Anatomy of the AGN in NGC 5548. VI. Long-term variability of the warm absorber

20 Pages·2016·0.48 MB·English

Astronomy&Astrophysicsmanuscriptno.ngc5548_archival (cid:13)cESO2016
January12,2016
Anatomy of the AGN in NGC 5548
VI. Long-term variability of the warm absorber
J.Ebrero1,J.S.Kaastra2,3,G.A.Kriss4,5,L.DiGesu2,E.Costantini2,M.Mehdipour2,S.Bianchi6,M.Cappi7,R.
Boissay8,G.Branduardi-Raymont9,P.-O.Petrucci10,11,G.Ponti12,F.PozoNúñez13,H.Seta14,K.C.Steenbrugge15,
andM.Whewell9
1 XMM-NewtonScienceOperationsCentre,ESAC,CaminoBajodelCastillos/n,Urb.VillafrancadelCastillo,28692Villanueva
6
delaCañada,Madrid,Spain
1
e-mail:[email protected]
0
2 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CA,Utrecht,TheNetherlands
2
3 LeidenObservatory,LeidenUniversity,P.O.Box9513,2300RA,Leiden,TheNetherlands

n 4 SpaceTelescopeScienceInstitute,3700SanMartinDrive,Baltimore,MD21218,USA
a 5 DepartmentofPhysicsandAstronomy,TheJohnsHopkinsUniversity,Baltimore,MD21218,USA
J 6 DipartimentodiMatematicaeFisica,UniversitàdegliStudiRomaTre,ViadellaVascaNavale84,00146,Roma,Italy
1 7 INAF-IASFBologna,ViaGobetti101,40129Bologna,Italy
1 8 DepartmentofAstronomy,UniversityofGeneva,16Ch.d’Ecogia,1290Versoix,Switzerland
9 MullardSpaceScienceLaboratory,UniversityCollegeLondon,HolmburySt.Mary,Dorking,Surrey,RH56NT,UK
] 10 UniversityGrenobleAlpes,IPAG,38000Grenoble,France
E 11 CNRS,IPAG,38000Grenoble,France
H 12 Max-Planck-InstitutfürExtraterretrischePhysik,Giessenbachstrasse,85748Garching,Germany
13 AstronomischesInstitut,Ruhr-UniversitätBochum,Universitätstrasse150,44801Bochum,Germany
.
h 14 DepartmentofPhysics,TokyoMetropolitanUniversity,1-1Minami-Osawa,Hachioji,Tokyo,Japan
p 15 InstitutodeAstronomía,UniversidadCatólicadelNorte,AvenidaAngamos0610,Casilla1280,Antofagasta,Chile
-
o Received<date>;accepted<date>
r
t
s
a ABSTRACT
[
Context.We observed the archetypal Seyfert 1 galaxy NGC 5548 in 2013-2014 in the context of an extensive multiwavelength

1 campaigninvolvingseveralsatellites,whichrevealedthesourcetobeinanextraordinarystateofpersistentheavyobscuration.
v Aims.Were-analyzedthearchivalgratingspectraobtainedbyXMM-NewtonandChandrabetween1999and2007inordertochar-
5 acterizetheclassicwarmabsorber(WA)usingconsistentmodelsandup-to-datephotoionizationcodesandatomicphysicsdatabases
8 andtoconstructabaselinemodelthatcanbeusedasatemplateforthephysicalstateoftheWAinthe2013observations.
3 Methods.WeusedthelatestversionofthephotoionizationcodeCLOUDYandtheSPEXfittingpackagetomodeltheX-raygrating
2 spectraofthedifferentarchivalobservationsofNGC5548.
0 Results.WefindthattheWAinNGC5548iscomposedofsixdistinctionizationphasesoutflowinginfourkinematicregimes.The
. componentsseemtobeintheformofastratifiedwindwithseverallayersintersectedbyourlineofsight.Assumingthatthechanges
1
intheWAaresolelyduetoionizationorrecombinationprocessesinresponsetovariationsintheionizingfluxamongthedifferent
0
observations,weareabletoestimatelowerlimitsonthedensityoftheabsorbinggas,findingthatthefarthestcomponentsareless
6
denseandhavealowerionization.Theselimitsareusedtoputstringentupper limitsonthedistanceoftheWAcomponentsfrom
1
thecentralionizingsource,withthelowestionizationphasesatseveralpcdistances(<50,<20,and<5pc,respectively),whilethe
:
v intermediatelyionizedcomponentslieatpc-scaledistancesfromthecenter(<3.6and<2.2pc,respectively).Thehighestionization
i component islocatedat∼ 0.6pcorclosertotheAGNcentralengine.ThemassoutflowratesummedoverallWAcomponents is
X ∼ 0.3M yr−1,aboutsixtimesthenominal accretionrateofthesource.Thetotalkineticluminosityinjectedintothesurrounding
⊙
r mediumisasmallfraction(∼0.03%)ofthebolometricluminosityofthesource.AfteraddingthecontributionoftheUVabsorbers,
a thisvalueaugmentsto∼0.2%ofthebolometricluminosity,wellbelowtheminimumamountofenergyrequiredbycurrentfeedback
modelstoregulategalaxyevolution.
Keywords. X-rays:galaxies–galaxies:active–galaxies:Seyfert–galaxies:individual:NGC5548–techniques:spectroscopic
1. Introduction coronainthe vicinityoftheSMBH (e.g.,Sunyaev&Titarchuk
1980; Haardt&Maraschi 1993). With the advent of medium-
Active galactic nuclei (AGN) are powered by gravitational ac- and high-resolution X-ray spectroscopy, it was discovered that
cretion of matter on to the central supermassive black hole absorptionlinesofphotoionizedspeciesblueshiftedwithrespect
(SMBH) that resides in their center (Rees 1984), emitting vast tothesystemicvelocityofthehostgalaxies(Kaastraetal.2000),
amounts of energy across the spectrum. X-ray emission is a were present in the soft X-ray spectra of more than 50% of
characteristicfeatureofAGNspectraandfindsitsoriginin the nearbySeyfert1 galaxies(Reynolds1997;Georgeetal. 1998).
inverse Compton scattering of lower energy photons in a hot This so-called warm absorber (WA) typically shows multiple
Articlenumber,page1of19
A&Aproofs:manuscriptno.ngc5548_archival
ionizationphaseswithtemperatures∼104−106K,columnden- 1
sitiesrangingfrom1020−1024cm−2,andoutflowvelocitiesofa
CLOUDY v.C13 (this work)
fewhundredtothousandsofkms−1(e.g.,Blustinetal.2005). XSTAR v1 (Steenbrugge et al. 2005)
CLOUDY v.C98
WhiletheirubiquitysuggeststhatWAareanimportantstruc-
0 XSTAR v2.2.1bk
tural feature in AGN, their contribution as sources of cosmic
feedback(the relation betweenthe SMBH growthand star for-
mation in the host galaxy) has been a matter of debate for V)
e
many years. Recent studies seem to indicate that the contribu- T (k −1
tionofWAwithlow-to-moderatevelocitiesisnegligibleinterms g
o
of feedback, with the exception of the so-called ultra-fast out- l
flows (UFOs), which would be able to carry enough momen-
2
tum to be a significant source of feedback (see Tombesietal. −
2013, and references therein). Current models show that ki-
neticluminosities(kineticenergycarriedbytheoutflowperunit
time) ranging between 0.5% and 5% of the bolometric lumi-
3
nosity (DiMatteoetal. 2005; Hopkins&Elvis 2010) must be − −0.5 0 0.5 1 1.5 2
log (ξ/T)
fed back in order to significantly affect the interstellar medium
(ISM)ofthegalaxyandreproducetheobservedM−σrelation Fig. 1. Stability curves obtained with different versions of XSTAR
(Ferrarese&Merritt2000;Gebhardtetal.2000). andCLOUDY(blacksolidline,CLOUDYv13;blackdot-dashedline,
CLOUDY v98; red dashed line, XSTAR v1; red dotted line, XSTAR
Akeyparameterforreliablyestimatingthemassoutflowrate
v2.2.1bk)forthesameinputspectralenergydistribution.Thesoliddots
(hence the kinetic luminosity injected into the ISM) is the ac- representtheWAcomponentsofSteenbruggeetal.(2005).
tuallocationoftheWA.Currentscenariosfortheoriginofthese
ionizedoutflowsare:(i)disk-drivenwinds,whicharelaunched
within hundredsof gravitationalradii (Murray&Chiang 1995; Thispaperisorganizedasfollows.InSect.2webrieflysum-
Elvis2000); and(ii) thermalwindsevaporatingfroman irradi- marize the motivation for the re-analysis of the grating X-ray
ateddustytorusthatsurroundstheAGNengine,whichlieatpc- spectra of NGC 5548, and the observations used in this paper.
scale distances (Krolik&Kriss 2001). Ideally, tight limits can InSect.3 we describethe analysisof thedata,while inSect. 4
be put on the distance by measuring variability on the ioniza- we discuss the results. Our conclusionsare reportedin Sect. 5.
tionpropertiesoftheWAinresponsetochangesintheincident Throughout this paper we assume a cosmological framework
ionizing flux (e.g., Detmersetal. 2008; Longinottietal. 2010; with H0 = 70 km s−1 Mpc−1, ΩM = 0.3, and ΩΛ = 0.7. The
Kaastraetal. 2012). High-density gas will ionize/recombine quotederrorsrefer to 68.3%confidencelevel (∆C = 1 for one
faster than low-density gas, allowing us to determine reliable parameterofinterest)unlessotherwisestated.
lower limits to the density and hence strict upper limits on the
distance where this gas crosses our line of sight. This is, how-
2. NGC5548revisited
ever, very challenging because the observations of a given ob-
jectareoftensparse,washingoutthepossibleeffectsofvariabil- 2.1.Motivation
ity,andlackingthesignal-to-noiseratiorequiredtosignificantly
The reasons to carry out a comprehensive re-analysis of the
measuretheexpectedchanges.
archival high-resolutiongrating spectra of NGC 5548 are two-
NGC 5548 is an archetypal Seyfert-1 galaxy (M =
BH fold.Themainconsequenceoftheextraordinaryobscurationob-
3.24 × 107 M , Pancoastetal. 2014), the first in which nar-
⊙ servedduringthe2013campaign(Kaastraetal.2014)isthede-
rowabsorptionlinesintheX-raysweredetected(Kaastraetal.
pressionofthesoftX-raycontinuum,whichmakesthedetection
2000). Since then, its ionizedoutflow has beenwidely studied,
ofWA featuresverychallenging,althoughitgivesusanexcel-
bothintheX-rays(Kaastraetal.2002;Steenbruggeetal.2003;
lentopportunitytostudythe X-rayemissionofthe narrowline
Steenbruggeetal. 2005; Detmersetal. 2008; Detmersetal.
region (NLR) of a Seyfert 1 galaxy with unprecedented detail
2009;Krongoldetal.2010;Andrade-Velázquezetal.2010)and
(Whewelletal. 2015). The depressed X-ray flux is not intrin-
in the ultraviolet(Crenshaw&Kraemer 1999; Brothertonetal.
sically low, since the hard X-ray continuum is comparable to
2002; Aravetal. 2002; Crenshawetal. 2003; Crenshawetal.
the historicalfluxesofNGC 5548in thatenergyband,butit is
2009).
causedbythepresenceofobscuringmaterialcrossingourlineof
Followingthe success of the multiwavelengthcampaignon sight.Moreover,accordingtoSwiftmonitoring,thisobscuration
Mrk509(introducedinKaastraetal.2011),NGC5548wasob- haslastedsinceatleastFebruary2012(Mehdipouretal.2015b).
served extensively in 2013-2014 in the context of a large ded- Ananalysisofthedatafromourmultiwavelengthcampaignre-
icated multiwavelength campaign led by XMM-Newton, com- vealsthattheobscurer,possiblyintheformofaclumpystream
prisingalsoobservationswithChandra,NuSTAR,INTEGRAL, ofgas,is locatedwithina few lightdaysof thecentralSMBH,
HST-COS, and including Swift monitoring, with the objec- extendingtooutsideoftheBLRbutcloserinthantheexpected
tive of characterizing the properties of the WA with unprece- location of the WA (Kaastraetal. 2014). The ionizing photons
dented detail. An overview of the campaign can be found in thatreachtheWAarethereforeseverelydiminished.
Mehdipouretal. (2015a)(PaperI).Interestingly,thesoftX-ray BycomparingthepropertiesoftheWAwiththosemeasured
flux of NGC 5548 during the campaign was remarkably low, earlier, we can take advantage of this unexpected situation to
about25timeslowerthanthemedianfluxofthesourceinpre- measuretheresponseoftheionizedgas,givingusanopportunity
viousobservations.Ouranalysissuggeststhatthisiscausedby to constrain the location of the WA componentswith unprece-
along-lastingstreamofobscuringmaterialsituatedatlight-day dentedaccuracy.This, however,posesanimportantproblemas
distances,partiallycoveringtheX-raysourceandthebroadline very few counts are left in the soft X-ray spectrum (where the
region(BLR),asreportedinKaastraetal.(2014). bulkof theWA featureslie) in spite ofthe largeexposuretime
Articlenumber,page2of19
J.Ebreroetal.:AnatomyoftheAGNinNGC5548.VI.
(∼660kswithXMM-NewtonRGSatthecoreofthecampaign). 0089960401) for ∼ 96 and 36 ks, respectively. In this
OurstrategytomodeltheWAduring2013reliedontheassump- paper we used solely the Reflection Grating Spectrometer
tionthatmostoftheparametersoftheclassicpersistentWAout- (RGS, denHerderetal. 2001) data, that were processed us-
flow willremainthe same with respectto the archivalobserva- ing the XMM-NewtonScience Analysis System (SAS) v13.5
tions,atleastthenumberofcomponents,theiroutflowvelocity, (Gabrieletal. 2004). The data were rebinned by a factor of 3
broadening of the lines and, possibly, their column density. If toavoidoversampling.Inwhatfollows,theRGSspectrawillbe
onegetsanaccuratecharacterizationoftheWAfrompastobser- identifiedinthepaperasR00x,R01y,andR01z,respectively.
vations,thisbaselinemodelcanthenbeappliedtotheobscured ThereareninearchivalobservationsofNGC5548takenwith
spectrumofNGC5548,dramaticallydecreasingthatnumberof thegratingsonboardChandra,sevenwiththeLowEnergyTrans-
freeparametersthatwouldpreventaspectralanalysisotherwise. mission Grating Spectrometer (LETGS, Brinkmanetal. 2000)
Our second motivation stems from the fact that different and two with the High EnergyGrating Spectrometer (HETGS,
atomic physics codes were used to derive the WA properties Canizaresetal. 2005). The HETGS/ACIS-S spectra were ex-
reported in the literature. The ionization state of the WA com- tractedfromtheTGCatarchive3.Thedatawerebinnedbyafac-
ponents is usually computed using a photoionization code that tor of two, and the Gehrels-statistic used by defaultin HETGS
calculates the transmission through a slab of material where data was replaced by the true Poissonian error. In order to ob-
all ionic column densities are linked through a photoioniza- tainconsistentresultsfortheMEGandHEGgratings,theMEG
tion balance model. Previous modeling of the NGC 5548 WA fluxeswerescaledbyafactorof0.954relativetotheHEGfluxes.
made use of XSTAR1 (Kallman&Krolik 1999) or CLOUDY2 Theobservationswereperformedin 2000,with an exposureof
(Ferlandetal.2013),whichuseslightlydifferentatomicphysics 82 ks, and in 2002 as part of an UV/X-ray campaign with an
databases. Furthermore, since past observations of NGC 5548 exposure of 153 ks (Steenbruggeetal. 2005). Throughout this
span many years, the published WA parameters are computed papertheseobservationswillbeidentifiedasH00andH02,re-
usingdifferentversionsofthesecodes,whichevolveandareup- spectively.
datedwithtime. The LETGS/HRC-S spectra were reduced using the Chan-
Differences between photoionizationcodes and versions of dra Interactive Analysis of Observations software (CIAO,
codes are not negligible. To illustrate this, in Fig. 1 we show Fruscioneetal. 2006) v4.5, until the level 1.5 event files were
fourstabilitycurvesobtainedfromtheoutputionizationbalance created. The rest of the procedure, until the creation of the fi-
providedby differentversionsof XSTAR and CLOUDY. In all nallevel2eventfiles,wasperformedfollowinganindependent
cases the input spectral energy distribution (SED) was that of methodfirstreportedinKaastraetal. (2002).Thisresultsinan
NGC5548reportedbySteenbruggeetal.(2005).Whilethefour improvedwavelengthaccuracydeterminationandeffectivearea
lines agree reasonablywell at low ionization values, the differ- generation.NGC5548wasobservedwithChandraLETGSonce
ences between them are dramatic for higher values of the ion- in1999for86ks(labeledasL99hereafter),andtwicein2002,
ization parameter ξ. In particular, the ionization balance used 2005,and 2007. In the latter three cases the observationswere
by Steenbruggeetal. (2005), computedwith XSTAR v1, lacks takeninconsecutiveorbits,allowingustoco-addthetwospectra
several unstable areas (the branches with negative slope) and toobtainahighersignal-to-noise.Therefore,weendedupwith
the Compton shoulderat high temperatures.Even the more re- a ∼ 340ks spectrumin 2002,∼ 141ks in 2005,and ∼ 138 ks
centversionsofCLOUDYandXSTAR(v13andv2.2.1,respec- in2007,thatarelabeledthroughoutthepaperasL02,L05,and
tively),althoughtheyreproducethesameoverallshapereason- L07,respectively.
ablywell,showstrongdeviationswithrespecttoeachother.The
solid dots overplotted on top of the curves represent the posi-
3. Dataanalysis
tionswheretheWA componentsdetectedbySteenbruggeetal.
(2005) fall, showing that a fit to the same spectrum with such 3.1.Strategy
differentionizationbalancesprovidesignificantlydifferention-
ThearchivalobservationsofNGC5548sumupto∼ 1.1Msof
izationparameterξvalues.
exposure time of grating spectra, although with great disparity
Therefore,inordertoobtainacomprehensivepictureofthe
indataqualityamongthedifferentdatasets.Amodelingstrategy
WApropertiesthatcanbeusedasatemplateforthe2013-2014
that makes the most of this large amount of data is therefore
campaign as well as consistentparameter values, it is essential
requiredinordertomaximizethescientificreturn.Ononehand,
tore-analyzethearchivalgratingspectraofNGC5548usingthe
fits to the individual observations in which all the parameters
same,andmostup-to-date,photoionizationcodeforallofthem.
are left free would lead to too large error bars for most of the
relevantparameters,orin thoseobservationswith a lowS/N in
2.2.Archivaldata completelyunconstrainedvalues.Ontheotherhand,ajointfitof
allthespectratogether(3RGS,2HETGS,and4LETGS)isnot
We used a total of 12 Chandraand XMM-Newtonindividual feasible, as it requiresa prohibitive amountof computing time
archivalobservationsin this paper.In some cases, the observa- dueto the largenumberof data channelsand modelsthatneed
tionsweretakeninconsecutiveorbitsandwiththesameinstru- tobeevaluatedtogether.
mentalconfiguration,allowingustoco-addthespectrainorder We thereforeadopteda mixedapproachformodelingthese
toobtainabettersignal-to-noiseratio.Belowwebrieflydescribe data.Sincethe2002dataconstitutethebulkofthearchivaldata
each of the observations. The observationslog is shown in Ta- (∼ 500 ks taking the L02 and H02 observations together), and
ble1. they are dominant in terms of S/N contribution, we used them
Priortothe 2013campaign,NGC5548was observedthree to create a model template for the rest of the archival obser-
times by XMM-Newton, once in 2000 (ObsID 0109960101) vations. We first obtained a best-fit model for the 2002 data,
for 26.1 ks, and twice in 2001 (ObsIDs 0089960301 and including the continuum, broad and narrow emission lines, ra-
diativerecombinationcontinua(RRCs),andabsorptiontroughs
1 http://heasarc.gsfc.nasa.gov/docs/software/xstar/xstar.html
2 http://www.nublado.org 3 http://tgcat.mit.edu
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Table1.NGC5548archivalobservationslog.
Observatory Grating ID StartTime(UTC) Exp.Time Labela
(yyyy-mm-dd hh:mm:ss) (ks)
XMM-Newton RGS 0109960101 2000-12-24 22:12:11 26.1 R00x
XMM-Newton RGS 0089960301 2001-07-09 15:45:59 95.8 R01y
XMM-Newton RGS 0089960401 2001-07-12 07:34:56 39.1 R01z
Chandra LETG 330 1999-12-11 22:51:20 86.0 L99
Chandra HETG 837 2000-02-05 15:37:29 82.3 H00
Chandra HETG 3046 2002-01-16 06:12:34 153.4 H02
Chandra LETG 3045 2002-01-18 15:57:02 169.4 L02b
Chandra LETG 3383 2002-01-21 07:33:57 171.0 L02b
Chandra LETG 5598 2005-04-15 05:18:18 116.4 L05b
Chandra LETG 6268 2005-04-18 00:31:12 25.2 L05b
Chandra LETG 7722 2007-08-14 20:57:57 99.6 L07b
Chandra LETG 8600 2007-08-17 03:54:51 37.2 L07b
Notes.(a)Labelusedtoidentifythespectrathroughoutthepaper; (b)Observationswiththesamelabelareco-addedtoproduceasinglespectrum
foranalysis.
produced by the WA (see the detailed modeling in Sect. 3.2).
This best-fit model was then used as a starting point to ana- 0−11
1
lyze the rest of the archivalspectra, leaving as free parameters
onlythoseofthecontinuum,thefluxoftheOviiforbiddenline,
andtheionizationparameterlogξ ofeachWA component.The
Ovii forbidden line flux was set free since it is known to vary 0−12
(toDemtmeaesrusreetsaigl.n2ifi00ca9n),twchhaenregaesstohrearreestnoofttehveenlinseigsnairfiectaonotlwyedaek- cm)−2 1
tected (partiallydue tothe wavenlengthcoverage,partiallydue g s −1
tothelowerS/N).Therefore,allotherparametersdescribingthe er
emission lines as well as the absorption lines (NH, outflow ve- νF (ν 0−13
1
locity,velocitybroadening)arekeptfrozen.
With this strategy we aim at determining the most relevant
parametersthatareknowntovarybetweenobservations,suchas
the spectralshape of the continuumand the WA ionizationpa- 10−141013 1014 1015 1016 1017 1018 1019
rameter.The dynamicsof the persistentoutflowsin NGC 5548 ν (Hz)
havebarelychangedover∼20years,i.e.thedifferentkinematic
Fig.2.SpectralenergydistributionofNGC5548.Datapointsarethose
componentscanbeeasilycross-identifiedbetweenthedifferent
ofSteenbruggeetal.(2005).
observations,buttheirionizationstatecanbesubstantiallydiffer-
ent.WethereforeassumethattheobservedvariationsintheWA
aremainlyduetoionizationchangesratherthanbeingcausedby 3.2.1. TheChandra2002spectra
thedissapearanceofanyoftheseoutflows,ortheappearanceof
newerones. AsmentionedinSect.3.1,wemodeledtheL02andH02datasets
inordertoobtaina baselinemodeltemplatethatcouldbeused
asastartingpointtoanalyzetheremainingarchivalspectra.The
methodologyreportedbelowwasfirstlydescribedinthesupple-
3.2.Modeling
mentarymaterial of Kaastraetal. (2014). The fits were carried
outin the 2−60 Å range for the LETGS spectrum,and in the
The archival grating spectra of NGC 5548 were analyzed
1.55−15.5Åand2.5−26ÅrangefortheHEGandMEGspec-
using the SPEX fitting package4 version 2.05 (Kaastraetal.
tra,respectively.Thecontinuumwasmodeledwithapowerlaw
1996). The fitting method was C-statistics (Cash 1979). The
plusa modified black body(mbb modelin SPEX), which con-
adopted cosmological redshift for NGC 5548 was z =
sidersmodificationsofa simple blackbodymodelby coherent
0.017175(deVaucouleursetal.1991).TheforegroundGalactic
ComptonscatteringbasedonthecalculationsofKaastra&Barr
columndensitywassettoN =1.45×1020cm−2(Wakkeretal.
H (1989).
2011)usingthehotmodelinSPEX,withthetemperaturefixed
Superimposedtothe continuumweaddedfourbroademis-
to 0.5 eV to mimic a neutral gas. Throughout the analysis we
sionlines,modeledwithGaussians,representingthe1s−2pand
assumedproto-SolarabundancesofLodders&Palme(2009).
1s−3ptransitionsofOvii,andthe1s−2ptransitionofOviiiand
Cvi.Theirparametervaluesweretakenfromthepreviousanal-
ysis of these datasets by Steenbruggeetal. (2005). In addition
tothemweincludedasetofnarrowemissionlines,namelythe
4 http://www.sron.nl/spex He-like triplets of Neix, Ovii, and Nvi, the forbidden line of
Articlenumber,page4of19
J.Ebreroetal.:AnatomyoftheAGNinNGC5548.VI.
L02 L02
Si XIII Mg XI Mg XII Mg XIMg IX Ne X Ne IX Ne IX Ne X Ne IXNe VIIINe VII O VIIIO VIII O VIII O VIIO VII O VII O VIIO VIII N VII O VII O VIO V
0 0
Å−1 1 Å−1 1
m s −2−1 m s −2−1
c c
nts nts
u u
o o
C 5 C 5
0 −4 0 −4
1 1
x x
0 0
6 8 10 12 14 16 18 20 22 24
Observed wavelength (Å) Observed wavelength (Å)
L02 L02
N VIIN VI C VI C VIN VI S XIII C V C VI C V S XII S X C V Si XII
0 0
Å−1 1 Å−1 1
m s −2−1 m s −2−1
c c
nts nts
u u
o o
C 5 C 5
0 −4 0 −4
1 1
x x
0 0
24 26 28 30 32 34 36 38 40 42
Observed wavelength (Å) Observed wavelength (Å)
L02 L02
S X S XI Si XI Si X S VIII S VII S IXS IX Mg IX S IX Si VIII Si X Si IX Mg VIII Ne VIII Si VIII
0 0
Å−1 1 Å−1 1
m s −2−1 m s −2−1
c c
nts nts
u u
o o
C 5 C 5
0 −4 0 −4
1 1
x x
0 0
42 44 46 48 50 52 54 56 58 60
Observed wavelength (Å) Observed wavelength (Å)
Fig.3.ChandraLETGSspectrumofNGC5548in2002(L02).Thesolidredlinerepresentsourbest-fitmodel.Somerelevant WAabsorption
featureshavebeenlabeled.
the He-like Cv, and the 1s− 2p transitions of Nex and Nvii, ther information we point the reader to Whewelletal. (2015),
as well as radiative recombination continua (RRCs). With the whopresentadetailedanalysisofthenarrowlinesandRRCsin
exceptionoftheforbiddenlinesofOviiandCv,theselinesare NGC5548.
difficulttodetectintheL02andH02spectra.Thereforewefixed
theirnormalizationtothevaluesobtainedinthe2013campaign TheWAwasmodeledusingthexabsmodelinSPEX,which
(Whewelletal.2015),wherethelinesareclearlyvisibleagainst calculates the transmission of a slab of material where all the
the strongly obscured continuum. The narrow lines and RRCs ioniccolumndensitiesarelinkedthroughaphotoionizationbal-
are convolved with a velocity dispersion of σ = 463 km s−1 ancemodel.AsshowninSect. 2.1,the choiceofthe photoion-
v
(seethesupplementarymaterialinKaastraetal.2014).Forfur- izationcodethat computesthis modelis critical. Here we used
CLOUDY version 13.01, and we provided as input the spec-
Articlenumber,page5of19
A&Aproofs:manuscriptno.ngc5548_archival
Table2.Continuumbest-fitparametersofthe2002observations.
Component Parameter LETGS2002(L02) HETGS2002(H02)
Powerlaw Photonindex 1.70±0.01 1.65±0.01
Flux(0.2−10keV)a 5.40±0.06×10−11 4.85±0.03×10−11
Luminosity(0.2−10keV)b 4.21±0.05×1043 3.66±0.05×1043
Mod.blackbody Temperaturec 0.140±0.004 0.115±0.010
Flux(0.2−10keV)a 2.8±0.2×10−12 5.4±2.3×10−12
Luminosity(0.2−10keV)b 3.2±0.2×1042 6.1±2.6×1042
Notes.(a)Inunitsofergcm−2s−1; (b)inunitsofergs−1; (c)inunitsofkeV.
Table3.Emissionfeaturesbest-fitparametersofthe2002observations.Valueswithoutuncertaintiesarekeptfrozen.
Component Line Restwavelengthb FWHMb Luminosityc
Broademission Cvi1s−2p 33.74 0.90 1.9
linesa Ovii1s−2p 21.60 0.58 3.4
Ovii1s−3p 18.63 0.50 1.4
Oviii1s−2p 18.97 0.51 2.8
Component Line Restwavelengthb σ d Luminosityc
v
Narrowemission Ovii f 22.101 382 4.7±0.4
lines Cv f 41.472 382 2.3±0.1
Component Line Restwavelengthb σ d Luminosityc
v
Narrowemission Nex1s−2p 12.134 463 0.09
linese Neixr 13.447 ′′ 0.22
Neixi 13.553 ′′ 0.14
Neix f 13.699 ′′ 0.78
Oviii1s−3p 16.006 ′′ 0.27
Oviir 21.602 ′′ 1.09
Oviii 21.807 ′′ 1.07
Nvii1s−2p 24.781 ′′ 0.33
Nvir 28.787 ′′ 0.26
Nvii 29.084 ′′ 0.25
Nvi f 29.534 ′′ 0.60
Component Ion Emissionmeasuref σ d Temperatureg
v
Radiativerecombination Cvi 90 463 5.6
continua(RRC)e Cvii 61 ′′ ′′
Nvii 11 ′′ ′′
Oviii 29 ′′ ′′
Oix 7.3 ′′ ′′
Nex 4.3 ′′ ′′
Nexi 1.6 ′′ ′′
Notes.(a)ValuesfromSteenbruggeetal.(2005); (b)inunitsofÅ; (c)inunitsof1040ergs−1; (d)convolvedvelocitydispersion,inunitsofkms−1;
(e)valuesfromWhewelletal.(2015); (f)inunitsof1059cm−3; (g)inunitsofeV.
tralenergydistribution(SED)of Steenbruggeetal. (2005)(see thehydrogencolumndensity N ,theoutflowvelocityv , and
H out
Fig. 2). The output photoionization balance was provided to theGaussianturbulentvelocityσ ,whilethecoveringfactorwas
v
SPEXtofittheabsorptiontroughscausedbyaninterveningWA. fixedtounity.Theionizationparameterξisameasureoftheion-
Since Steenbruggeetal. (2005) used an older version of their izationstateofthegas,anditisdefinedas
photoionizationcode,welookedforfurtherWAcomponentsthat
mighthaveremainedundetectedintheiranalysis.
ξ= L/nR2, (1)
We started from the four WA components reported in
Steenbruggeetal. (2005), and added additional xabs compo- whereListheionizingluminosityinthe1−1000Rydrange,n
nents on a one-by-one basis until the fit no longer improved. is the hydrogendensity of the gas, and R is the distance of the
The xabs free parameters were the ionization parameter logξ, gastotheionizingsource.
Articlenumber,page6of19
J.Ebreroetal.:AnatomyoftheAGNinNGC5548.VI.
The L02 and H02 spectra were fit simultaneously, but we 2 keV) and hard (2−10 keV) fluxes, and their hardnessratios
allowedthepowerlawandmodifiedblackbodyparameterstobe (HR=(H−S)/(H+S)).
differentforeachdataset(toaccountforcontinuumvariability), In the 1999−2002 period, NGC 5548 experienceda max-
aswellastheionizationparameterslogξoftheWA(toaccount imum flux variability of a factor of ∼ 2, with a median flux
forpossibleshort-termvariations,astheobservationsweretaken of ∼ 5 × 10−11 erg cm−2 s−1. In 2005, the overall flux de-
3.5daysapart).Therestoftheparameterswerecoupledsothat creased by almost a factor of 3 with respect to the L02 obser-
theyhadthesamevaluesforbothdatasets.Weassumedthatthe vation,andevenfurtherin2007whenitreachedaminimumof
WA componentsabsorb the continuumand the broad emission ∼ 10−11 erg cm−2 s−1. The behavior of the soft (0.2− 2 keV)
lines,butnotthenarrowemissionlinesandRRCs. and hard (2 − 10 keV) X-ray fluxes is very similar, following
WefoundthattheWAinNGC5548iscomposedofsixdis- the same trends as the overall X-ray flux over the years. This
tinctionizationphases,labeledfromAto F inorderofincreas- suggests that the source variability is due to intrinsic changes
ing ionization. The final C-stat/d.o.f. was 2402/2090 for L02, rather than obscuration processes such as the one reported in
and2890/2560forH02.Thebest-fitvaluesofthejointL02and Kaastraetal. (2014). During the obscurationeventthe hard X-
H02analysisarereportedinTables2,3,and4.Thelogξvalues rayfluxstayedatthehistoricalvalueswhilethesoftX-rayflux
oftheseobservationspresentedheredifferfromthoseshownin decreaseddramatically.Furthermore,thissupportsourapproach
TableS2inthesupplementarymaterialofKaastraetal.2014by of using the 2002 SED for the WA analysis of all the archival
∼0.25dexduetoabuginCLOUDY,whichcausedtheionizing observations, as the variations only involved the normalization
flux to be calculated in the 1−∞ range instead of the conven- but did not significantly changed the shape of the SED. More
tionalrangeof 1−1000Ryd. The L02spectrum togetherwith detailed studies of the broadband continuum of NGC 5548 is
thebest-fitmodelareshowninFig.3. reported in Mehdipouretal. (2015a) (Paper I) and Ursinietal.
(2015)(PaperIII).Anin-depthdiscussionontheoriginandvari-
abilityofthesoftexcessinNGC5548isalsoreportedinPapers
3.2.2. Theremainingarchivalspectra IandVI(Mehdipouretal.2015a,b).
After obtaining a best-fit model for the 2002Chandraspectra,
we used it as a template for modeling the remaining archival 4.2.Thermalstabilityofthewarmabsorber
datasets. As we mentioned in Sect. 3.1, we left as free param-
eters in the fits only the continuumparameters(powerlaw and Some clues on the structure of the X-ray WA can be drawn
modifiedblackbodyemission),thefluxofthenarrowOviifor- fromthethermalstabilitycurvesofthephotoionizedgas.These
biddenemissionline,andthe ionizationparameterofeachWA curves, also known as cooling curves, are determined by the
component.Theremainingparameterswerekeptfixedto those ionizing SED that illuminates the gas. They are typically rep-
obtainedinthejointfitoftheL02andH02datasets.Inmodeling resented as plots of the pressure ionization parameter Ξ as a
theWA,theionizationbalanceusedasinputbyxabswasbased functionofthe electrontemperatureT. Thepressureionization
on the SED of Steenbruggeetal. (2005), because it is the one parameterΞ(Kroliketal.1981)isdefinedas
with a wider contemporaneous frequency coverage. We there-
foreadopteditasatemplateSEDfortherestofthearchivalob-
servations.Wenotethat,althoughideallyonewouldneedtode- Ξ= L/4πr2cp=ξ/4πckT, (2)
riveaSEDforeachoftheindividualobservations,theassump-
tionofatemplateSEDwillnotsignificantlyaffectthexabspa- where ξ is the ionization parameter defined in Eq. 1, c is the
speedoflight,kistheconstantofBoltzmann,andT istheelec-
rametersdetermination.Intheabsenceofhugespectralchanges
(such as those originated by obscuration events), the different tron temperature. The differentvalues of Ξ were computedus-
ing CLOUDY version 13.01, which provided a grid of ioniza-
SEDs may changetheir normalizationdue to the intrinsic vari-
tionparametersξandtheircorrespondingelectrontemperatures
abilityofthesourcebutnottheirshape,whichisthekeyfactor
T for a thin layer of gas irradiated by the ionizing continuum.
incomputingthexabsparameters.Thebest-fitresultsareshown
ForconsistencywiththeanalysisweperformedinSect.3.2,we
inTable5.
usedasinputSEDthatofSteenbruggeetal.(2005)forallofthe
TheionizationstructureoftheWAinNGC5548depictedin
archivalobservations,and assumedthe proto-Solarabundances
the 2002Chandraobservations,showingsix distinct ionization
ofLodders&Palme(2009).
components,is also seen in the rest of the archivaldatasets. In
The curve thus generated divides the Ξ − T plane in two
some cases, some of the componentsare either not detected or
regions: below the curve heating dominates cooling, whereas
areblendedwithothercomponentsduetoapoorsignal-to-noise
abovethecurvecoolingdominatesheating.Inthepointsonthe
ratioofthespectrum.ThisisthecaseforR00x(veryshortexpo-
curve,the heatingrate equalsthecoolingrateso thatthegasis
sure,∼ 26ks)orL07(verylowintrinsicflux).Amoredetailed
inthermalequilibrium.Thebranchesofthecurvewithnegative
view ofthe parametervariabilitybetweenthe differentarchival
derivativedT/dΞ < 0 (thosewherethecurveturnsbackwards)
observationsisdiscussedinSect.4.
areunstableagainstisobaricthermalperturbations(e.g.,vertical
displacementsfromthecurve).Thismeansthatfora photoion-
izedgasinanunstablebranch,ifitissubjecttoasmallpositive
4. Discussion
temperatureperturbationitstemperaturewillkeepincreasingun-
til it reaches a stable branch of the curve (those with positive
4.1.Spectralproperties
derivative dT/dΞ > 0). Conversely, a small negative tempera-
The archival observations of NGC 5548 span almost 8 years, tureperturbationwillcausethegastemperaturetodecreaseun-
withthebulkofthedatasetstakenintheperiod1999−2002.In til it falls in a stable region of the Ξ−T plane. Moreover,gas
Fig.4weshowtheevolutionintimeoftheoverall0.2−10keV componentssharingthesameΞvaluearenominallyinpressure
fluxofthesourcetoprovideareferenceonthefluxstateofthe equilibriumwitheachother,andthereforetheyarelikelypartof
sourceattheepochofeachobservation,alongwiththesoft(0.2− thesamelong-livedstructure.
Articlenumber,page7of19
A&Aproofs:manuscriptno.ngc5548_archival
Table4.Warmabsorberbest-fitparametersofthe2002observations.
Component logξa(L02) logξa(H02) N b σ c v c
H v out
A 0.51±0.08 0.60±0.16 2.0±0.6 210±40 −588±34
B 1.26±0.05 1.42±0.06 7.0±0.9 61±15 −547±31
C 1.90±0.03 2.18±0.10 15.3±3.3 19±6 −1148±20
D 2.10±0.03 2.26±0.05 10.7±1.6 68±14 −254±25
E 2.68±0.08 2.64±0.05 28.0±8.1 24±12 −792±25
F 2.87±0.05 3.10±0.04 57.1±17.0 34±13 −1221±25
Notes.(a)Inunitsofergcms−1; (b)inunitsof1020cm−2; (c)inunitsofkms−1.
Table5.Best-fitparametersoftheremainingarchivalobservations.
Dataset L99 H00 R00x R01y R01z L05 L07
PLΓ 1.81±0.02 1.56±0.01 2.25±0.05 1.83±0.05 1.91±0.07 1.53±0.04 1.70±0.06
PLF a 7.00±0.12 3.90±0.06 4.24±0.15 6.75±0.15 8.48±0.25 1.90±0.06 1.05±0.06
0.2−10
PLL b 5.46±0.10 2.84±0.04 4.36±0.16 5.40±0.12 6.97±0.21 0.57±0.02 0.95±0.05
0.2−10
mbbTemp.c 0.138±0.004 0.134±0.012 0.090±0.014 0.145±0.006 0.144±0.010 0.070±0.027 ...
mbbF a 0.64±0.05 0.47±0.19 0.16±0.10 0.47±0.05 0.54±0.11 <0.04 ...
0.2−10
mbbL b 0.68±0.06 0.48±0.20 0.35±0.22 0.52±0.06 0.61±0.12 <0.15 ...
0.2−10
Ovii f Lum.d 5.3±1.0 8.4±1.7 7.2±1.3 7.7±0.7 4.9±1.3 2.4±0.4 1.9±0.4
WAAlogξe 1.01±0.08 1.19±0.15 ... 0.52±0.09 0.58±0.15 0.07±0.42 1.01±0.35
WABlogξe 1.71±0.06 1.84±0.09 0.80±0.20 1.31±0.06 1.32±0.08 0.01±0.24 −2.08±0.34
WAClogξe 1.93±0.06 2.43±0.10 1.74±0.11 1.86±0.02 1.78±0.03 0.75±0.20 0.22±0.35
WADlogξe 2.26±0.08 2.52±0.08 2.14±0.09 2.09±0.05 2.08±0.08 1.80±0.14 1.98±0.30
WAE logξe 2.69±0.07 3.06±0.08 2.37±0.11 2.62±0.10 2.57±0.11 1.96±0.10 2.33±0.33
WAF logξe 3.09±0.10 3.19±0.08 2.93±0.14 2.83±0.07 2.81±0.06 2.59±0.13 2.30±0.21
C-stat/d.o.f 2299/2090 2764/2560 1517/1102 1445/1069 1461/1067 2697/2214 1731/1659
Notes.(a)Observedflux,inunitsof10−11ergcms−1; (b)inunitsof1043ergs−1; (c)inunitsofkeV; (d)inunitsof1040ergs−1; (e)Inunitsoferg
cms−1.
In Figs. 5 and 6 we show the thermal stability curves of whichwereinasimilarionizationstate.InH00,anobservation
the archival observations of NGC 5548 analyzed in this paper. takenalmost2monthslater,theoverallfluxdecreasedbyafac-
The curves are the same in all of the observations since the torof2butalltheWA componentswerein a higherionization
same input SED was used to generate the curve. Overplotted state.ComponentC "jumped"theunstablebranchthatwasjust
onthecurvesarethedifferentWAcomponentsdetectedineach aboveits positionin the L99curve,almostblendingwith com-
archival observation. While in most of the cases the WA com- ponent D. Similar jumps happen for components E and F. In
ponentsBtoF seemtobeinpressureequilibrium,theextentof thiscase,thedropinfluxbetweenL99andH00couldhavehap-
theuncertaintiesmakestheoppositeequallylikely,andtherefore penedshortlybeforetheobservation,notgivingthegasenough
therearenogroundstoclaimpressureequilibrium.Thelargeer- timetorespondtothischange.Insupportofthisargument,R00x
rorbarsassociatedwithmostoftheobservationsaremainlydue wasobservedalmost11monthsafterH00,maintainingasimilar
to the uncertaintiesin thedeterminationofξ. Itispossible that fluxlevel,butwithallcomponentssignificantlylessionized.The
someofthecomponentsmightbetrulyinpressureequilibrium, uncertaintiesinthisobservation,however,arequitelargedueto
buttheirdifferentoutflowvelocitiesandturbulencesuggestthat theshortexposure(only26ks).R01y,observed∼200dayslater,
theylikelyoriginatefromdifferentspatialregions. hasafluxhigherbyafactorof∼2comparedtoR00x,andithas
all six components in an ionization state fully consistent with
Ontheotherhand,wecanexaminetheevolutionoftheWA
L02.SimilarlyforR01z,observedtwodayslater.H02,takensix
components as they move along the curve between the differ-
monthslaterwithafluxhalfofthatinR01z,showslittlechange
entepochsofthearchivalobservations.Sincetheuncertaintyof
intheWAcomponents,exceptforDandFwhichclimbedinthe
theWAcomponentsintheverticalaxisismuchsmallerthanin
curvetowardtheunstablebranchesabovethem.Again,welack
the horizontal axis, it is possible to examine vertical displace-
hereinformationaboutthefluxhistoryofthesourceinthedays
ments along the curve. For this reason, for comparison among
or weeks prior to this observation, which could provide a hint
the different datasets, we also show as horizontal dashed lines
onwhatinitiatedthischange.Interestingly,whentheL02obser-
in Figs. 5 and 6 the location of the WA components in obser-
vationwastaken4dayslaterthesecomponentshadreturnedto
vation L02, the one with the best determined WA parameters.
theirpositionsinthestablebranchesofthecurve,whichtellsus
Inthefirstgratingobservation(L99)thefluxofthe sourcewas
somethingaboutthetimescalesatwhichtheyrespond(butsee
slightlyhigherthaninL02.TheWA componentsarehigheron
thediscussioninSect.4.3).Finally,onmuchlongertimescales,
thecurvethanattheL02epochexceptforcomponentsCandE,
Articlenumber,page8of19
J.Ebreroetal.:AnatomyoftheAGNinNGC5548.VI.
10−10
L99 R01z
L02
V) 5×10−11 R01y
e
k H02
10 H00 R00x
−
2
0. 2×10−11
Flux ( L05 L07
10−11
5×10−11
V) 2×10−11
e
k
−2 10−11
2
0.
x ( 5×10−12
u
Fl
2×10−12
5×10−11
V)
e
k
0 2×10−11
1
−
2
x (
u 10−11
Fl
0.6
0.4
0.2
R
H
0
−0.2
0 500 1000 1500 2000 2500
Time (days)
Fig.4. Fluxesinthe0.2−10keV,0.2−2keV,and2−10keVbands,andhardnessratios(HR=(H−S)/(H+S))asafunctionoftimeindays
sincethefirstobservation(L99)foralloftheobservationsusedinthispaper.
L05 was observed ∼ 3 years later when the source had a flux archivalobservationusingtheL02parametersasreferenceval-
level ∼ 3 times lower than in the L02 epoch. Even though the ues. We chose to use L02 for its quality and because the WA
errorbarsareratherlarge,onecanseeadramaticchangeinthe parameterswere bestdeterminedfor thisobservation.Foreach
locationofthe WA componentsonthe curve,allof themmov- componentthe dotted horizontalline indicatesthatthe WA did
ingtowardlowerpositionsinthecurve.Thelastobservationin notrespondtovariationsinfluxwithrespecttothatofL02(i.e.
2007hasthelowestoverallflux,severaltimeslowerthanthatof the ionization parameter ξ is the same as that of L02 for each
L02,andtheassociatederrorbarsaresolargethatnousefulcon- component).A dasheddiagonallineshowsξ proportionalto L,
clusioncanbeextractedfromitotherthantheWAcomponents and marks the trend that the WA would follow if the material
seemtobelocatedatsimilarpositionsasin2005. respondsinstantaneouslytocontinuumchanges.
Itisworthnotingagainthateachobservationisjustasnap- The lowest flux observations, L05 and L07, have typically
shotofadynamicalpictureinwhicheachgascomponentisre- largeuncertaintiesin the ionizationparameterξ for all the WA
actingdifferentlytochangesintheionizingfluxovertheyears, components.Overalltheyseem to beconsistentwithan imme-
a picture that is incomplete since we do not have a continuous diate response for components A to C, while components D to
monitoring of the flux history of the source between observa- F couldbeconsistentwithanytrendatthe2σconfidencelevel.
tions. However, this provides support for the fact that the re- AlltheWAcomponentsofH00aresystematicallymoreionized
sponse to flux variations on different time scales is motivated that those of L02, even though the ionizing luminosity at the
by the different densities of the absorbing gas components. If H00 epoch was ∼20% lower than that of L02. Similarly most
theirdensitiescouldbeconstrainedthroughtheirvariability,we componentsaremorehighlyionizedinH02,whichhasanion-
wouldhaveapowerfultooltoalsoconstraintheirlocationwith izing flux close to thatof L02.The exceptionsare components
respecttothecentralionizingsource. Aand E,whichareinconsistentatonlythe 2σlevel.A similar
behaviorisseenfortheWA inR00x(ionizingfluxcomparable
toL02)butwithsystematicallylowerionizedcomponents,with
4.3.Warmabsorbervariability
the exception of components D and F. The observations with
MotivatedbythebehavioroftheWAcomponentsalongthesta- higher ionizing luminosity, the RGS observations in 2001 and
bilitycurvesshowninSect.4.2,welookedforadditionalsigna- L99,showdifferenttrends.Ingeneral,bothR01yandR01zare
turesofvariabilitythatwouldhelptoconstrainthetypicaltime closertothenoresponsetrend(i.e.theirWAcomponentshavea
scales at which each component responds to flux changes. In ξ valueclosetothoseofL02).However,forcomponentsAand
Fig. 7 we show the values of the ionization parameter ξ plot- Bthe errorsare largeenoughthatthey are within 2σ also con-
ted against the ionizing luminosity L for each component and sistentwithanimmediateresponsetrend.Ontheotherhand,the
Articlenumber,page9of19
A&Aproofs:manuscriptno.ngc5548_archival
0 L99 0 H00
F
F
E
V) −1 V) −1 D
ke E ke
g T ( D g T ( C
o o
l l
C
−2 B −2 B
A A
3 3
−0 0.5 1 1.5 2 −0 0.5 1 1.5 2
log Ξ log Ξ
0 R00x 0 R01y
F F
V) −1 V) −1
T (ke E T (ke E
log D log D
2 2 C
− C − B
B A
3 3
−0 0.5 1 1.5 2 −0 0.5 1 1.5 2
log Ξ log Ξ
0 R01z 0 H02
F
F
V) −1 V) −1
T (ke E T (ke E D
g g
lo D lo
C
2 C 2 B
− −
B
A A
3 3
−0 0.5 1 1.5 2 −0 0.5 1 1.5 2
log Ξ log Ξ
Fig.5.ThermalstabilitycurvesoftheNGC5548archival observations (L99andH00inthetoppanels;R00xandR01yinthemiddlepanels;
R01zandH02inthebottompanels)showingthepressureionizationparameterasafunctionoftheelectrontemperature(solidline).Thecurves
havebeencomputedfromtheSEDofSteenbruggeetal.(2005).TheX-rayWAcomponentsineachobservationarerepresentedassolidsquares
(AtoF).Forcomparison,thedashedhorizontallinesrepresentineachpanelthelocationoftheWAcomponentsintheL02observation.
WAcomponentsinL99showavarietyofbehaviors:AandBare inTable5.Ifthesechangesareduetophotoionizationorrecom-
significantly more ionized than in L02, D and F are consistent binationoftheionizedgasinresponsetocontinuumchanges,it
with the gas respondingimmediately,whileC and E would be is then possible to estimate a lower limit on the density of the
closertoanoresponsescenario. absorbinggasviathefollowingequation(Bottorffetal.2000):
It is clear that, in spite of the large uncertaintiesassociated
withthedeterminationofξ,theWAcomponentsshowsomekind t (X)= α (X)n f(Xi+1) − αr(Xi−1) −1, (3)
ofvariabilityasseeninFigs.5,6and7,andinthevalueslisted rec i r i " f(X) α (X) #!
i r i
Articlenumber,page10of19