DRAFTVERSIONFEBRUARY2,2008 PreprinttypesetusingLATEXstyleemulateapjv.7/15/03 HIGHRESOLUTIONX-RAYSPECTROSCOPYOFTHEFEKCOMPLEXINIC4329A BARRYMCKERNAN1,2 ANDTAHIRYAQOOB2,3 DraftversionFebruary2,2008 ABSTRACT We reportthe detection of complex Fe K line emission from a Chandra High EnergyTransmission Grating Spectrometer(HETGS ) observationof the Seyfert1 galaxyIC 4329A. The line emission is double-peaked, one peak centered at ∼6.3 keV, and the other at ∼6.9 keV in the source rest frame. When modeled by Gaussians,thelowerenergypeakisresolvedbytheHEGat>99%confidence,whilstthehigherenergypeak isresolvedatonly<90%confidence.Thebest-fittingwidthsare∼21,000kms- 1and∼4000kms- 1FWHM 4 for the ∼6.3 keV and ∼6.9 keV peaksrespectively. If the peaks correspondto two distinct emission lines, 0 0 thenthepeakenergiesareredshiftedwithrespecttotheexpectedlineenergiesofFeIKαandFeXXVILyαby 2 atleast650kms- 1and950kms- 1 respectively. Alternatively,theFeKlineprofilemaybeduetoasingleline n fromarelativisticaccretiondisk. Inthatcasetheinclinationangleofthediskisrequiredtobe24+- 91 degrees, theouterradiusconstrainedtoseveraltensofgravitationalradii,andtheradiallineemissivityflatterthanr- 0.7. a J Another possibility is that both peaks are due to distinct lines but each one relativistically broadened by a disk. Inthatcase thelowerenergypeakcouldcorrespondto emissionfromFe in a lowionizationstate, and 0 3 the high-energypeak to Fe XXVI Lyα emission. Then, the inclination angle is even less, restricted to a few degrees. However, the radial emissivity law is allowed to be steeper (∼r- 2.5) and the outer radius does not 1 have to be fine-tuned. Yet another scenario is that the lower energy peak originatesin a disk but the higher v energy peak originates in more distant matter. The disk inclination angle is then intermediate between the 9 last two cases but the emissivity is again requiredto be flat. We cannotrule outFe XXV He-like absorption 3 modifyingtheobservedlineprofile.However,thedata,andinferredemission-lineparameters,areinsensitiveto 6 thepresenceofaComptonreflectioncontinuum.IncludingComptonreflectiondoes,however,allowasteeper 1 radialemissivity law forthe relativistic line. Futuremissions such as Astro-E2 will be able to breaka lot of 0 thedegeneracyinthephysicallydistinctmodelsthatcanallaccountfortheChandradata. SinceIC4329Ais 4 oneofthebrightestSeyfert1galaxiesitshouldbeagoodastrophysicallaboratoryforstudyingtheionization 0 structureofaccretiondisksaroundsupermassiveblackholes. / h Subjectheadings:galaxies: active–galaxies: individual(IC4329A)–galaxies: Seyfert–techniques: spec- p troscopic–X-rays:line–emission:accretion–disks:galaxies - o r t 1. INTRODUCTION FeKlinescanoriginatein,orhaveasignificantcontribution s from,theputativeaccretiondisk(Petruccietal. 2002;Leeet a SomeoftheFeKαfluorescentemissionlineinactivegalac- : ticnuclei(AGNs)isbelievedtooriginateinarelativisticac- al. 2002;Yaqoobetal. 2003a). NarrowFeKlinesoriginat- v ingfroma diskcanbe interpretedin termsof truncateddisk cretion disk around a supermassive black hole (see e.g. re- Xi viewsbyFabianetal.2000;Reynolds&Nowak2003).Early emission(e.g. Done,Madejski,&Z˙ycki2000)orintermsof a flattened radialline emissivity where the intensity per unit ar rsetuvdeiaelsedofatwyipdee1vaAriGetNyoufsiFnegKXα-raliyneobwsiedrtvhastiaonndsswhaitphesA(SeC.gA. area falls off less steeply than ∼r- 2 (Yaqoob et al. 2003a). Alternatively, the narrow Fe K lines could be due to local- Nandra et al. 1997; Lubin´ski, P. & Zdziarski 2001; Perola izedemissionfromthedisk(Turneretal. 2002;Yaqoobetal. et al. 2002; Yaqoob et al. 2002). These results have been 2003a).Thereare,ofcourse,otherpossibleinterpretationsof confirmedwiththeadventoftheChandraandXMM-Newton the origin of narrow Fe K lines (e.g. Sulentic et al. 1998; X-rayobservatories,butamorecomplexpictureisemerging. Elvis2000;Hartnoll&Blackman2000). New,highresolutionobservationsarerevealingthatanarrow FeKline(<10,000kms- 1 FWHM)componentiscommon Earlier observationswith ASCA also identified Fe K-shell emissionduetohighlyionized(H-like)Feintwotype1AGN, intype1AGN(Yaqoobetal.2001;Kaspietal.2002;Pounds PG1116+215(Nandraetal. 1996)andTonS180(Turneret etal. 2001; Turneretal. 2002; Fangetal. 2002; Reeveset al. 1998;Comastrietal. 1998).ObservationswiththeChan- al.2002;Yaqoob&Padmanabhan2004),oftensuperimposed on top of a broad(∼ 10,000–100,000km s- 1 FHWM) com- drahighenergytransmissiongratingspectrometer(HETGS) andtheXMM-NewtonCCDdetectorsarenowrevealingFeK ponent. Traditionally,narrowFe K lines are associatedwith emission due to H-like and He-like Fe in other AGN (e.g. matter at least several thousand gravitational radii from the black hole (one gravitational radius, r , defined as GM/c2). Reeves et al. 2002; Dewangan 2002; Page, Davis, & Salvi g 2003;Yaqoobetal.2003a).Asmoreexamplesofhighlyion- However,thereisincreasingevidencetosuggestthatnarrow izedFespectralfeaturesaredetectedinthespectraoftype1 1 Present Address: Department of Astronomy, University ofMaryland, AGN, it is clear that we need a better understanding of the CollegePark,MD20742 complexionizationphysicsoftheaccretiondisk(e.g. Nayak- 2DepartmentofPhysicsandAstronomy,JohnsHopkinsUniversity,Balti- shin&Kallman2001;Ballantyne,Ross&Fabian2001;Bal- more,MD21218 lantyne&Ross2002). 3LaboratoryforHighEnergyAstrophysics,NASA/GoddardSpaceFlight HerewereportonthespectralresultsofaChandraHETGS Center,Greenbelt,MD20771 observation of the type 1 AGN IC 4329A, in which we ob- 2 serveacomplex,broad,double-peakedline profile. Prelimi- entiredurationofthe campaign. Forexample,forHEGplus naryresults of this observationwere presentedin Yaqoob& MEG combined first-order lightcurves binned at 128 s, the Padmanabhan(2004)andwe nowpresentdetailedmodeling excessvarianceabovetheexpectationforPoissonnoise(e.g. of the Fe K line profile at the highestspectralresolution yet see Turner et al. 1999) was (3.4±18.2)×10- 5, consistent available(FWHM∼1860kms- 1 at6.4keV).IC4329Aisa withzero. Onlysinglespectraperinstrumentweretherefore brightSeyfert1.2AGNatz=0.016054(Willmeretal. 1991) extracted. embedded in a nearly edge-on host galaxy. It is one of the We made effective area files (ARFs or ancillary response brightest Seyfert galaxies known, and is sometimes brighter files),photonspectraandcountsspectrafollowingthemethod than 3C 273 in the 2–10 keV band. IC 4329A is luminous, of Yaqoob et al. (2003b). A net exposure time of 59,087 s with L (2–10keV) ∼ 4×1043 ergs s- 1 typically, where we was obtained (including a deadtime factor of 0.0162). The haveconvertedfromtheluminositygivenbyReynolds(1997) HEG bandpass is ∼0.8–10 keV and the MEG bandpass is usingH =71kms- 1Mpc- 1(Spergeletal. 2003)andq =0. ∼0.5–10keVbuttheeffectiveareaofbothinstrumentsfalls 0 0 IC4329AwasobservedpreviouslywithASCA(Mushotzky off rapidly at either end of the bandpass. Background was etal. 1995;Cappietal. 1996),GRO(Madejskietal. 1995), not subtracted since it is negligible in the energy region of BeppoSAX (Perola et al. 1999), ASCA simultaneously with interest. TheHEG hastwice the spectralresolution(0.012Å RXTE(Done,Madejski&Z˙ycki2000)andXMM-Newtonsi- FWHM) of the MEG (0.023Å FWHM) and has a larger ef- multaneouslywithBeppoSAX(Gondoinetal.2001).AGaus- fective area than the MEG in the Fe K band so our analysis sian modelfitted to the ASCA observations(Done, Madejski willfocusontheHEGdata,sinceweareinterestedinstudy- & Z˙ycki 2000) revealed strong (EW=180±50 keV), broad ing the Fe K regionwith the highestspectralresolution cur- (FWHM=42,960±11,015 km s- 1) Fe K emission peaking rentlyavailable.TheMEGspectrumofIC4329Aisdiscussed near∼6.4keV.TheXMM-Newtonobservations(Gondoinet elsewhere (McKernan, Yaqoob, & Reynolds2004, in prepa- al. 2001)revealedanarrowFeKαline(FWHM<8785km ration). s- 1,EW=43±1eV)originatinginmatterwithalowioniza- tionstate(thelinepeakedatE=6.42+- 00..0043keV). 3. MODELINGOFTHEFEKCOMPLEXINIC4329A Thepaperisorganizedasfollows.In§2wediscusstheob- We used XSPEC v.11.2.0 for spectral fitting to the HEG servationanddata. In§3wediscussfittingdifferentphysical spectruminthe3–8keVband. Thischoiceforthelowerend modelsofthecomplexspectrumintheFeKband. In§4we ofthebandpassavoidstheknowncomplexitiesinthesoftX- discusstheimplicationsofourresultsandpresentourconclu- ray spectrum due to absorption and the higher end ensures sions. thatbackgroundwillindeedbenegligblesincetheinstrument efficiencyareavanishesrapidlyathighenergies.Galacticab- sorption of 4.55×1020cm- 2 (Elvis et al. 1989) has a neg- ligibleeffectinthisenergyrangeandsowasnotincludedin ourmodeling.Therehasbeenawell-documenteddegradation inthequantumefficiencyofChandraACISduetomolecular contamination.However,theeffectofthisdegradationismost pronouncedatenergiesbelow∼1keVandisnegligibleover the energyrange discussed here so we do not include ACIS degradation in our modeling. The C–statistic (Cash 1976) wasusedforminimizationinallmodelsandallstatisticaler- rorsquotedare90%confidenceforoneinterestingparameter (∆C=2.706),unlessotherwisestated. Firstweinvestigatedhowthe3.0–8.0keVHEGdatacom- pared with a simple model consisting of a single power law plusintrinsicabsorption,withthespectrumbinnedat0.16Å. We included absorption in this initial fit, just to ensure that it can be neglected in the bandpass that we used. The best– fitting power-law index for this model was 1.66+0.09 and the - 0.05 absorbing column density was zero with an upper limit of 5.1×1021 cm- 2. Figure 1(a) shows the HEG photon spec- trumofIC4329Awiththemodelsuperimposed. Figure1(b) FIG. 1.—IC4329AHEGspectrumbetween3.0and8.0keVbinnedat shows the ratio of the data to the model. The single power 0.16Åcomparedwiththebest-fittingpower-lawmodel.(a)Photonspectrum, (b)Ratioofthedatatothemodel. law is clearly a good overall fit to the hard X-ray spectrum butthereisa broaddouble-peakedfeatureinthe Fe K band. Thepeaksoccurat∼6.2keVand∼6.8keVintheobserved 2. OBSERVATIONANDDATA frame,andthelowerenergyfeatureappearstobebroaderthan thehigh-energyone. To investigatethe Fe-K emissioncom- IC4329AwasobservedwiththeChandraHETGS(Markert plexinmoredetail,weusedspectrabinnedat0.005Å,which etal. 1995)on2001August26fromUT06:14:07foradu- correspondsapproximatelytothe 1σ HEG resolution. Since rationof∼60ks. TheChandraHETGSconsistsoftwograt- noabsorptionisrequiredinthe3–8keVband,weusedonlya ingassemblies, ahigh-energygrating(HEG)andamedium- powerlawforthecontinuum.Severalmodelsofthecomplex energygrating(MEG).Weusedonlythecombined±1orders FeKemissionarediscussedbelow. ofthegratingdata. ThemeanChandratotalHEGandMEG countrateswere1.158±0.0051ct/sand2.244±0.0067ct/s 3.1. DoubleGaussianModel respectively.Thesourcefluxshowedlittlevariabilityoverthe 3 We added two Gaussian emission-line components to the of each Gaussian component adds three free parameters to power-lawcontinuummodelinordertofitthe∼6.2keVand a fit, so the lower energy Fe K feature was detected with a ∼6.8keVemissionpeaks.WedidnotincludeaComptonre- confidence level > 4 σ and the higher energy feature was flectioncontinuuminthefitsincewewillshowpost-factothat detectedatonly>95%confidence. theemission-lineparametersareinsensitivetothepresenceor absence of a Compton reflection continuum(see §3.6). The best-fitting parameters for this model are given in Table 1. NotethatinTable1weshowthevelocityshiftfromsystemic when the lower and higher-energy peaks are identified with Fe I Kα emission (at 6.400 keV in the rest frame) and H- likeFeXXVILyαemission(at6.966keVrestframe)respectively. Evenwhenwetakeintoaccountthesystematicuncer- taintyoftheHEGwavelengthscale(∼±420,±460kms- 1at ∼6.2keVand∼6.8keVrespectively)4,thelow-energyfeature(ifFe I Kαemission)isredshiftedby>650kms- 1 and the higherenergyfeature(if Fe XXVI Lyα ) is redshiftedby >950kms- 1 (bothare90%confidencelowerlimits). Ifthe higherenergyfeatureisduetoFe XXV forbiddenorFe XXV resonanceemission,thentheblueshiftis∼12,200kms- 1 or ∼9,220kms- 1respectively,relativetosystemicvelocity. FIG. 3.—Confidencecontoursofequivalent width(EW)versusFWHM for(a)thelowerenergyFeKemissionpeakand(b)thehigherenergypeak fromadouble-Gaussian modelfittotheHEGspectrum ofIC4329A (see §3.1andTable1). Thetwo-parameterjointconfidencelevelscorrespondto 68%(innermost),90%and99%(outermost). Fig. 2(a) shows a two-parameter joint confidence contour of line intensity versus rest-frame energy for the lower en- ergyFeKemissionandFig.2(b)showsasimilarplotforthe higher energy emission. Consistent with the spectral fitting results, it can be seen that the lower energy Fe K emission peak is marginally redshifted from the rest-frame energy of Fe I Kα (dashed line) at ∼90% confidence. If we identify the higher energy feature with Fe XXVI Lyα emission, this lineisalsomarginally(at>90%confidence)redshiftedwith respect to systemic velocity (dashed line). Fig. 3(a) shows FIG. 2.—Confidencecontoursoflineintensityversusrest-frameenergy a two-parameter joint confidence contour of the equivalent for(a)thelowerenergyFeKpeakand(b)thehigherenergypeakfroma doubleGaussianmodelfittotheHEGspectrumofIC4329A(see§3.1and width(EW) versusFWHM forthe lower energyFe K emis- Table1).Thetwo-parameterjointconfidencelevelscorrespondto68%,90%, sionandFig.3(b)showsa similarplotforthehigherenergy and99%confidence.Thedashedlinein(a)denotestherest-frameenergyfor emissionpeak. FeIKαemission(6.400keV)andthatin(b)correspondstotherest-energy ofFeXXVILyαemission(6.966keV).Thecontoursin(a)indicatethatthe Fig. 4(a) shows a close-up of the best-fitting Gaussian lowerenergyfeatureisredshifted fromtheFe IKαrest-energy at∼90% model fit (see Table 1) to the IC 4329A HEG spectrum. confidence. From(b)itappearsthatthehigherenergyfeatureisredshifted Again, the offset of the Gaussian peak energies from the fromtherest-energyoftheFeXXVILyαemissionline. dashedlinesinFig.4(a),whichmarktheenergiesofFeIKα andFe XXVILyαemissionintheobservedframe(6.299keV The low-energy Fe K feature is moderately broad and 6.856keV respectively), show that both the lower and (FWHM=20,825+10,110 km s- 1), as we saw from higher energy peaks (if Fe I Kα and Fe XXVI Lyα respec- - 7375 Fig. 1, whereas the higher energy feature is narrower tively) are indeed marginally redshifted in IC 4329A. If the (FWHM=3680+5200 km s- 1). We estimated the significance higher energy feature is Fe XXVI Lyα emission, then un- - 2455 of each feature by comparing the values of the C–statistic der some circumstances we should also expect significant obtained from fitting a power-law continuumonly and from FeXXVILyβemission(e.g.seeBautistaetal. 1998;Bautista addingthefeatureinquestiontothepower-lawonlyfit. The &Titarchuk1999).WethereforeaddedathirdGaussiancom- resulting values of ∆C are shown in Table 1. The addition ponent to the model in Table 1 and we extended the HEG data energy range to 3.0–8.3 keV since Fe XXVI Lyβ oc- 4http://space.mit.edu/CXC/calib/hetgcal.html curs at 8.251 keV in the rest-frame (for all successive mod- 4 TABLE1. DOUBLEGAUSSIANMODEL Low-Ionization High-Ionization FeKPeak FeKPeak σcE∆((keEVeV/)E)0(kms- 1)a 64.36041108+-+-00633..+45002677900630 62.9508653+-+-00611..+52005912310087 EIFWWb H(eMV)(kms- 1)a 20,821591+-.1170030+-,7+-1865146..09066 368604.+-2252-++-24343052..140054 Γ 1.76±0.-0460 -.2.1. ∆Cc - 26.1 - 10.6 C(d.o.f.)d 518.2(508) ... NOTE. — Modelparametersforthebest-fittingdouble-Gaussianmodelforthe double-peakedFeKcomplexintheHEGspectrumIC4329A(see§3.1). Errorsare 90%confidenceforoneinterestingparameter(∆C=2.706). Allparametersarein thesourcerestframe(z=0.016054). aWeshowredshiftofthelinecentroidenergy asavelocityoffsetwhenthelowandhighenergypeaksareidentifiedwithFeIKα emissionandFeXXVILyαrespectively.FeIKαemissionandFeXXVILyαemission haverestenergiesof6.400keVand6.966keVrespectively.Velocitiesareroundedtothe nearest5kms-1.ComparethesevalueswiththesystematicuncertaintyintheHETGS energyscaleof±420,±460kms-1attheobservedenergiesoftheemissionpeaks.b Gaussianemission-lineintensityinunitsof10-5photonscm-2s-1. cChangeintheC– statisticuponadditionoftheparticularGaussianmodelcomponenttoasimplepower- FIG. 4.—IC4329AHEGphotonspectrabetween5.0and7.3keV(ob- lawmodelforthecontinuum(therearethreeadditionaldegreesoffreedomforeach served energy), binned at 0.01Å. The solid lines correspond to physically Gaussiancomponent).dDegreesoffreedom. different(anddegenerate)models(seetextfordetails).Notethatthedataare notunfolded: thespectraweremadebymultiplyingtheratioofthecounts ineachspectralbintothepredicted modelcountsinthatbin, bythebest- fitting model. In each case the continuum is a simple power law and the Next, we replaced the double-Gaussian emission-line emission-linemodelsare(a)adoubleGaussian(see§3.1andTable1);(b)a singlediskline(see§3.2andTable2),and(c)adoublediskline(see§3.4 model described above with a single, broad emission line andTable3).Dashedlinesindicatetheobserved-frameenergiesexpectedfor modelfrom a relativistic disk around a Schwarzschild black FeIKα(6.400keV)andFeXXVILyα(6.966keV)emissionrespectively. hole(e.g. Fabianetal. 1989). Thesedatacannotdistinguish betweenKerrandSchwarzschildmetrics(see§3.6). Thein- ner radius of the disk was fixed at 6r since it could not be g constrained. The outer disk radius and line intensity were els we shall return to fitting the data range 3.0–8.0 keV). Fe XXVI Lyβ is hardly detected (∆C improvesby only ∼1 freeparameters.Theradiallineemissivityperunitareawasa powerlaw,rq, whereqistheemissivityindex. Thistimewe fortheadditionofaGaussianwiththreefreeparameters,and also included another disk-line component to model Fe Kβ EW<49eV).The valueofthe Fe Lyβ:Lyαratio isa power- emission. Although it makes little difference to the results, fuldiagnosticsinceitcandeterminetheprocessofexcitation. we included it for completeness. The rest energy was fixed However,themeasurementerrorsontheFeLyβ:Lyαratioin at7.056keV(correspondingtoFe I emission)forsimplicity thesedataaretoolargetoobtainanymeaningfulconstraints. eventhoughtheenergyofFeKαwasallowedtovary(butthe On the other hand, since IC 4329A is one of the brightest proximityofthelatterenergytotheFeIKαenergy,combined Seyfertgalaxies(sometimesit is brighterthan 3C 273), it is withtherelativeweaknessofKβ easilyjustifiesthisassump- one of the best candidates to constrain the Fe Lyβ:Lyα ration).TheFeIKβdisk-lineparameterswerecoupledtothose tio with future missions such as Astro-E2. We obtained the ofthelowerenergyKαline,withtheintensityofKβ tiedto observed-frame 2–10 keV flux (for comparison with values (17/150) times the Kα line intensity. There were a total of intheliterature)fortheChandraHEGspectrumbyextrapo- five free parameters for this emission-line model, including lating the abovebest-fitting modeldown to 2 keV and up to 10keV,andthisyieldedavalueof16.2×10- 11ergcm- 2s- 1, theintrinsicrestenergyofthemonochromaticemissionline. Thebest-fittingresultsforthismodelaregiveninTable2and corresponding to an intrinsic, source-frame 2–10 keV lumi- nosityof9×1043ergs- 1. thedataandmodelareshowninFig.4(b).ThecomplexFe-K emissioncanclearlybefitted with a singlebroadline given: Sincewedetectedanemissionlinepeakthatcouldbedue a flat radial emissivity (q>- 0.72)with emission from ∼6– to Fe XXVI Lyα we searched for evidence of other features 70 r and θ ∼24+9 degrees (90%, one-parameteruncertain- due to highly ionized Fe (as detected e.g. in NGC 7314 by g - 1 ties).Thebest-fittingrest-frameenergyforthisrelativistically Yaqoobetal. 2003a)byaddingathirdGaussianmodelcom- broadenedlineis6.74+0.22keV,whichisindicativeofHe-like ponent.WefindEW<17eVforFeXXV(f)(atthesameoffset - 0.13 Fe.TheC–statisticforthesingledisk-linemodelishigher(by velocityandthewidthofthehigherenergyFeKfeatureinTa- ∼8.5,correspondingto>99%confidence)thanthatforthe ble1)andwefindmarginallysignificant,narrowFeXXV(f) double-GaussianfitinTable1. However,thesingledisk-line emissionatthe galaxysystemic velocity(detectedat>95% confidence, since ∆C improvesby ∼5.6 for the addition of modelweusemaybetoosimplistic(inparticularwithrespect to the assumed simple emissivity law and the assumption of onefreeparameter). axisymmetricemission),sowecannotruleitoutonthebasis ofthefitstatisticalone. Notealsothattheflatline-emissivity 3.2. SingleRelativisticDiskLineModel index may be a result of fixing the inner disk radius at 6r . g 5 theC–statisticbyonly6.2fortheadditionoftwofreeparam- TABLE2. SINGLEDISKLINEMODELANDHE-LIKEFEABSORPTION eters (correspondingto a significance of <99%), we do not pursuethematterfurther. Diskline DiskLine Absorption (NoFeXXV)a (WithFeXXV)b Line 3.4. DoubleDiskLineModel qEθ((◦k)eV) 6.74>+- 002-..214023+.79c 6.8052.6+-2+-00+31..017..3787 6.7(fixe..d....) dinisNake-vlxeitnr,yewsmeimofidptelitlshcteiocFmwepaKoyn,eethmnetissp.soiToshsnie-bliiinldieteyacoohfmelripenleeisxetmwoiiistnhsviotewnstofigrKoamtαe FIΓrE∆oWdWuCt/He(erMgV)(kms- 1) 12.71346.75+-31+-0052+-.76..-+-0044....3101996581. 313.72.8544+-+-987100-+-+-..1...14220058...24516552.. 12,120+-416156,3+-0...0722...028... aaicrozamoudemmtdipeserlektetero,fawrrptsdohiimtcleihaofyhraaoeirgrervc,hsteohpewroeortlihneectiwdrnalslielolntaryggydthsyeittsoerrpkabte-ltholeaiifiewnkneeieecndaroccthiouleoimlnnntdheaipzerctogiaoohnytmnioeolnetanitnnlfsasergytowlreeemuerm,c.rtaethiuPsehtrsaieiieiigo.nrdhsnnFsleooyocrfroitaopuehnxnaladd---t C(d.o.f.) f 526.7(509) 520.5(507) ... theywereforcedtovarytogether.Bothdisk-linemodelcom- ponentshad inner radii fixed at 6r . The rest-energyfor the g NOTE.—Modelparametersforthebest-fittingrelativisticSchwarzschild lower energy line component was a free parameter but the diskline models(withandwithoutFe XXVHe-like resonance absorption) rest energy of the higher energy line was fixed at the rest– fittedtotheFeKcomplexintheHEGspectrumofIC4329A(see§3.2).All parametersareinthesourcerestframe(z=0.016054). Errorsare90%con- energy of Fe XXVI Lyα (6.966 keV). Thus, the radial line fidenceforoneinterestingparameter(∆C=2.706). Theinnerradiusofthe emissivities of the two disk-line components were indepen- disk-linemodelcomponentwasfixedat6rg.aModelfittedwithnoFeXXV dent.Theabovesimplifyingassumptionsarenecessary,given He-likeresonanceabsorption. b Modelincludes aninvertedGaussianwith the limitationsof the data, in ordernot to over-parameterize centerenergyfixedat6.7keVtomodelFeXXVHe-likeresonanceabsorption(best-fittingabsorption-lineparametersaregiveninthelastcolumnofthe themodel. Again,forcompletenesswe addedanextradisk- table).cTherearelocalminimaallthewayuptotherestenergyofFeXXVI linecomponenttomodelFeKβ emissionassociatedwiththe Lyα(6.966 keV). d Fe-K emission-line intensity in units of 10- 5 photons lower energy peak, as described in §3.2 for the single disk- cm- 2s- 1.eChangeintheC-statisticupontheadditionofthedisk-linemodel line model. However,inclusion of the Fe Kβ emission does componenttoasimplepower-lawmodel. f Degreesoffreedom. notadd any more free parametersto the model. Thus, there werea totalof six freeparametersin thismodelofthe Fe K emissioncomplex. Thebest-fittingresultsforthismodelare If the line emission is truncated at a larger radius, a steeper giveninTable3andthedataandmodelareshowninFig4(c). emissivitylawisallowed(e.g. seeDoneetal. 2000). When Clearlybothdisklineshaveverysimilarradialemissivitypro- wefixedqat- 2.5(avaluetypicallyfoundinrelativisticFeK filesandarestronglyconstrainedtoanearface-oninclination linefitstoCCD data)andallowedtheinnerradiustovary,a (θ <6.0◦ at >95% confidence). The EWs of the low and fitwasobtainedwhichwasonlymarginallyworse(|∆C|<3), highenergyfeatures,asmodeledbydisklines,arelargerthan andyieldedaninnerradiusof28+11r . the correspondingEWs in Table 1 (double-Gaussianmodel) - 10 g andtheir combinedEW is largerthan the single disk-line fit 3.3. SingleDiskLinewithFeXXVHe-likeAbsorption totheFeKregion(at90%confidence). Thefitstatisticisan improvement over the double-Gaussian model fit at >95% It is conceivable that between the low-energy and high- confidence and is an improvement over the single disk-line energypeaksoftheFeKemission-linecomplex,wemayhave modelat>3σconfidence(∆C=- 12.6foroneadditionalfree Fe XXV He-likeresonanceabsorptionat6.7keV.Suchreso- parameter). nance absorption cutting away the Fe K emission-line profile has been suggested for NGC 3516 (Nandra et al. 1999) and MCG - 6-30-15 (Fabian et al. 2002) and discussed in 3.5. RelativisticDiskLineandGaussianModel a theoreticalcontextby, for example, Ruszkowski & Fabian Next, we modeled the two peaks in the Fe K band with (2002). Therefore,wetestedthedataagainstasimplemodel a relativistic disk line forthe lower energyemission compo- ofFeXXVHe-likeresonanceabsorptionbyaddinganinverted nentandaGaussianforthehigherenergycomponent.Again, Gaussian componentto the single disk-linemodeldescribed we included Fe Kβ emission as described in §3.2 for com- in §3.2. The center energy was fixed at 6.7 keV in the rest pleteness. The best-fitting results for this model are given frameofthesource,whichmeantthattwomorefreeparame- in Table 4. The disk inclination angle is tightly constrained terswereaddedtothemodel(thewidthandequivalentwidth to be 12±2 degrees and q is flat. The Gaussian best-fitting oftheabsorptionline). TheresultsforboththerelativisticFe parametersto the high-energyfeature are similar to those in KemissionlineandtheabsorptionlineareshowninTable2 Table1,asweshouldexpect. Thismodelisnotsignificantly sothattheycanbedirectlycomparedwiththefitwiththerel- betterthanthedouble-Gaussianmodelstatistically(Table1), ativistic Fe K emission line only (§3.2). It can be seen that butit is better than the single broaddisk-line fit (Table 2) at theadditionoftheabsorptionlinehaslittleeffectonthedisk >95% confidencesince the C–statistic decreasesby ∼10.3 inclinationangle,thediskouterradius,andtheradialemissiv- forthreeadditionalfreeparameters(relativetothesingledisk- ityderivedfromthebroadFeKline. Naturally,theintensity linemodel). Notethattheradialemissivitypower-lawindex and equivalentwidth of the Fe K emission line are required (q)forthedisk-lineemissionisveryflat. to be larger due to the presence of the absorption line. The derived equivalent width of the absorption line is 46+22 eV, 3.6. ComplexContinuum - 28 and its width is 12,120+11,070 km s- 1. These values depend So far, we have not included the effects of a Compton- - 6530 somewhat on how the underlying emission line is modeled. reflection continuum, or indeed any other complexity in the However, since the additionof the absorptionline decreases continuum. First of all, we note that there is little evidence 6 TABLE3. DOUBLEDISK-LINEMODEL TABLE4. DISKLINEPLUSGAUSSIANLINEMODELS Low-IonizationFeK FeXXVI Lyα DiskLine Gaussian DiskLine Gaussian (simple)a (simple)a (complex)b (complex)b lineNrOegTioEn.o—fCE∆ΓθqEIrthoPa((Wceu◦(akCtdrH)/e(a.bdVeromEgVb.G)eft.)e)srpseefcotrrutmheibneIsCt-fi4t35t62i1-n.9442g1A24..6.1d47.1033o-4T9<2.+-(3u65+-+-200h0+-b+-00..000e56+-0053l....66e72251003376..m..2024066576429)roeldaetliviinsc6til-.cu92dd61.ei2614ss-0k0.afi5-+-21nl+-x00i2+-n..F....67e3344.e......e9d373049....fiKtβtoethmeisFseioKn ΓREIFrσqEθcdo∆(WeW((u◦ketEV)/eH(Vr/)eEMgV)0(()kkmmss--11))cc 16.-.740511.3±1621790.+-21+-00065±+-21...+-+-.00...54..032375042......520908190852.... 642.97801450003.4+-+-+-400663311+-..+-+-610025.....538733953233.......120560504161..... 6.<3114.5>829.97.8+->91+-+--0080016..82-+20.......311+2196773......43607721687.... 642.90809460504.4-++-+-000004211-+..+-+-172500....239695332242......935550062117.... componentforthelower-energyline(see§3.4forfittingdetails).Errorsare90%confi- ∆C - 30.2g - 12.2h - 32.6g - 9.6h denceforoneinterestingparameter(∆C=2.706)exceptwherenoted.Theinnerradius C(d.o.f.)f 516.4(506) ... 516.8(505) ... ofalldisk-linemodelcomponentswasfixedat6rg.Allparametersareinthesourcerest frame(z=0.016054).TheenergyoftheFeXXVILyαdisklinecomponentwasfixedat itsrestenergyof6.966keV.aPower-lawindexforthelineradialemissivity.Thelower NOTE.—Modelparametersforthebest-fittingrelativisticdisklineplusGaussian energyFeKαandFeXXVILyαdisklinecomponentswerecoupledinallparameters modelstotheFeKcomplexintheHEGspectrumofIC4329A(see§3.5forfitting exceptthisone(q). bBothθandrout/rgfortheFeXXVILyαdisklinewereforced details). ThemodelalsoincludesFeKβemissionforthelowerenergyline(see§3.2 totheircorrespondingvaluesforthelowerenergydisklineduringthefitting. TheC- fordetails).Errorsare90%confidenceforoneinterestingparameter(∆C=2.706).The statisticvariesnon-monotonicallyclosetothebestfit.Forthediskinclinationanglethe innerradiusofthelowerenergyFeKαdisk-linemodelwasfixedat6rg.Allquantities 95%confidenceupperlimitisshown.Formally,θ<0.35◦at90%confidencebutthe arequotedinthesourcerestframe(z=0.016054). aSimplemodelwithapower-law 95%confidencevalueismorereliablesinceitisintheregionwhereCiswell-behaved. continuum,andarelativisticdisklineforaSchwarszchildblack-hole. b Relativistic Forrout/rg,theerrorsrepresent90%confidencevalues,butonlywhen∆Cneverdrops disklinefrommaximallyrotatingKerrblack-hole,withapower-lawplusCompton- below2.706again. cLineintensityinunitsof10-5photonscm-2s-1. dChangeinC– reflectioncontinuumblurredbytherelativisticlinekernel. FeXXVILyαhasarest statisticupontheadditionoftheparticulardisklinemodelcomponenttoapower-law energyof6.966keV.cVelocitiesareroundedtonearest5kms-1. d Lineintensityin continuumonly.eDegreesoffreedom. unitsof10-5photonscm-2s-1. eEffective‘reflectionfraction’(i.e. R=1corresponds tothesteady-stateComptonreflectionamplitudefromaninfinitedisksubtending2π solidangleattheX-raysource).fDegreesoffreedom.gChangeintheC–statisticupon additionofonlythedisklinetothecontinuum.hChangeintheC–statisticuponaddition ofaGaussianemission-linecomponenttothedisk-linepluscontinuummodel. of an Fe K edge above 7 keV in the data (Fig. 4). An Fe K edge could be produced either in transmission or reflection from optically-thickmatter. We added a simple edge model tothedisk-lineplusGaussianmodel(§3.5),at7.11keV(cor- energypeak. ThisisbecauseifitisduetohighlyionizedFe, respondingto neutral Fe) and found a best-fit value of zero, thecurvatureinthereflectedcontinuumismuchlessthanthe and a 90%, one-parameterupper limit of τ <0.054. This caseforadiskwhichisnotionized,especiallyfordatainthe Fe iscomparablewithτ ∼0.03fromtheXMM-Newtonobser- restrictedbandpass(3–8keV)thatwearedealingwithhere.If Fe vationofIC4329AbyGondoinetal. (2001). Thisimpliesa thehigherenergyfeatureisnotaseparateline,thenthesingle totalneutralabsorbingcolumnofN <4.9×1022cm- 2inthe Compton-reflectioncontinuumalreadyincludedisadequate. H line-of-sight. Thisiscertainlyconsistentwiththelow-energy The free parametersof the laor modelwere the disk in- MEGdata(McKernan,Yaqoob,&Reynolds2004,inprepa- clinationangle,θ,thepower-lawemissivityindex,q,andthe ration). outerdiskradius,r . Sinceeventhelowerenergyemission out Dueto therestrictedbandpassandlimitedsignal-to-noise, peakis notextremelybroad, we foundthe fit to be quite in- the data are not sensitive to the Compton-reflection contin- sensitivetotheinnerdiskradius,soagainfixeditat6r . The g uum, which becomes important above ∼ 7 keV. However, Compton-reflectionmodel(pexrav)doesnotallowthedisk we demonstrated this directly by fitting a model consisting inclinationangletobetiedtothatofthelaormodel,andalso of a power law with a high-energy exponential cut-off, and doesnotallowinclinationangles<18◦. Thereforewe fixed Compton reflection from optically-thick, neutral matter (us- the inclination angle at 30◦ for the Compton-reflectioncon- ing the XSPEC model pexrav, as described in Magdziarz tinuum. Thisis adequatefor ourpurpose. We also assumed & Zdziarski1995), a Fe Kα line to model the lower energy solar element abundancesand an exponentialcut-off energy peak in the Fe complex, and anotheremission line to model of 200 keV for the intrinsic power-law continuum. This is thehigherenergypeak. AFeKβ linewasalsoincluded,for consistentwith broadbandobservationsofIC4329Aaswell completeness,andtiedtotheFeKαlineasdescribedin§3.2. asresultsforotherAGNs(e.g. Madejskietal. 1995;Malizia TheFe Kαline, the Fe Kβ line, andthe Comptonreflection etal. 2003;Perolaetal. 2002). Inanycase,ourconclusions continuumwereallblurredwitharelativistickernelbasedon arenotsensitivetothisassumption. TheCompton-reflection theKerrmetric(Laor1991;seealsoFabianetal. 2002). The continuumthenhadonlyonefreeparameterinthefit,thera- reasonfornow usingthe Kerrmetric (fora maximallyspin- tio,R,oftheamplitudeofthereflectedcontinuumtothatex- ningblackhole),insteadoftheSchwarzschildmetric,istodi- pectedfromacentrallyilluminatedinfinitedisk,subtendinga rectlydemonstratethatthedataarenotsensitivetotheblack- solid angleof2π atthe X-raysource. In all, thismodelhad holeangularmomentum.Sincethehigherenergypeakofthe elevenfreeparameters,includingthe overallcontinuumnor- Fecomplex(possiblyduetoFe XXVI Lyα)intheIC4329A malization. Thisisonemorefreeparameterthantheequiva- HEGdataisquitenarrow,itwasmodeledwithaGaussianand lentmodelthatutilizedtheSchwarzschildmetricandthatdid notsubjectedtotherelativisticblurring.Also,weincludedno not include a Compton-reflectioncontinuum(i.e. the model Compton-reflectioncontinuumassociatedwiththethehigher describedin§3.5). 7 TheresultsareshowninTable4sothattheycanbedirectly preparation; see also Gondoin et al. 2001). Such a column comparedwith the resultswhen Comptonreflectionwas not densitycanatmostproduceanEWof<50eV,evenifitfully included, and the continuum was modeled only with a sim- coveredthesky(e.g. Krolik&Kallman,1987;Yaqoobetal. ple power law. It can be seen that the quality of the fits for 2001). ToinvestigatethecontributiontotheFe Klineemis- the two models, the best-fitting values for the emission-line sion from distant matter, we added an extra (narrow) Gaus- parameters and their statistical error ranges are virtually in- sianline componenttothe twoGaussian modeldescribedin distinguishable. Thisappliestoboththerelativisticandnon- Table 1. The energy of the new Gaussian component was relativistic emission lines. It is important to note, however, fixed at 6.4 keV in the rest–frame of the AGN since we are that inclusion of the Compton-reflectioncontinuum does al- testingthedataforalinewhichoriginatesfrommattersuffi- low the line radial emissivity index to be steeper, relieving ciently distantthat there is no gravitationalredshift. Also, a the requirement that the emissivity law be flat (which may lineoriginatingfromsuchdistantmatterwouldbeunresolved bephysicallydifficulttoachieve). Anothernoticeablediffer- byChandra,sowefixedthewidthoftheGaussianatavalue ence between the simple and complexcontinuum fits is that (1 eV) much less than the instrument resolution. The two the latter fit yields a somewhat steeper intrinsic X-ray con- other Gaussian model components were allowed to vary as tinuum power-law index. For the fit with a complex con- before. The best–fittingmodeldid notresult in a significant tinuum, the reflection fraction, R, was consistent with zero, improvementinthefit–statistic (∆C=2.3foroneadditional andweobtainedanupperlimitofR<2.9(90%confidence, free parameter). Thus, the valuesand errorrangesof all the one parameter). This confirms that the data are insensitive model parameters did not change significantly compared to to the Compton-reflection continuum. The result also justi- thevaluesshowninTable1,whichwereobtainedwithoutthe fies using only a simple power-lawfor the continuumin the distant–matterline.The90%confidence(one–parameter)up- model-fitting described in §3.1 – §3.5. Since the relativis- perlimitonthedistant–matterlineEWweobtainedis18eV, tically smeared Compton-reflection continuum is smoother so that it could contribute at most ∼1/4 of the low energy and produces less curvature in the net spectrum than an un- (6.2–6.4keV)Fe KlineemissioninIC4329A.Thisandthe smeared continuum (in the energy band relevant here), we fact that the low-energy peak is at 6.3 keV and resolved by tested the data against the same model with the smearing theHEGsuggeststhatthecontributionfromdistantmatteris oftheCompton-reflectioncomponentturnedoff. Physically, not as important as in some other AGN in which the Fe K such a component could originate in optically-thick distant lineemissionpeaksat6.4keVandisunresolvedbytheHEG matter,suchasaparsec-scaletorus. Again,noreflectionwas (see Yaqoob & Padmanabhan 2004). Thus, it is likely that formally requiredby the data, with a 90% confidence upper the bulk of the ∼6.3 keV line emission originates in an ac- limit of R<1.7. These results further justify using only a cretiondisk. Itisdifficulttodeducewhattheionizationstate simple powerlaw forthe continuumin the model-fittingde- of the matter must be because of the possibility of a peak at scribedin§3.1–§3.5. energieshigherthan6.4keVbeinggravitationallyredshifted downto∼6.3keV.Notethataradiusof∼65r isrequiredto 4. DISCUSSION g gravitationallyredshifta6.4keVlineto6.3keV. In a Chandra HETGS observation of IC 4329A, we have The interpretationof the overall, double-peakedFe K line measuredbroadandcomplexlineemissionintheFeKband profileisdifficult. Severalphysicallydistinctmodelscande- usingthehighestspectralresolutioncurrentlyavailableinthe scribethedata.ThetwopeaksmaycorrespondtotheDoppler FeKband(∼1860kms- 1FWHMat6.4keV).Thelinepro- hornsofasingledisklineortheymaybeartificialinthesense file is double-peaked, the peaks occurringat ∼6.3 keV and thatHe-likeFeKabsorptionhasremovedthecentralportion ∼ 6.9 keV in the source rest frame. Modeled with Gaus- ofadisklineprofilewhichmaynothaveprominentDoppler sians, the lower and higher energy peaks have widths of peaks. Alternatively,thehigherenergypeakat6.9keVmay 20,825+- 170,3,71510 km s- 1 FWHM and 3680+- 52240505 km s- 1 FWHM correspondtoFeXXVILyαemissionwhichisindependentof respectively. The lower energypeak is resolvedby the high the line emission associated with the lower energy peak. In energygrating(HEG)at>99%confidence,whilstthehigher thatcase,theFeXXVILyαlinemayormaynotoriginateina energypeakisonlyresolvedat<90%confidence. Thelow- disk.Againthetwoscenariosaredegenerateasfarasthedata energypeakisredshiftedfromtheexpectedrest-frameenergy areconcerned. IfthereisFe XXVI Lyαlineemissionfroma ofFe I Kαbyatleast650kms- 1 andthehigh-energypeak disk itmay comefroma layerwhich is hotterthanthat pro- isredshiftedfromtheexpectedrest-frameenergyofFeXXVI ducingthelower-energyline(forexamplethehotcorona,e.g. Lyαbyatleast950kms- 1. Bothoftheselowerlimitsonthe seeNayakshin&Kallman2001;Ballantyne,Ross,&Fabian redshiftcorrespondto90%confidencelevelsforoneparame- 2001). Althoughthe∼6.9keVlineappearstobefairlynar- ter. Theemission-linepeakat∼6.9keVthatwefoundinthe row we have shown by directmodelfitting that it couldstill ChandraHETGS data hasnotbeendetectedin anyprevious originatefromadiskwhichextendsdownto6rg,witharadial observationsof IC 4329A.We also detected a much weaker lineemissivityperunitareawhichisnotflat. emission peak at the expectedenergyof the Fe XXV forbid- Thelineemissionpeakat6.9keVisreminiscentofasim- denlineenergyatthesystemicvelocityofIC4329A,butthe ilar feature found in a long XMM-Newton exposure of the featureisdetectedonlywithamarginalsignificance(>95% Seyfert galaxy MCG - 6-30-15 (e.g. Fabian et al. 2002). confidence). In that case there is also an ambiguitybetween Fe XXV He- TheamountofFeintheline-of-sightmaybeinsufficientto likeabsorptionandandFeXXVILyαemission,andthepeak producealloftheobservedEWofthelowerenergy∼6.3keV energy is slightly redshifted from the expected energy of Fe K feature (110+- 4460 eV from Table 1) since the measured FeXXVILyα,justasinIC4329A.FeXXVHe-likeabsorption (HEG)upperlimitontheopticaldepthofanFeKedgecor- hasactuallybeenobservedwithlessambiguityinsomeAGN responds to an absorbing column density <4.9×1022cm- 2 (PG 1211+143,Poundset al. 2003a;PG 0844+349,Pounds andthisisconsistentwiththelow-energyMEGdatafromthe etal. 2003b;NGC3783,Reevesetal. 2003). same observation (McKernan, Yaqoob & Reynolds 2004 in Although there are several interpretations of the complex 8 Fe K emission revealedby the HEG spectrum of IC 4329A, AST0205990 (BM) and NASA grants NCC5-447 (TY) and eachofthedifferentscenariosisinterestingfromthepointof NAG5-10769(TY).We thankA. C. Fabian for providingus viewthatfutureobservationswithAstro-E2willbreakmuch witharelativisticblurringroutine.WealsothankUrmilaPad- ofthedegeneracyandwhicheverscenarioturnsouttobecor- manabhanforhelpwithsomeofthedataanalysis. Wemade rect,wewouldlearnsomethingnewaboutthecentralengine. useoftheHEASARCon-linedataarchiveservices,supported Inparticular,observationalconstraintsontheionizationstruc- by NASA/GSFC and also of the NASA/IPAC Extragalac- tureoftheaccretiondiskinAGNarecurrentlyverypoorand tic Database (NED) which is operated by the Jet Propulsion IC4329Amaybeagoodastrophysicallaboratoryforimprov- Laboratory,CaliforniaInstituteofTechnology,undercontract ingthem. with NASA. We are gratefulto the Chandra instrumentand We gratefully acknowledge support from NSF grant operationsteamsformakingthisobservationpossible. 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High Resolution X-ray Spectroscopy of the Fe K complex in IC 4329A

Similar High Resolution X-ray Spectroscopy of the Fe K complex in IC 4329A