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

0.22 MB·

by  B. McKernan

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)respec-
tively. Evenwhenwetakeintoaccountthesystematicuncer-
taintyoftheHEGwavelengthscale(∼±420,±460kms- 1at
∼6.2keVand∼6.8keVrespectively)4,thelow-energyfea-
ture(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 ra-
tion).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-likeresonanceabsorp-
tion(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 pro-
file 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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