the L6/L2−10 relation by Stern (2015) relevant for
high luminosity AGN, the predicted L2−10 decreases by a fur-
ther 0.1dex. The angular resolution of the mid-infrared observa-
tionsused(seeing-limitedat≈0.4arcsec)correspondstoaphysi-
4 DISCUSSION calscaleof≈0.24kpcfortheunresolvednuclearemissionatthe
4.1 Intrinsicluminosity distance of NGC7674, and represents the best direct measure of
theintrinsicAGNpower,withemissionfromsurroundingstarfor-
OurthreemodelsreturnintrinsicL2−10valuesrangingover ≈(3– mationbeinglargelyresolvedout.Afurthercheckonanycontam-
5)×1043ergs−1forthebestfitparametersinTable2.Thenarrow
inationbynon-AGNcomponentsmaybeobtainedfromthemid-
rangeoftheseluminositiesisnoteworthy,despitethedifferingge-
infrared colour of NGC7674 as tabulated in the WISE/AllWISE
ometriesinherenttothesemodels.Placingthisrangeinthecontext catalogue (Wright et al. 2010; Cutri et al. 2013). The W1–W2
ofotherwell-knownCTAGN,theluminosityofNGC7674issim- colouris1.16±0.03mag,whichplacesthesourceabovethecolour
ilar to NGC1068, and is factor of ≈2 lower than Mrk34 (Bauer thresholdofW1–W2>0.8identifiedbySternetal.(2012)where
etal.2014;Gandhietal.2014).Theformerobjectistheprototypi-
themid-infraredemissionislikelytobeAGN-dominated.
calreflection-dominatedAGN,whilethelatteristhemostluminous
Comparing next to the [OIII] emission line, Bassani et al.
knownbonafideCTAGNwithin∼250Mpc.Fortheobserved(i.e. (1999) present the [OIII]λ5007 luminosity of the source as
aLbo2s−bos1r0be=d1).s5p×ec1tr0a4,2wereghsa−v1e–Fa2o−bfas1c0t=or7o.7f×≈1200−–3130etirmgse−s1locwme−r2th,aonr cLo[OrreIIcIt]i=ng3.5fo(±r 0d.u2s)t×r1e0d4d2enerigngs−1b.asTedhisupvoanlueaisBadlemrievreddeacfrtee-r
theinferredintrinsicpower. ment of 4.80. Using the L2−10/L[OIII] relationship presented
Inordertoestimaterealisticuncertaintiesontheintrinsiclu-
in Lamastra et al. (2009) with a scatter of 0.6dex, we expect
misaintioosniti(eNs,)wfoersttehpepiendtroinvseircaP2L-dAiGmNen–siothnealtwgroidpoafraΓmaentedrsnowrmhiaclh- Lw2it−h1o0u=r5sp+−e74c×tra1ll0y43meordgesl−le1daitnt6r8in%sicclounmfiidneonscitey,roavnegrela.pping well
determinetheabsorption-correctedflux(andhencedirectluminos-
Insummary,thebestfitX-rayluminositymeasuredfromour
ity).Carryingoutfitsoverthegridyieldsaχ2valueforeachcom-
spectral analysis agrees well with the multiwavelength compar-
binationofΓandN,andthuseffectivelyforeachvalueofL2−10. isonsabove,especiallywhenconsideringthefullrangeofrealistic
Different combinations of the two starting parameters can return
uncertaintiesonthespectralmodelling(Fig.5).Thisisencouraging
identicalL2−10values,sothe1-dimensionalspaceofχ2asafunc- giventhatthissourceappearstobeheavilyCompton-thickandthat
tion of L2−10 is not unique. But the overall uncertainties can be thetruetorusgeometryisunknown.Broadbandspectralmodelling
gaugedfromtheenvelopeofχ2contoursforallcombinations.This
of high signal-to-noise X-ray data is what enables us to get such
envelopeisplottedinFig.5forbothmodelsMandT.Thefigure
goodagreement.
showsthatrealisticL2−10 uncertaintiesspantherangeof≈(1.3–
13)×1043ergs−1,oraboutanorderofmagnitude.
Our estimate of the mass of the supermassive black hole 4.1.2 ThesoftX-raycomponent
(SMBH)inNGC7674isMBH<107.43M(cid:4) (seeAppendix).Us-
ainbgooloumrbeetrsitc-fictoLrr2e−ct1i0onralnikgeelyofra(3n–g5e)o×f1L0B4o3l/eLrg2−s−101≈tog1e0t–h2e0rw(Vitah- TLhA0.eP5−aEbC2so≈rp4t.i5on×-c1o0r4r2ecetregds−lu1m.iTnohsisityisinfotrheboXtIhSA0P.5E–C2ckoeVmpboannedntiss
combined, but is dominated by a factor of 4 by the lower tem-
sudevan&Fabian2007),weestimateanEddingtonratiorangeof
perature component. Using the relation for star-forming galaxies
LBol/LEdd>0.09–0.29.IncludingthefullrangeofX-rayluminos-
byMineoetal.(2012)betweenthethermalcomponentluminosity
ityuncertaintiesexpandsthisrangetoLBol/LEdd>0.04–0.74,im-
and the star formation rate (SFR), we estimate an X-ray derived
plyingthepresenceofanefficientlyaccretingAGN. SFRX−ray≈9×103M(cid:4)yr−1.
This may be compared to the infrared derived SFR, based
4.1.1 Multiwavelengthcomparisons uponthefar-infraredcontinuumluminosityLIRanditsrelationto
SFRIR (Kennicutt1998).ForNGC7674,LIR≈1011.56L(cid:4) (Koss
Multiwavelength scaling relations are very useful for compari- etal.2013),whichyieldsSFRIR≈60M(cid:4)yr−1.Thisismorethan
son of intrinsic luminosity estimates, especially in the heavily twoordersofmagnitudelowerthantheestimatedSFRX−ray,and
Compton-thickregimewherethedirectX-rayemissionisnotde- impliesthatstarburst-poweredAPECcomponentsaloneareanun-
tected.Twocommonlyusedmultiwavelengthindicatorsofintrinsic physical representation of the soft X-ray emission in NGC7674.
AGNpowerarethemid-infraredcontinuumandtheopticalforbid- Photoionisationcouldinsteadpowersomeofthis,aswehaveal-
denemissionlineluminosities,inparticularthe[OIII]λ5007A˚ line readyalludedtoonseveraloccasions.Wealsonotethatuncertain-
doublet(e.g. Gandhietal.2009;Lamastraetal.2009, andrefer- tiesrelatedtoabsorptioncorrectionofsoftX-rayscannotaccount
encestherein).WecompareNGC7674totheserelationshere. fortheextremelyhighSFRX−ray.Ignoringabsorptioncorrections
NGC7674 has been observed at high angular resolution in andusingtheobserved(i.e.absorbed)LA0.P5−EC2 directlystillresults
the mid-infrared using 8m class telescopes. It is found to have a inSFRX−ray≈950M(cid:4)yr−1,farhigherthanSFRIR.
12μminfraredluminosityofL12=1.8(±0.3)×1044ergs−1 (As- We note, however, that the estimated value of SFRIR is it-
musetal.2014).UsingthisvalueofL12,therelationbetweenin- selflarge.Forexample,itisabout10timesabovetheSFRofthe
fraredandX-rayluminositiesforlocalAGN(Asmusetal.2015; prototypicalstarburstgalaxyM82(e.g. Telesco&Harper1980).
(cid:2)c 0000RAS,MNRAS000,000–000
NuSTARobservationsofNGC7674 9
Table2.ResultsofX-rayspectralfittingtoNGC7674
Component Parameter ModelP ModelT ModelM Units
PrimaryAbsorber/Reflector NH(nuc) 3.4+−00..86 36+−u10 5.5+−u2.3 1024cm−2
NH(eq) (cid:2)(cid:2) (cid:2)(cid:2) 10.0+−u4.1 1024cm−2
θinc 85+−u8 87.1f 65+−94 deg
θtor – 62+−1151 – deg
R –1f – –
AFe 0.51+−00..1101 –‡ –‡
EW(FeKα) ................0.38+−00..1009................† keV
EW(FeXXVI) ................0.20+−00..1019................† keV
PLAGN Γ 2.07+−00..1151 1.80+−00..1151 1.93+−00..2182
Additionalcomponents
Hostabsorption NH(host) 4.0+−21..16 3.1+−11..71 3.4+−00..22 1021cm−2
APECa1 kTapec1 0.11+−00..0032 0.12+−00..0033 0.11+−00..0044 keV
Aapec1 0.1+−u0.08 0.1+−00..609 0.1+−00..807
APECa2 kTapec2 0.56+−00..0077 0.59+−00..0077 0.59+−00..0098 keV
Aapec2 0.3+−00..81 0.2+−00..51 0.3+−00..52
APECa3 kTapec3 0.17+−00..0054 0.15+−00..0064 0.14+−00..0064 keV
Aapec3 0.01+−0u.07 0.01+−0u.11 0.01+−0u.16
PLsoft Γsoft – 2.0f 2.0f
Norm – 2.5+−11..25 2.5+−11..45 10−5phkeV−1cm−2s−1
Scattering fscatt – 3.5±1.2 6.1+−33..61 10−2
NH(scatt) – 9.4±2.5 10.6+−32..33 1022cm−2
CXXIISSFBIIcross-calib CONST 1.00±0.07 1.01±0.07 1.00+−00..0077
CXFPISMFAI cross-calib CONST 1.06+−00..0087 1.11±0.08 1.12+−00..0098
CXFPISMFBIcross-calib CONST 1.04+−00..0097 1.10+−00..0058 1.11+−00..0099
CXXIRSTFIcross-calib CONST 0.98+−00..1133 1.01±0.14 1.00+−00..1154
χ2/dof 325/305 320/300 325/300
ModelP:PEXMONcomponentfit(Nandraetal.2007).
ModelM:MYTORUScoupledcomponentfit(Murphy&Yaqoob2009).
ModelT:TORUSmodelcomponentfit(Brightman&Nandra2011).
uunconstrainedtowithinthemodellimits.NHupperlimitsare1025cm−2and1026cm−2forMYTORUSandTORUS,respectively.APECisdefinedbetween
abundancesof0to5,relativetoSolar.
ffixed.
†EquivalentwidthmeasuredusingasimpleGaussianatopalocallyfittedpowerlawcontinuum.
aAPEC1andAPEC2representthermalcomponentsinSuzakuXIS,andAPEC3isforSwiftXRT.
‡ThesemodelsaredefinedforabundancesfixedtoSolaronly.
Such high star formation is likely to power extended ionised gas withfluxF14−195=9.9+−52..14×10−12ergs−1cm−2consistentwith
emission,andwewillreturntopossibleimplicationsofthisinSec- NuSTAR.Forinstance,ourmodelT,whenextrapolatedtotheen-
tion4.3.3. ergyrangeof14–195keV,impliesbestfitfluxesrangingover(8.4–
9.3)×10−12 ergs−1cm−2 between the mission cross-calibration
uncertainties.
4.2 Thenatureofthelong-termfluxchanges
Here,weexaminetheviabilityoftheinferredlong-termde-
Fig.6showsthelong-termX-raylightcurveofNGC7674overa clineanditsimplications.
period of about 37years. As first noted by Bianchi et al. (2005),
the source showed a decline by a factor of ∼3 in the 2–10keV
4.2.1 Onthepossibilityofcontaminationbyanothersource
band between the first detection by HEAO in the late 1970s and
the Ginga measurement in 1989, followed by a further order of Couldthepre-BeppoSAXfluxdeclinebeassociatedwithacontam-
magnitude flux decrease in 1996 when BeppoSAX identified the inatingsource,unrelatedtotheAGN?Theobservedluminositiesat
sourceasaCTAGN(Malagutietal.1998).Thereafter,thesource theHEAOandGingaepochsareabove1042ergs−1 –alreadytoo
fluxhasremainedconstantwithnosignificantspectralorfluxvari- hightobeeasilyassociatedwithX-raybinaries(XRBs)withinthe
ation for the past ≈20 years. This now includes the most recent hostgalaxy.AnXRBwithinourGalaxywhichjusthappenstolie
Suzaku and NuSTAR observations. The individual Swift observa- alongthesamel.o.s.asNGC7674cannotberuledout,thoughthe
tionsspanningtheperiodof2011–2014detailedintheAppendix highGalacticlatitude(b=–48◦)makesthisunlikely.
also show fluxes or detection limits broadly consistent with Bep- RegardingcontaminationbyotherAGN,thereisonlyonepos-
poSAX, Suzaku and NuSTAR. Finally, the custom analysis of the siblebrightsourceintherecent70monthallskySwift/BAThard
Swift/BAT maps by Koss et al. (2013) shows a weak detection X-ray survey that could potentially have contaminated both the
(cid:2)c 0000RAS,MNRAS000,000–000
10 P.Gandhi etal.
Fe K(cid:31)
Fe XXVI
eV−1 10−3 APEC TORUS
k
s −1
m −2 PLscatt
c
V 2 10−4
e
k
PL
soft
or
err 2
el)/
d 0
o
m
− −2
a
at
d
(
1 10
Observed frame Energy (keV)
Figure3.Suzaku,SwiftandNuSTARdatafitswiththereflectionmodelT.ColoursareasinFig.2.
MYT(L0)
Fe XXVI
s keV)−1−1 10−3 APEC MYT(Z9M0)Y*PTL(SAG0N)
m −2
c
V (2 10−4
e
k PL
scatt
10−5
or
err 2
el)/
d 0
o
m
− −2
a
at
d
(
1 10
Observed frame Energy (keV)
Figure4.DecoupledMYTORUSmodelfitwiththedirectcomponentabsorber(MYTZ90)orientationbeing90deg,andthereflectionbeingdominatedbythe
face-onscattererMYTS0anditscorrespondingfluorescenceemissionfromMYTL0.
HEAOandGingafieldsofview.ThisisPKS2325+093whichlies 4.2.2 Backgrounduncertainties
at a separation of 0.9deg from NGC7674 and shows a BAT flux
of F14−195≈3×10−11ergs−1cm−2 (Baumgartner et al. 2013).
Measurementswithnon-imagingdetectorssuchastheHEAOA–
However, this source lies outside the HEAO positional error box
1 Large Area Sky Survey instrument (Wood et al. 1984) and the
of NGC7674 (Bianchi et al. 2005). Moreover, PKS2325+093
Ginga Large Area Counter (Turner et al. 1989) are prone to un-
shows a very hard spectrum, at least in the BAT band, with
certainbackgroundestimates,especiallyatlowsourcefluxlevels.
ΓBAT=1.29±0.28. This is much harder than the spectral shape
However, as discussed by Bianchi et al. (2005), the background
inferred for the Ginga observation below ≈10keV by Bianchi
fortheobservationofNGC7674isbasedonascanningobserva-
etal.(2005).Althoughadrasticchangeinspectralcurvaturearound
tionobtainedcloseintimetothetarget(Awakietal.1991).These
10keVcannotberuledout,thecombinedweightofevidenceap-
backgroundscansresultinnoiseestimateswhicharemorereliable
pearstodisfavourcontamination.
thanmodelestimatestypicallyadoptedfornon-imagingdetectors.
Therefore,theGingaflux,atleast,isconsideredtobereliable
andissignificantlyhigherthanthatseenbysubsequentmissions,
by about a factor of 10. The HEAO flux, on the other hand, may
(cid:2)c 0000RAS,MNRAS000,000–000