On the inability of Comptonization to produce the broad X-ray iron lines observed in Seyfert nuclei

5 Pages·0.09 MB·English

by  C. S. Reynolds

DRAFTVERSIONFEBRUARY1,2008
PreprinttypesetusingLATEXstyleemulateapjv.04/03/99
ONTHEINABILITYOFCOMPTONIZATIONTOPRODUCE
THEBROADX-RAYIRONLINESOBSERVEDINSEYFERTNUCLEI
CHRISTOPHERS.REYNOLDS1,2ANDJO¨RNWILMS3
DraftversionFebruary1,2008
ABSTRACT
Ithas recentlybeen suggestedthatComptondownscatteringmay giverise to the broadironlines seen in the
X-rayspectra ofSeyfert1 galaxies. Thischallengesthe standardmodelinwhich these linesoriginatefromthe
innermostregionsof the black hole accretion disk with Doppler shifts and gravitationalredshiftsgiving rise to
0
the broadenedline profile. Here, we apply observationalconstraints to the Compton downscatteringmodelfor
0 MCG−6-30-15 and NGC 3516, the two best cases to date of Seyfert galaxies with relativistically broad lines.
0
WeshowthatthecontinuumsourceinMCG−6-30-15requiredbytheconstrainedmodelviolatestheblackbody
2
limit. InthecaseofNGC3516,onlyaverysmallregionofparameterspaceiscompatiblewiththeconstraints.

n Hence,weconcludethattheComptonizationmodelisnotaviableoneforthebroadlineseeninthesetwoobjects.
a
Theaccretiondiskmodelremainsthebestinterpretationofthesedata.
J
Subjectheadings:galaxies:Seyfert,galaxies:individual:MCG−6-30-15,galaxies:individual:NGC3516,
1
2 line:formation,X-ray:galaxies

2 1. INTRODUCTION photons. F95 rejected this modelon the basis that the Comp-
v tonizingcloudmusthavea radiusofR < 1014cminorderto
0 The X-ray study of Seyfertnucleiand other typesof active
maintaintherequiredhighionizationstateandthat,withsucha
12 bgyaltahcetiocbnsuecrlveait(ioAnGoNf)rehlaastibveisetniceanlleyrgbirzoeaddfiorornthKeαpalsintefeswinytehaerisr smallradius,gravitationaleffectsfromacentral107M⊙ black
holewouldbeimportantanywayfordeterminingthelinepro-
2 X-rayspectra(Tanakaetal.1995;Nandraetal.1997;Reynolds
1 1997). Inparticular,theSeyfertgalaxyMCG−6-30-15hasbe- file. Theprincipalaim of F95was to demonstratethe need to
9 comeanimportanttestinggroundformodelsofbroadironline include strong gravity in any model of the iron line, so they
9 formation. A long observation of MCG−6-30-15 by the Ad- terminatedtheirchainofreasoningatthatpoint. Thequestion
/ remains,however,astowhetherComptondownscatteringhasa
h vanced Satellite for Cosmology and Astrophysics (ASCA) re-
significantaffectonthelineprofileorwhetherwecaninterpret
p vealed a high signal-to-noise broad iron line with a velocity
ironlineobservationsintermsofnakedaccretiondiskmodels.
- width of ∼ 105kms−1 and a profile which is skewed to low
o Misra & Kembhavi (1998) and Misra & Sutaria (1999;
energies(Tanakaetal. 1995). Theexcitementstirredbythese
r hereafter MS99) have recently developed the Comptonization
t studiesisduetothewidelyheldbeliefthattheironlinesorig-
s modelfurther. Intheircurrentmodel,theysuggestthatacloud
inate from the surface layers of an accretion disk which is in
:a orbit abouta supermassiveblack hole, and that the line width with opticaldepthτ = 4 andtemperaturekT ∼< 0.5keV sur-
v roundsthe centralengine. The upper limit to the temperature
andprofileprovideadirectprobeofthevelocityfieldandstrong
i oftheComptoncloudcomesfromthefactthattheironKαline
X gravitationalfieldwithinafewSchwarzschildradiioftheblack
photonsneedtobeprimarilydownscattered,ratherthanupscat-
hole. Modelsoflineemissionfromtheinnerregionsofablack
r tered,inordertoreproducetheobservedlineprofile. Thecen-
a holeaccretiondisk(e.g.,Fabianetal. 1989;Laor1991)fitthe
tral engine producesthe continuumemission which keeps the
observedlineprofileswell.
cloudionized,andanarrowironlinewhichisComptonbroad-
Thesuggestionthatweareobservingtheimmediateenviron-
ened to the observedwidth. They show that the resultant line
mentofanaccretingsupermassiveblackholeisaboldoneand
profilescanbebroughtintogoodagreementwiththeASCAob-
certainlywarrantsa criticalexamination. Inthisspirit, Fabian
servations.
et al. (1995; hereafterF95) examineda numberof alternative
A direct prediction of the Comptonization model (F95,
hypotheses for the origin of these broad iron lines including
MS99) is that the multiple Compton scatterings should pro-
modelsin whichthe line isproducedin anoutflowor jet, and
duce a break in the spectrum of the power-law continuumra-
modelsinwhichthelineisintrinsicallynarrow(orevenabsent)
diationatapproximatelyE ∼ m c2/τ2 (i.e.,∼ 30–40keV).
and a complex underlying continuum mimics the broad line. br e
Recently, it has been reported that BeppoSAX (Guainazzi et
Bothoftheseclassesofmodelswerefoundtobeunphysicalor
al. 1999)observationsconstrainthelocationofthecontinuum
didnotreproducetheobservedspectrum.
break to be at energies greater than 100 keV , thereby argu-
Another alternative model, first proposed by Czerny,
ing against the Comptonization model (Misra 1999). How-
Zbyszewska & Raine (1991) but also considered by F95, is
ever, a robust determination of the continuum break is not
one in which the iron line is intrinsically narrow (i.e., emit-
completely straightforward since it depends upon the param-
tedinslowlymovingmaterialwhichisveryfarfromacompact
etersassumedfortheComptonreflectioncomponent(e.g.,see
object) and then broadened to the observed profile by Comp-
Lee et al. 1999). Thus, while the lack of a spectral break at
ton downscatteringin matter that surroundsthe source of line
1JILA,CampusBox440,UniversityofColorado,BoulderCO80303
2HubbleFellow
3Institutfu¨rAstronomieundAstrophysik–Astronomie,UniversityofTu¨bingen,Waldha¨userStraße64,D-72076Tu¨bingen,Germany
1
2
30–40keV remainsthe most compellingargumentagainst the ThefluxattheinneredgeoftheComptoncloudisthengiven
Comptonizationmodel,itisinterestingtoconsiderconstraints by
ontheComptonizationmodelthatareindependentofacontin-
uumspectralbreak. hν3Lbb Lplf(ν)
Inthispaper,weapplyanumberofobservationalconstraints Fν = 2σ T4c2R2(exp(hν/kT)−1) + 4πR2 Ξ, (2)
SB in in
to the MS99 model. We focus on the case of the iron line in
MCG−6-30-15, but also address the line in NGC 3516, the where Rin is the inner radius of the Compton cloud, Lbb is
other high signal-to-noise case of a relativistically broad line. the luminosity of the black body component, σSB = 5.67×
WeshowthatthecontinuumsourceinMCG−6-30-15required 10−5ergcm−2s−1K−4theStefan-Boltzmannconstant,Lplis
by the constrained model violates thermodynamic limits (i.e., theluminosityinthepower-lawcomponent,f(ν)=ν−1inthe
the “black body” limit). We also show that only a very small rangeνmin < ν < νmax (andzeroelsewhere), andΞ isgiven
regionofparameterspaceisopentotheComptonizationmodel by
inthecaseofNGC3516. Hence,weconcludethattheComp- νmax
Ξ=ln . (3)
ton downscattering model is not a viable model for the broad (cid:18)ν (cid:19)
min
ironlinesinone,andpossiblyboth,ofthesesources.
GuidedbythehardX-rayobservationsofMCG−6-30-15(e.g.,
2. CONSTRAINTSFROMCONTINUUMVARIABILITYIN Leeetal. 1999),theparametersdescribingthepower-lawcom-
MCG−6-30-15 ponentarefixedtohavethefollowingvalues:
TheironlineinMCG−6-30-15hasbeenobservedtochange
hν =0.1keV, (4)
flux and profile on timescales of 104s (Iwasawa et al. 1996, min
1999). This is the shortest timescale on which detailed line hνmax =50keV, (5)
changescancurrentlybeprobedandtheremayindeedbeline L =5×1043ergs−1 (6)
pl
variabilityonshortertimescales. MS99notethatsuchvariabil-
ityisconsistentwiththelineoriginatingfromaComptoncloud TheresultingComptontemperatureisgivenby
ofsizeR∼1014cm.
1 h(ν −ν )
However,intheComptonizationmodel,thecontinuumpho- T = T L+ max min , (7)
tonsalso passthroughthesame Comptonizingmediumas the C 1+L(cid:18) 4kΞ (cid:19)
iron line photons. Thus, continuumvariability can be used to
whereListheratiooftheblackbodyluminositytothepower-
place much tighter constraints on the size of the cloud. Any
lawluminosity:
variability of the central source would be smeared out as the
L
photonsrandomwalkthroughthecloudonatimescaleof L= bb. (8)
L
pl
Rτ
tMS ∼ c . (1) The line corresponding to a Compton temperature of TC =
0.5keVonthe(L,T)-planeisshownonFig.1a,andtheforbid-
Appreciable continuum variability in MCG−6-30-15 is ob-
denregionofparameterspace(givingT >0.5keV)isshaded
served on timescales down to t ∼ 100s (Reynolds et al. C
obs withlinesofpositivegradient.
1995; Yaqoobet al. 1997). Since we must have tobs ∼> tMS, For completeness, itshouldbe notedthatthe aboveexpres-
anupperlimitontheComptoncloudisRcloud =1012cm,two sion for the Compton temperatureis only strictly valid due to
ordersofmagnitudeless thanthe size assumed in MS99. As- thesoftnatureofourspectrum. TheComptontemperaturede-
suming a geometricallythick cloud and solar abundances, the pends, of course, on the form of the radiation field inside the
densityofthematerialisnH ∼>5×1012cm−3. cloud. Ignoring downscattering, this field is greater than the
In assessing the robustness of this constraint, it should be external radiation field by a factor of τ. For the high-energy
noted that the iron line in MCG–6-30-15 is always observed radiation (hν ∼> 50keV), τ has an energydependencedue to
to be broad (although the width of the line does indeed vary, Klein-Nishinacorrections,therebyaffectingtheComptontem-
e.g. Iwasawa et al. 1996), and the source is always observed perature. Theneglectofdownscatteringisalsoinvalidatthese
to vary its flux with a temporal power spectrum that extends energies. However,thesecorrectionstotheComptontempera-
downto100stimescales(Leeetal. 1999b;Nowak&Chiang turehaveanegligibleeffectinourcase.
1999; Reynolds1999). Thus, itis difficulttosupporta model The ASCA observationshows no evidence for a soft excess
in which the Compton cloud is sometimes present (producing componentinMCG−6-30-15acrosstheentirewell-calibrated
a broad line and a slowly varying continuum) and sometimes spectral range of the solid-state imaging spectrometers (SIS;
absent (producinga narrow line and a rapidly varying contin- 0.6–10keV). Thus, we impose the condition that the black-
uum). bodyfluxat0.6keVislessthanthepower-lawfluxatthesame
energy:
3. THECOMPTONTEMPERATUREANDTHEBLACK-BODYLIMIT
InthesituationpostulatedbytheMS99model,thetempera- hν3Lbb Lplf(ν)
< (9)
tureoftheComptoncloudwillbelockedtotheComptontem- 2σ T4c2R2 (exp(hν/kT)−1) 4πR2 Ξ
SB in in
peratureofthe(local)radiationfield. Wemodelthecontinuum
spectrum of the central source as the superpositionof a black The regionon the (L,T)-planeforbiddenby this constraintis
bodyspectrum(whichmayrepresentthermalemissionfroman shadedwithlinesofnegativegradientinFig.1a.
accretion disk) and a power-law spectrum with energy index Finally,wemaketheobservationthatthereisafundamental
α=1whichextendsuptohardX-rayenergies(whichmaybe limittotheblackbodyluminositywhichisimposedbythermo-
identifiedasaccretiondiskphotonsthathavebeensubjectedto dynamics:
multipleComptonupscatteringbyanaccretiondiskcorona). L <4πR2 σ T4 (10)
bb max SB
3
0.08 0.08
MCG-6-30-15 NGC 3516
0.06 0.06
V] V]
ke ke
T [ 0.04 T [ 0.04
k k
0.02 0.02
0.00 0.00
0 2 4 6 8 10 0 2 4 6 8 10
L /L L /L
bb pl bb pl
FIG. 1.—ConstraintsdiagramsfortheComptonizationmodelappliedtoMCG−6-30-15(left)andNGC3516(right). Thealmostverticallinecorrespondstoa
ComptontemperatureofkTC = 0.5keV,withtheregionleftofthelinebeingexcludedsinceitwouldproducetoomuchComptonupscatteringoftheironline
photons.TheregionshadedwithlinesofnegativeslopeisforbiddensinceitwouldproduceasoftexcessintheASCA(MCG−6-30-15)orBeppoSAX(NGC3516)
bands(whichisnotobserved).Theshadedregionisforbiddensincethesourcewouldviolatetheblackbodylimit.
where R is the maximum allowed size of the black body higher energies. Either of these effects will raise the Comp-
max
source. Sincethecontinuumsourceishypothesizedtobeinte- ton temperature of the power-law component and require an
riortotheComptoncloud,wemusthaveRmax ∼< Rcloud. The evencoolerblackbodycomponentinordertocooltheComp-
regionofthe(L,T)-planeforbiddenbythisconstraintisshown toncloudbelowthe0.5keVlimit. Itshouldalsobenotedthat
insolid-shadeinFig.1a. we have ignored any infra-red emission from the continuum
Weseethatapplyingthesethreeconstraintseliminatesallre- source. Due to the high densities of the matter in the Comp-
gionsofthe(L,T)-plane.OnemustconcludethattheCompton toncloud,IRemissionsredwardsof∼ 10µmwillbefree-free
cloudmodeldiscussedbyMisra&Kembhavi(1998)andMS99 absorbed and act to heat the cloud rather than Compton cool
isnotvalidinthecaseofMCG−6-30-15. it. Again,theneglectoftheIRemissionsisaconservativeas-
NGC 3516 also displays a strong broad iron line that has sumptionforourpurposes.
been observed at high signal-to-noise with ASCA (Nandra et Thereisanother,independent,problemfacedbytheComp-
al. 1999). We have also examined constraints on the Comp- ton cloud model: it is very difficult to maintain the required
tonization model for this iron line. Continuum variability in ionization state. F95 treated this problem by considering the
thisobjectisobservedontimescalesdownto∼ 2000s(Edel- required cloud size necessary to acheive some critical ioniza-
son&Nandra1998;K.Nandra,privatecommunication),giving tion parameter ξc ≡ Lion/nR2. According to F95, the AGN
amaximumsizeofRcloud ∼2×1013cmfortheComptonizing spectrumofMathews&Ferland(1987),ξc = 104ergcms−1
cloud,ratherlargerthanthecaseofMCG−6-30-15.Also,Bep- canbeconsideredthepointatwhichaphotoionizedplasmabe-
poSAXobservationsfailtoseeasoftexcessintheX-rayspec- comes completely ionized. Using the observed luminosity of
trumallofthewaydownto0.2keV(Stirpeetal.1998).Noting MCG−6-30-15,they deducedthat the cloud must have a size
that L ≈ 1×1044ergs−1 producesthe constraintdiagram R<1014cminordertoachieveatleastthiscriticalionization
pl
showninFig.1b. Itisseenthattheseconstraintseliminateall parameter. As we will now show, this is a very conservative
butaverysmallregionofparameterspace. Thus,althoughthe argumentand, in fact, ionization balance imposes much more
broad line in NGC 3516 could in principle be explained with severelimitsonthecloudsize.
theComptonizationmodel,theamountoffinetuningnecessary Whiletheformalionizationparametermaybeveryhigh,the
forfindingthelineparametersmakesthemodelimprobablein very soft continuum spectrum postulated by MS99 may still
thiscase. havetroublefullyionizingtheironthroughoutthewholecloud.
Toseethis,notethatallcontinuumphotonscapableofionizing
4. DISCUSSION hydrogenlike iron (FeXXVI) reside in the power law compo-
Itshouldbestressedthatwehaveusedconservativeparam- nentofthecontinuum. ThecontinuumsourceinMCG−6-30-
eters in our assessment of these observational constraints. In 15emitsFeXXVIionizingphotonsatarate
particular,weassumethatthepower-lawcomponentofthecon-
L
tinuum emission possesses an energy index of α = 1 (corre- Nion ≈ pl , (11)
E Ξ
spondingtoaphotonindexofΓ =2)andahighenergycutoff ion
of 50keV. In fact, the overall X-ray spectrum is harder than where Eion = 9.3keV is the ionization potential of FeXXVI.
this (especiallyoncethe Comptonreflectioncomponentis ac- ThisevaluatestoN ≈3×1050s−1. Theradiativerecombi-
ion
countedfor)andthehighenergycutoffmaywelloccuratrather nationrateofthepostulatedComptoncloud,ontheotherhand,
4
isgivenby samepartsoftheComptoncloudthatbroadenstheironline(in
order to Compton cool it), one concludes that the black body
4π T −Xrad photonsand broad iron lines photonswill follow very similar
Nrec ≈ 3 R3n2Arad(cid:18)104K(cid:19) (12) paths through the system. Hence, it is impossible to hide the
soft excess emission from view in a system in which we ob-
serveaComptonbroadenedironline.
wherethecoefficientsA andX aregivenbyShull&van
rad rad Thirdly,theblackbodylimitcanbebypassedifthesoftcon-
Steenberg (1982). For a temperature of kT = 0.5keV and
tinuumsourceisplacedoutsideoftheComptoncloud.Whileit
R =1012cm,thisgivesN =3×1050s−1. Thus,thereare
rec isdifficulttoconstructrigorousargumentsagainstthiscase,we
just enough ionizing photons present in the entire power law
considerthatplacingapowerful(L /L >3)softcontinuum
tail toionize the hydrogen-likeiron. Ifthe temperatureof the bb pl
sourceatlargedistancesfromthecentralhardX-raycontinuum
Compton cloud is below 0.5keV, or the radiusof the cloud is
sourceisanad-hocsolution.
larger4, it will be impossible to photoionize the cloud. Very
large iron edges would then be present in the observed X-ray
5. CONCLUSIONS
spectrum, contrary to observations. Thus, ionization balance
imposesasizelimitofR ∼< 1012cm,independentlyofcontin- Inthiswork,wehaveconstrainedtheComptoncloudmodel
forthe broadironline in bothMCG−6-30-15andNGC 3516
uumvariabilityconstraints.
by considering two observational constraints which are inde-
Finally, we address whether there are reasonable modifica-
pendent of the detection of a spectral break in the continuum
tionsthatcanbemadetotheMS99scenariothatwillavoidthe
spectrum: thecontinuumvariabilitytimescaleandtheabsence
constraintsimposedinthispaper. Therearethreesuchmodifi-
ofanobservedsoftexcess. Wehavethendemonstratedthatthe
cationsthatweshouldconsider.Firstly,ifthegeometryissuch
constrainedmodelrequiresacontinuumsourcewhichviolates
thattheX-raycontinuumsourceisvieweddirectly(ratherthan
the black body limit. We also point out that the difficulty of
throughtheComptoncloud),onemightimaginethatthesizeof
photoionizingtheComptoncloudtotherequiredlevels. Thus,
theComptoncloudandtheX-raycontinuumvariabilitywould
weruleouttheComptonizationmodelforthebroadironlinein
bedecoupledtherebyrelaxingtheconstraintsdiscussedabove.
MCG−6-30-15,andshow thatfine tuningis requiredin order
AnexampleofsuchageometryisiftheComptoncloudforms
for the model to explain the line in NGC 3516. We conclude
atorusaroundthecentralX-raysource.Insuchageometry,the
thatthe combinationof relativistic Dopplershiftsand gravita-
X-ray continuum source illuminates and ionizes the observed
tionalredshiftsstillprovidesthebestexplanationforthebroad
face of the Compton cloud and powers iron line fluorescence
from an optical depth of τ ∼ 4 into the cloud. However, in ironlinesseeninAGN.
thiscase,onewouldexpectionizedironlines(fromtheionized
zonesthatoverlaythenear-neutralzonesintheComptoncloud) We are indebted to Jim Chiang, Andrew Fabian, Mike
ratherthanthe observedcoldironlines. Also, the illuminated Nowak, and Firoza Sutaria for insightfuldiscussionsthrough-
surface of the Compton cloud, which must be highly ionized outthecourseofthiswork. Wearealsogratefultotheanony-
so as notto be a strong narrowironline emitter, wouldact as mousrefereeforseveralusefulsuggestions. We thanktheAs-
aComptonmirrorandsmearouttheobservedcontinuumvari- penCenterforPhysicsfortheirhospitalityduringtheX-rayAs-
ability, even though the continuum source is viewed directly. trophysicsWorkshopinAugust1999,atwhichtimethiswork
Of course, anysuchmodificationto the basic Comptonization wasstarted. CSRappreciatessupportfromHubbleFellowship
modelinwhichtheComptoncloudisallowedtobebiggerthan grantHF-01113.01-98A.ThisgrantwasawardedbytheSpace
R ∼ 1012cm must be subject to the ionization problem de- Telescope Institute, which is operated by the Association of
scribedabove. UniversitiesforResearchinAstronomy,Inc.,forNASAunder
Secondly, a large regionof parameterspace would openup contractNAS5-26555.WealsoappreciatesupportfromNASA
iftheComptoncloudexperiencedadifferentsoftcontinuumto under LTSA grant NAG5-6337 and the RXTE guest observer
thatobserved(i.e.ifthesoftexcesscanbe‘hidden’fromview). grantNAG5-7339aswellasDeutscheForschungsgemeinschaft
Notingthattheblackbodycomponentmustscatterthoughthe grantSta173/22.
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