An XMM-Newton Observation of Abell 2597

0.68 MB·

by  R. Glenn Morris

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed2February2008 (MNLATEXstylefilev2.2)
An XMM-Newton Observation of Abell 2597
R. Glenn Morris⋆ and A. C. Fabian
InstituteofAstronomy,MadingleyRoad,CambridgeCB30HA
2February2008
ABSTRACT
We report on a 120ks XMM-Newton observation of the galaxy cluster Abell 2597. Results
5 from both the European Photon Imaging Camera (EPIC) and the Reflection Grating Spec-
0 trometer (RGS) are presented.From EPIC we obtain radial profilesof temperature,density
0 andabundance,andusethesetoderivecoolingtimeandentropy.Weillustratecorrectionsto
2
theseprofilesforprojectionandpointspreadfunction(PSF)effects.Atthespatialresolution

n availabletoXMM-Newton,thetemperaturedeclinesbyaroundafactoroftwointhecentral
a 150kpc or so in radius, and the abundance increases from about one-fifth to over one-half
J
solar.Thecoolingtimeislessthan10Gyrinsidearadiusof130kpc.EPICfitstothecentral

7 regionareconsistentwithacoolingflowofaround100solarmassesperyear.Broad-bandfits
1 totheRGSspectraextractedfromthecentral2arcminarealsoconsistentwithacoolingflow

ofthesamemagnitude;withapreferredlow-temperaturecut-offofessentiallyzero.Thedata
1
appeartosuggest(albeitatlowsignificancelevelsbelowformaldetectionlimits)thepresence
v
oftheimportantthermometerlinesfromFeXVIIat15,17A˚ restwavelength,characteristicof
7
gasattemperatures∼0.3keV.Themeasuredfluxineachlineisconvertedtoamassdeposi-
4
tionestimatebycomparisonwithaclassicalcoolingflowmodel,andonceagainvaluesatthe
3
1 levelof100solarmassesperyearareobtained.Thesemassdepositionrates,whilstlowerthan
0 thoseofpreviousgenerationsofX-rayobservatories,areconsistentwiththoseobtainedfrom
5 UVdataforthisobject.Thisraisesthepossibilityofaclassicalcoolingflow,atthelevelof
0 around100solarmassesperyear,coolingfrom4keVbymorethantwoordersofmagnitude
/ intemperature.
h
p
Keywords: galaxies:clusters:individual:A2597–X-rays:galaxies:clusters–coolingflows
-
o
r
t
s
a
: 1 INTRODUCTION observationsofthecentral0.3arcsec(Tayloretal.1999)show an
v invertedspectrum(a =0.6)core,togetherwithstraight,symmetric
i Abell2597isarelativelynearby(z≈0.08),Abellrichnessclass0
X jetsemanatingfrombothsides.
clusterofgalaxies.Richinfeatures,ithasbeenstudiedextensively
r inmanywavebands.
a Abell 2597 has previously been observed in X-rays by
ThecentralcDgalaxycontainstheradiosourcePMNJ2323-
various observatories, for example: Einstein (Crawfordetal.
1207 (PKS B2322-12) (Wright&Otrupcek 1990; Griffithetal.
1989); ROSAT (Sarazinetal. 1995; Sarazin&McNamara 1997;
1994).Thetotalfluxdensityofthissourceis∼1.7Jyat1.5GHz
Peresetal.1998);ASCA(White2000);Chandra(McNamaraetal.
(McNamaraetal.2001;Owenetal.1992),and∼0.3Jyat5.0GHz
2001); XMM-Newton (Still&Mushotzky 2002). Traditionally, it
(Balletal.1993),implyingafairlysteepspectralindexa =−1.3
hasbeenclassifiedasamoderatelystrongcoolingflow(e.g.Fabian
(Sn (cid:181) n a ). The source is physically small, ∼7×5,5×2arcsec 1994)cluster,withinferredX-raymassdepositionrate(M yr−1):
⊙
saotl1ve.5d,a5t.t0hGesHezfrereqsupeencctiievse,lyb.utTahtethseohuirgcheerisfreoqnulyenmcyodhearsaatenlyelorne-- 270±41 (ROSAT PSPC; Peresetal. 1998, rcool =152+−6578kpc);
259+176(ASCA;White2000).
gatedappearancesuggestingadouble-lobedstructure.At8.44GHz −178
(Sarazinetal. 1995), the source is resolved into a nucleus and
A short (∼ 20ks of good time) Chandra observation
two lobes (∼5arcsec across), orientated roughly NE–SW. A jet
(McNamaraetal.2001)highlightedtheinteractionbetweenthera-
is clearly seen leading from the nucleus on the SW. This jet un-
dio source and the X-ray gas, through the presence of so-called
dergoes a sharp (∼90◦) bend about 0.5arcsec from the nucleus.
‘ghost cavities’ of low surface brightness, seemingly coincident
Assumingatypicalspectralindexforthejetsof−0.7,theinferred
withspursoflowfrequency(1.4GHz)radioemission.Suchbright-
lobespectralindexis≈−1.5,suggestiveofconfinementandsyn-
nessdepressions arebelievedtobebuoyantly risingbubbles(e.g.
chrotronageing.VeryLongBaselineArray(VLBA)1.3,5.0GHz
Bˆırzanetal.2004)associatedwithaprior(∼108yrpreviously,in
thiscase) outburstofthecentralradiosource.Thesmallphysical
⋆ E-mail:[email protected] sizeoftheradiosourcemeans, however, thattheseissuesarenot
(cid:13)c 0000RAS
2 R.G. Morrisand A.C. Fabian
amenable to study with XMM-Newton, given its ∼5arcsec point sively)thecaseincoolingflowclusters.TheHa +[NII]luminos-
spreadfunction(PSF). ity is 2.7×1042ergs−1, making it among the most luminous in
In the Chandra and XMM-Newton era of high spatial and thesampleofHeckmanetal.(1989).TheHa luminosityismany
spectral resolution, the cooling flow model is undergoing re- (∼300) times greater than that expected from extrapolating the
interpretation. The cause is quite simply an absence (or at best classical∼107KX-raymassflowratesdownto∼104K,ageneral
a severe weakness) of the spectral features expected from gas at featureofsuchnebulae.Thediscrepancyismadeevenworseifthe
the low-end (.1keV, say; though there does not appear to be a relative sizes of the nebulae and X-ray cooling regions are taken
fixedabsolutecut-off)oftheX-raytemperatureregimeintheob- intoaccount. Thedeep optical spectra of Voit&Donahue (1997)
servedspectraofclustercentralregions.Reductionsintemperature reveal that the Abell 2597 nebula has T ∼104K, Z ∼0.5Z⊙,
byfactorsofafew(e.g.Kaastraetal.2004)withradiusareseen. ne∼200cm−3,and issignificantlyreddened by(intrinsic,owing
Theexpectedemissionlinesforseveralimportantlow-temperature tothelowGalacticcolumn)dust.Thelowinferredcolumndensity
species (e.g. the FeXVII 15 and 17A˚ lines) do not seem to be NHII≈3×1019cm−2relativetothatofHIsuggeststhatthinion-
present at the levels predicted by simple models. This is a trend izedHII layerssurround coldneutral HI cores.Thenebulahasa
thatappearstoberepeatedinmanyclustersthathavetraditionally lowionizationparameter,mostspeciesbeingonlysinglyionized.
been thought toharbour cooling flows(e.g. Petersonetal.2003). Photoionization from hot stars is the only ionization mechanism
Many explanations (e.g. Fabianetal. 2001; Petersonetal. 2001) notruledout bytheobservations, buteven thehottest Ostars(in
for this effect have been put forward; some involving suppres- isolation)wouldnotbeabletoheatthenebulatotheobservedtem-
sionof thegascooling, some involvingdepartures fromstandard perature.Someformofheattransferfromtheintraclustermedium
collisionally-ionizedradiativecooling.Afteraflirtationwithther- (ICM)maysupplyacomparableamountofenergytothenebula.
mal conduction (but see e.g. Soker 2003), theoretical studies at Thecoreof thecentral cDexhibitsasignificant blueoptical
presentseemtobeconcentratedonsomeformofdistributedheat- excess (e.g. McNamara&O’Connell 1993; Sarazinetal. 1995).
ing from a central AGN (e.g. Vecchiaetal. 2004; Reynoldsetal. HST observations(Koekemoeretal.1999)resolvethisexcessinto
2005; Ruszkowskietal. 2004), effected by buoyant plasma bub- continuum emission around and across the radio lobes, and sev-
bles. eral knots tothe south-west. Line-dominated (thereby not optical
Observations at ultra-violet (UV) wavelengths suggest that synchrotronorinverseComptoninorigin)excessappearsinbright
Abell2597maybeaparticularlyinterestingobjectforstudyinlight arcsaroundtheedgeoftheradiolobes,andmorediffuselyacross
oftheseissues.Abell2597,togetherwiththez=0.06clusterAbell the lobe faces. Such geometry is suggestive of an emission shell
1795,wasobservedwiththeFarUltravioletSpectroscopicExplorer surroundingtheradiolobes.Abrightopticalknotisalsoseenco-
(FUSE;Moosetal.2000)missionbyOegerleetal.(2001).FUSE incidentwiththesouthernradiohot-spot,consistentwithamassive
is sensitive to the OVI 1032, 1038A˚ resonance lines, which are clumpresponsiblefor thedeflectionofthesouthernjet.Thecen-
strong diagnostics of thermally radiating gas at temperatures ∼ tralregionalsoshowsstrongdustobscuration,resolvedintoseveral
3×105K. In a traditional cooling flow model, in which the ulti- large(severalhundredparsecs)regions.
matefateofthehotgasistocooltoverylowtemperatures,strong In the infra-red, vibrational molecular hydrogen emission
UVemissionisexpectedinsuchlinesasgascoolsoutoftheX-ray lines characteristic of ∼ 1000–2000K collisionally ionized gas
temperatureregime;therebypotentiallyformingabridgebetween have been detected from the innermost ∼3arcsec of the central
the hot, X-ray emitting gas and the cool, Ha (see below) radiat- clustergalaxy(Falckeetal.1998;Donahueetal.2000;Edgeetal.
inggas.OVI1032A˚ emissionwasdetectedinAbell2597,withan 2002). Aswith the optical nebula, there istoo much emission as
inferred luminosity3.6×1040ergs−1.Using simplecooling-flow compared to the expectation from even an inordinately massive
theory,Oegerleetal.(2001)calculatetheassociatedmassflow-rate classicalcoolingflow,andyettoolittlemassforthistobethefinal
as40M yr−1(withintheeffectiveradiusoftheFUSEapertureat reservoirofastandard,long-livedflow.Furthermore,themolecular
⊙
thisredshift,∼40kpc),foranintermediatecasebetweenthelimits gasisdusty, andthereforeunlikelytobethedirectcondensate of
of isobaric and isochoric cooling. TheROSAT HRI mass deposi- thehot ICM(wheredust israpidlydestroyed). Againaswiththe
tionrateofSarazinetal.(1995),≈330M⊙yr−1 withinacooling optical light, there is a complex filamentary structure that seems
radius(definedbytcool<10Gyr)≈130kpc,isaroundthreetimes to be spatially associated with the radio source. The optical and
larger(applying asimplelinear M˙ (cid:181) r scaling).Theweakness of infra-redpropertiesarebothconsistentwithUVheatingofthegas
cluster cooling flows in comparison to the predictions of the tra- byapopulationofyoung,hotstars(Donahueetal.2000)(SFR∼
ditional model is thus again demonstrated, but extended down to few M yr−1).Abell2597alsohasapossibleCOdetection(Edge
⊙
theUVregime.Thisunder-luminosityatUVtemperaturesmaybe 2001);indicativeofverycold,moleculargas.
contrastedwiththesituationatHa temperatures,wheretheAbell The Galactic column density in the direction of Abell
2597emissionnebula(seebelow),likeothersuchnebulae,isover- 2597 (l =65.4, b=−64.8) has the relatively low value 2.5×
luminous, even whencompared toclassical,high X-raymassde- 1020cm−2(Dickey&Lockman1990;Starketal.1992).Recently,
positionrates(Voit&Donahue1997). Barnes&Nulsen(2003)haveshownthatthe5s limitonanyfluc-
In contrast, Abell 1795 was not detected in OVI. Similarly, tuation in the foreground HI column in this direction, on scales
LecavelierdesEtangsetal.(2004),alsousingFUSE,wereunable ∼1arcmin,is0.43×1020cm−2(17percent).
todetectOVIemissionfromeitherofthelowredshift(z≈0.07), IntrinsicHI21cmabsorptiontowardsthecentralradiosource
low Galactic column (NH ∼4×1020cm−2), rich clusters Abell was detected by O’Deaetal. (1994). A narrow (∼200kms−1),
2029 and Abell 3112. These results highlight the apparently un- redshifted (∼300kms−1), spatially unresolved component coin-
usual (within the currently favoured non-cooling flow paradigm) cident withthenucleusisinferredtobeagasclumpfallingonto
natureofAbell2597,andmotivateamoredetailedX-raystudyof the cD. A broad (∼400kms−1), spatially extended (∼3arcsec,
itsproperties. i.e. the size of the radio source) component at the systemic ve-
The central (∼15arcsec in diameter) regions of Abell 2597 locity is consistent with being associated with the (hence photon
harbour an optical emission line nebula, as is often (and exclu- bounded) Ha . The inferred column densities and masses for the
(cid:13)c 0000RAS,MNRAS000,000–000
XMM-Newton Observationof A2597 3
narrowandbroadcomponentsareNH∼{8.2,4.5}×1020Tscm−2;
M∼{3.5,7}×107TsM⊙;withTsthespintemperatureinunitsof
100K.Intrinsic,redshiftedHI wasalsoseenintheVLBAobser-
vations of Tayloretal. (1999), and interpreted as a turbulent, in-
wardly streaming, atomic torus (scale height .20pc) centred on
thenucleus.
Our chosen cosmology has H0 =50kms−1Mpc−1, W M =
1.0, and W L = 0.0. Adopting a redshift z = 0.0822 (see Sec-
tion5.2.1),theluminositydistanceisDL=503Mpc,theangular
diameterdistanceisDA=429Mpc,andtheangularlengthscaleis
2.08kpcarcsec−1.DataprocessingwascarriedoutusingtheXMM-
NewtonScienceAnalysisSystem1(SAS)version5.4.1(akaxmm-
sas 20030110 1802).Spectralanalysesemployed XSPEC2version
11.3.Unlessotherwisestated,quoteduncertaintiesare1s (68per
cent)onasingleparameter.
Figure1.MOS2lightcurveformedfrom100stimebins.Upperpanel:total
exposure,withrejectedperiodsofhighcount-rateingrey,acceptedperiods
2 EPICDATAREDUCTION
inblack.Lowerpanel:onlytheselectedGoodTimeInterval(GTI)isshown.
Abell 2597 was observed by XMM-Newton on 2003 June 27–28
(observation ID 0147330101), during revolution 0650. The nom-
inal length of the observation was 120ks, unfortunately the ex- removesveryfewcounts,andmeansthattheonlybackgroundline
posure was heavily affected by background flaring (see Fig. 1). remaininginthepnisnotspatiallyvarying.Thisfreesusfromthe
Thethinopticalblocking filterwasappliedtotheEuropeanPho- needtoextractbackgroundspectrafromthesamespatialregionof
tonImagingCamera(EPIC).TheMOS(Turneretal.2001)andpn thepndetector, i.e.enables ustousealocal background. Thisis
(Stru¨deretal. 2001) detectors were both operated in Full Frame desirable because it removes the vagaries of scaling and varying
mode. cosmicbackgroundnecessarilyassociatedwiththeuseofablank-
EPICeventsfileswereregeneratedfromtheObservationData skybackground(seebelow);anditallowsusgreaterfreedominthe
Files (ODFs) using the SAS meta-tasks epchain and emchain. filteringofbackgroundflares.Additionally,comparisonoftheoff-
Foremchain,standardoptionswereused,exceptthatbadpixelde- sourcepn spectrafortheAbell 2597 andblank-sky fieldsreveals
tectionwasswitchedon.epchainwasrunintwostepsinorderto that the8keV fluorescence linesareshiftedtoalower energy by
mergeintheinstrumenthouse-keepingGoodTimeInterval(GTI) about20eVinthelatter.Thisalmostcertainlyrepresentsachange
data(whichareotherwiseignored)beforethefinaleventslistwas in calibration between the processing of the blank-sky templates
created. and our re-processing of the ODFs for Abell 2597 (the effect of
suchashiftwouldofcoursebelargelyrestrictedtoanincreasein
c 2inthosechannelslyingaroundthefluorescentlineenergies).
2.1 Backgroundfilteringandsubtraction TheonlysignificantfluorescentlinesintheMOSdataarethe
AlandSilinesat1.5and1.7keVrespectively(theCr,Mn,andFe
The EPIC background is comprised of a number of components:
linesaround6keVareseentobenegligible).Bothofthesefeatures
proton,particle,photon.SoftprotonsintheEarth’smagnetosphere
exhibitstrongspatialvariation(Lumb2002)overtheMOSfieldof
scatterthroughthemirrorsystem,andareresponsibleforperiodsof
view (hereinafter fov). Using a local background for the MOS is
intensebackgroundflaring,wheretheinstrumentcountratecanrise
thereforenotasviableapropositionasitisforthepn.
byordersofmagnitude.Suchintervalscanonlybedealtwithbyex-
Inordertobeabletomakeuseoftheblank-skyEPICback-
cisingthemfromfurtheranalysisusingarate-basedGTIfiltering.
ground templates of Lumb (2002), we use the same form of fil-
Energeticchargedparticlesthatpassthroughthedetectorexciteflu-
teringprocessasemployedintheconstructionofthosefiles.That
orescent emission lines in several of the constituent components.
is, we make a light-curve (e.g. Fig. 1) in 100s time bins of the
Thestrongspatialvariationofsomecomponents ofthisemission
high energy (PI > 10000) single pixel (PATTERN == 0) events
(Freybergetal. 2002a, Freybergetal. 2002b, Lumb 2002) makes
acrossthewholedetector.ThestandardXMMeventselectionflags
itpreferabletoextractabackground fromthesameregion ofthe
#XMMEA EMand#XMMEA EPareappliedfortheMOSandpnrespec-
detectorasthesource.Fortheanalysisofextendedsources,theuse
tively(theseexcludecosmicrays,eventsinbadframes,etc.).
ofblank-skybackgroundtemplates(Lumb2002)isthereforetobe
Our preferred filteringmethod is toform ahistogram of the
desired.
number of events in the 100s time-binsof the light curve, fit the
For Abell 2597, cluster emission is detected up to around 8
coreofthedistribution(i.e.ignoringthehighcount-ratetail)witha
and9keVintheMOSandpnrespectively.Thepnexhibitsstrong
Gaussian,andexcludeperiodswherethecount-rateliesmorethan
fluorescencelinesofNi,CuandZnaround 8keV.Theonlynon-
ns awayfromthemean.Unfortunately,forMOS1(andthepn,but
negligiblefluorescentlinevisibleinthepndatabelow8keVisthe
thisisnotanissuesinceweusealocalbackgroundinthiscase)the
1.5keV Al line. Unlike the Cu (for example) line, this line does
2s count-ratethresholdslieabovethefiltrationlimitsusedinthe
notvaryspatiallyoverthedetector(Freybergetal.2002b),sinceit
creationofthebackgroundeventstemplates(0.2and0.45cts−1for
originatesinthecamerahousing.Cuttingoffthespectraat7.3keV
theMOSandpnrespectively).InordertomakeuseoftheMOS1
backgroundtemplatewethereforeapplythesameabsolutecount-
1 http://xmm.vilspa.esa.es/sas/ ratefilteringtothisinstrumentaswasusedtomakethebackground
2 http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/ events file. For MOS2, the2s count-rate threshold is 0.13cts−1,
(cid:13)c 0000RAS,MNRAS000,000–000
4 R.G. Morrisand A.C. Fabian
andthisabsolutevalueisusedtofilterboththeobservationevents background fields. For the MOS, we make use of single, double,
file,andtore-filterthebackgroundtemplateeventsfiletothesame triple, and quadruple pixel events (i.e. PATTERN 6 12). For the
level.Forthepn,the2s count-ratethresholdis0.54cts−1. pn,weusesingleanddoubleevents(PATTERN 6 4).Inbothcases
We strengthen the filtering by requiring that all GTIs be at onlyeventswithFLAG == 0areconsidered.
least 10 minutes long. The GTI files are aligned on the frame
boundaries of the relevant events files before being applied. The
meanremainingONTIMEoftheeventsfilesafterfilteringinthis 2.2 Outoftimeevents
wayare39(pn),37(MOS1),and38ks(MOS2).
WhenthepnisoperatedinFullFramemode,asisthecasehere,
It is known that the particle background can vary from
brightsourcescanbesignificantlyaffectedbyso-calledoutoftime
observation to observation at around the ten per cent level
(OOT)events.Thesearecausedbyphotonswhicharrivewhilethe
(Freybergetal. 2002a). We therefore scale the products (images
CCDisbeingreadout(chargeshiftedalongcolumnstowardsthe
and spectra) of the blank-sky MOS events file before they are
readout node). Such events are assigned a wrong RAWY coordi-
used to correct the source products. The scaling factor is ob-
nateandhenceanimproperCTI(chargetransferinefficiency)cor-
tained from the ratio of the count-rates outside the fov, where
rection.Inanuncorrectedimage, theOOTeventsduetoastrong
the only events should be the result of particle-induced fluores-
sourcearevisibleasabright streaksmeared out inRAWY. OOT
cence rather than from actual photons passing through the mir-
eventsacttobroadenspectralfeatures.
ror system. In detail, we find the number of counts remaining
OOT events can only be corrected for in a statistical sense.
in the GTI-filtered data after applying the evselect filtering
We follow the standard procedure of re-running the SAS meta-
criterion#XMMEA 16 && FLAG & 0x766a0f63 == 0 && PI in
task epchain with the option withoutoftime=y passed to the
[200:12000] && PATTERN <= 12, where the flag expression
epeventstask.Thisgeneratesaneventsfilethatconsistsentirely
selectseventsoutsidetheMOSfovbutwithnoothernon-zeroflag
ofOOTevents,distributedovertherangeofRAWYpresentinthe
settings.Combiningthisfigurewiththeexposuretimegivesanout-
data.OOTimagesandspectracanthenbeproducedinthenormal
of-fovcount-ratewhichisusedtonormalizethebackgroundtem-
way,andscaledandsubtractedfromsourceimagesandspectrato
plateproducts. Inthisway, weobtain scalingfactors of 1.02 and
correctthemfortheOOTevents.
0.91 for the MOS1 and MOS2 backgrounds respectively. To al-
The appropriate scale factor is obtained from the expected
lowforsomeuncertaintyinthenormalization,weassigna10per
fraction of OOT events, which depends on the ratio of the read-
centsystematicerrortotheblank-skyproducts,whichisaddedin
outtimetotheframetime.ForthepninFullFramemode,thisis
quadraturetothestatisticalerrors.
4.6/73.3ms=6.3per cent (Stru¨deretal.2001,table1). Aslight
The SAS task attcalc was used to reproject the blank-sky
correctiontothisfactorisneededbecausetheversionofepevents
templatestothesameskypositionastheAbell2597observation,so
useddoesnottakeaccountofthefactthatinpnFullFramemode,
thatthesameskycoordinateselectionexpressionscouldbeapplied
thefirst12(of200)rowsaremarkedasbad.Accordingly,thecor-
toboththescienceandbackgroundfields.
rectscalingfactoris(1−12/200)∗6.3=5.9percent.Inpractice,
Below5keVorso,thecosmicX-raybackground(CXB)dom-
OOTeventsmakelittledifferencetothepnspectralfits.
inates(Lumb2002)overtheinternalcamerabackground. Ifthere
were no significant variation of the CXB over the sky, then the
blank-skytemplateswouldalsorepresentthisbackgroundcompo-
2.3 Responsefiles
nent. To allow for any changes in the CXB in the region of the
target,aso-calleddoublebackgroundsubtractionapproachcanbe The lower limit for the spectral fits was placed at 500eV for the
used. The large field of view of XMM-Newton, together with the MOS(followingtheadviceofKirsch2003forpost-revolution450
redshift of Abell 2597, result in significant areas of the detectors data;inanycasetheMOSresultsarerelativelyinsensitivetothe
beingessentiallyfreefromclusteremission.Thisenablesustoex- adopted low-energy cut-off). For consistency (and to avoid low-
amine the background spectrum from such regions and compare energy residuals) the same limit was applied to the pn data. The
it with those of the (scaled) blank-sky templates extracted from highenergycut-offwasplacedat7.3keV,slightlybelowthemaxi-
the same detector region. For this purpose we extracted spectra mumdetectedenergyfromthecluster,toavoidthebackgroundpn
fromanannuluslyingbetween7and10arcmin(avoidingregions fluorescencelinecomplexat8keV(asdescribedinSection2.1).
near the edge of the fov) from the cluster centre, which we take Spectraweregroupedtoaminimumof20countsperchannel
to be located at the emission peak, in sky coordinates (X,Y)= toensureapplicabilityofthec 2statistic.
(23720.5,23720.5).Non-ICMsourcesinthisregionwereexcluded Redistributionmatrixfiles(RMFs)andancillaryresponsefiles
withcircularmasksofradiusatleast30arcsec.TheBACKSCAL (ARFs)weregeneratedusingtheSAStasksrmfgenandarfgen
FITSheaderkeyswerecorrectedfortheareasoremoved. respectively.RMFsweremadeusingthestandardSASparameters.
Initial comparison of the spectra extracted from this region ARFsweremadeinextended-sourcemode,usingalow-resolution
oftheAbell2597fieldrevealsthattheagreement withtheblank- detector-coordinateimageofthespectralregion.Theimagepixel
skyspectrascaledaccordingtotheout-of-fovcount-ratesisgood; size was chosen according to the size of the extraction region
despite therelativelypoor quality of thescience observation asa so as to adequately sample the variation in emission, bearing in
whole(i.e.theheavybackgroundflaring).Thereissomethingofa mind that the version of arfgen employed does not model vari-
slightexcessofsoft(.2keV)countsintheAbell2597field,but ations on scales .1arcmin. Strictlyspeaking, the images should
nottoanygreatdegree. beexposure-weighted(i.e.de-vignetted;Saxton&Siddiqui2002),
In summary, we use a local (extracted from 7–10arcmin in but we find this makes almost no difference in practice in this
radius)backgroundforthepn.FortheMOS,weusetheblank-sky case.Badpixellocationsweretakenfromtherelevanteventsfile.
templatestodealwiththespatiallyvaryingSiandAlbackground TheapproachofgeneratingARFsappropriatetotheemissionpat-
lines, corrected by a double-background subtraction approach to tern in the spectral extraction region differs from the ‘weight-
accountforany(minor)varianceinCXBbetweenthescienceand ing’ method (e.g. Arnaudetal. 2001) more frequently employed
(cid:13)c 0000RAS,MNRAS000,000–000
XMM-Newton Observationof A2597 5
Table1.Global(1–4arcmin)spectralproperties.
A B C
-12d00m00.0s NkTH 3.525.+490.05 31..6497+−+000...0337 3.525.+490.05
−0.05 −0.07 −0.05
M1 Z 0.25+0.02 0.26+0.02 0.26+0.02
−0.02 −0.02 −0.03
z 0.0822 0.0822 0.078+0.001
−0.002
c 2 349.6/343 340.5/342 342.1/342
09m00.0s NH 2.49 1.91−+00..33 2.49
kT 3.43+0.05 3.50+0.07 3.42+0.08
−0.05 −0.07 −0.05
M2 Z 0.22+0.02 0.23+0.02 0.22+0.02
−0.02 −0.02 −0.02
z 0.0822 0.0822 0.081+0.002
−0.002
c 2 297.3/348 294.2/347 296.8/347
18m00.0s
NH 2.49 0.01−+00..021 2.49
kT 3.24+0.030 3.59+0.04 3.22+0.02
−0.003 −0.04 −0.03
pn Z 0.20+0.01 0.25+0.01 0.22+0.01
26m00.0s 30.0s 23h25m00.0s 24m30.0s −0.01 −0.01 −0.01
z 0.0822 0.0822 0.077+0.0004
−0.001
Figure 2. pn count image of Abell 2597, selected with FLAG == 0 && c 2 1321.2/1003 1118.8/1002 1293.6/1002
PteAreTdTEaRnNd6cor4rec&te&dPfIoriOnO[T20e0v:e1nt5s00a0s]d.eTschreibiemdaigneSheacstiboenen2,GbTuItfinlo- NkTH 3.325.+409.01 30..6519+−+000...0113 3.323.+490.02
vignetting or background correction has been applied. Image pixels are All Z 0.212−+00.0.00053 0.25+−00..0013 0.23+−00..0012
4arcsecacross,roughlythesizeofthePSFcore. z 0.08−202.006 0.08−20.201 0.079−+00..00101
−0.001
c 2 2000.6/1698 1815.8/1697 1968.0/1697
inXMM-Newtonspectralanalyses.Wefindthatthedifferencesin NHisGalacticcolumninunitsof1020cm−2;kT temperatureinkeV;Z
thetemperatureprofilesetc.obtainedusingthetwoapproachesare metallicityrelativetosolar.Errorsare1s .Quantitieswithouterrorswere
negligible(asperGastaldelloetal.2003). fixed.ModelsA–Carephabs×mekal,with:GalacticNH;freeNH;freez.
Whenfittingallthreedetectorssimultaneously,thenormalizationswere
untied(thoughtheeffectsofthiswereslightinthisinstance).
3 GLOBALPROPERTIES
threedetectorsadoptdisparateredshifts,withMOS1andthepndis-
Thepnimageof thecluster isshowninFig.2.AsintheROSAT agreeing(atthe1s level)withthenominalopticalredshift,0.0822.
observationofSarazin&McNamara(1997),thereareanumberof AnalysisoftheRGSdata,however,withitshigherspectralresolu-
non-ICM sources in the field of view, though only two of these tion(seeSection5.2),resultsinaredshiftconsistentwiththeopti-
arewithin6arcminoftheclustercentre.Theclusteremissionitself calvalue.ComparingtheresultsofmodelsAandC(whichdiffer
displaysarelaxed,somewhatellipticalform. onlybywhetherredshiftisfreeornot),weseethattheonlyeffects
Inordertoexaminetheglobalspectralpropertiesoftheclus- offreeingzaretoreduce c 2 somewhat–theresultsforotherpa-
ter,andtochecktheagreementbetweenthethreeEPICdetectors, rameterssuchasT andZareunchanged.Accordingly,wehaveleft
weextractedspectrafromanannulusofradius1–4arcmincentred zfixedattheopticalvalueinallsubsequentEPICfits.
on the cluster emission peak (J2000: RA = 23h25m19.8s, Dec = In Table 2 are shown the results of fitting the entire central
-12d07m26s;skycoordinates:X =23720.5,Y =23720.5).Thisra- 4arcmin.Nowthatweincludethecoolingregion,awidervariety
dial range was selected to excise any cooling flow region and so ofspectralmodelsbecomerelevant.ModelsAandBarethesame
thatthebackgroundcontributionwasinsignificant,inordertohave asinthe1–4′ case.Thefitsinthe0–4arcmin regionadopt lower
themostsimplespectralshapepossible.Theresultsofbasicsingle temperaturesandhigherabundances,sothatwemayexpecttosee
temperaturefitsarepresentedinTable1. temperatureandabundanceprofilesthatdecreaseandincreasere-
Whenafixed,Galacticabsorptionisused(modelA),thepn spectivelywithdecreasingradius(seeSection4).
has atemperature somewhat below that of theMOS instruments, Theresultsof model Bcanbecompared directlywiththose
andahigherreducedc 2.Iftheabsorbingcolumnisallowedtovary ofAllenetal.(2001),whofound,usingASCA:T=3.38+−00..0067,Z=
(modelB),thenallthreedetectorspreferavaluesignificantlybe- 0.35−+00..0043,andNH=0.99−+00..0087(90percentconfidencelimits).
lowGalactic,withthepnN beingmuchlowerthanthatofeither InmodelsDandEweaddasecondmekalcomponent,with
H
MOS. Importantly, however, freeing N results in the MOS and anindependenttemperatureandnormalization.Forallthreedetec-
H
pnT andZ valuescomingintomuchcloseragreementwitheach tors, the fits are significantly improved. For example, for MOS1
other.Theconclusionwemaydrawfromthisisthatallthreedetec- withafreeNH, thechange infitstatisticisD c 2=−17.9 for the
torsexhibitsomeformofsoftexcess(whoseoriginisunclear)with removal of two degrees of freedom. According to the F-test, the
respecttoaGalacticabsorbingcolumn,moresointhepn;butthat significanceofsuchanimprovementis(1−3.4×10−4).Adhering
whentheabsorbingcolumnisallowedtobefree,thethreedetectors to strict mathematical formalism, the F-test is not really valid in
agreereasonablywellwithregardstotheotherclusterproperties. casessuchasthis(testingforthepresenceofasecondcomponent),
Whenallthreedetectorsareanalysedsimultaneously,thefitsadopt whereone ofthehypotheses liesontheboundary of theparame-
acompromisebetweenthepnandMOSvalues(thereareroughly terspace(Protassovetal.2002).Inpractice,however,MonteCarlo
equalnumbersofcountsinthepnandtwoMOScombined). simulationsshowthatitgivesacceptableanswers(Johnstoneetal.
Model C has a fixed N , but freely varying redshift z. The 2002).
H
(cid:13)c 0000RAS,MNRAS000,000–000
6 R.G. Morrisand A.C. Fabian
Table2.Global(0–4arcmin)spectralproperties.
A B D E F G
NH 2.49 1.49+−00..22 2.49 2.52−+00..44 2.49 2.11−+00..33
kT 3.35+0.03 3.47+0.04 3.46+0.04 3.45+0.04 3.56+0.05 3.56+0.05
−0.03 −0.04 −0.03 −0.03 −0.05 −0.05
kT2 - - 0.47−+00..0077 0.46−+00..0088 - -
M1 K2/K - - 0.015−+00..000022 0.015−+00..000055 - -
M˙ - - - - 93+15 73+20
−15 −19
Z 0.35+0.02 0.37+0.02 0.39+0.01 0.39+0.02 0.37+0.02 0.37+0.02
−0.01 −0.02 −0.01 −0.02 −0.02 −0.02
c 2 474.9/396 450.0/395 432.0/394 432.1/393 437.4/395 435.4/394
NH 2.49 1.78+−00..22 2.49 2.13−+00..33 2.49 2.56−+00..33
kT 3.22+0.03 3.30+0.03 3.32+0.03 3.35+0.05 3.40+0.05 3.40+0.05
−0.03 −0.03 −0.03 −0.04 −0.04 −0.04
kT2 - - 0.75−+00..019 0.86−+00..12 - -
M2 K2/K - - 0.016−+00..000033 0.015−+00..000066 - -
M˙ - - - - 92+16 96+22
−16 −22
Z 0.30+0.01 0.31+0.01 0.32+0.02 0.32+0.02 0.31+0.01 0.31+0.01
−0.01 −0.01 −0.02 −0.02 −0.01 −0.01
c 2 404.3/400 391.9/399 375.5/398 374.0/397 372.0/399 371.9/398
NH 2.49 0.47+−00..11 2.49 0.87−+00..11 2.49 1.11−+00..11
kT 3.04+0.02 3.29+0.02 3.20+0.02 2.29+0.08 3.43+0.03 3.43+0.03
−0.02 −0.02 −0.02 −0.08 −0.04 −0.03
kT2 - - 0.31−+00..0022 6.78−+00..65 - -
pn K2/K - - 0.035−+00..000033 0.51−+00..0077 - -
M˙ - - - - 159+8 86+11
−10 −11
Z 0.278+0.008 0.318+0.009 0.325+0.01 0.27+0.01 0.301+0.009 0.322+0.01
−0.008 −0.009 −0.006 −0.01 −0.009 −0.009
c 2 1781.8/1190 1482.2/1189 1537.3/1188 1307.4/1187 1504.2/1189 1417.2/1188
Models:A,Bphabs×mekal;D,Ephabs×(mekal+mekal);F,Gphabs×(mekal+mkcflow).Thefirst,secondmodelineachpairhaveGalactic,free
NHrespectively.K2/Krelativenormalizationofsecondtemperaturecomponent;M˙ massdepositionrate(M⊙yr−1);Zmetallicity(solarunits).
Therelativenormalizationofthesecondtemperaturecompo- thesecondcomponent,alsoprovidesasignificantimprovementin
nent is a few per cent of that of the bulk hot gas. If the hot and fit over a one-component model. The data are not able to distin-
cold phases are in pressure equilibrium, then nCTC =nHTH. The guishbetweenthesetwopossibilitieswithanydegreeofaccuracy.
normalizationofthemekalmodel variesasK(cid:181) n2V.Hence,the For example, for MOS1 a cooling flow provides a very slightly
relativefillingfactorofthecoldphaseisgivenby worsefitthanasingletemperature,whereasfortheMOS2thesit-
uation is reversed. The cooling flow fit does have only one extra
VC/VH=(KC/KH)∗(nH/nC)2=(KC/KH)∗(TC/TH)2; freeparameter though. Whentheabsorbing column isallowedto
befree,allthreedetectorsindependentlyfitamassdepositionrate
w∼hfiecwha×pp1l0ie−d4toinththeisrecsauslets.inTable2foreachdetectorgivesvalues ∼90M⊙yr−1.AnanalysisoftheReflectionGratingSpectrome-
ter(RGS)data(Section5)producesverysimilarmassdeposition
Interestingly,whenasecondtemperaturecomponentisavail-
results.
able, then the fitted N for the MOS detectors becomes consis-
H InFig.3weshowtheresultsofallowingtheminimumtem-
tent with the Galactic value. The pn results, however, for model
perature of the cooling flow component, T , to vary freely; for
min
Earenotsensiblyconstrained.Itisnoticeablethatthebest-fitting
bothfixedandfreeN (i.e.modelsFandGinTable2respectively).
H
low-temperature components for the twoMOS detectors in mod-
WhenT isallowedtovaryfreely,avaluesomewhatabove‘zero’
min
elsDandEaresomewhatdiscrepant.If(aswewillarguelateron)
(i.e.themodel minimum, 0.081keV)ispreferred; but azerocut-
therereallyisacoolingflowoperatingdowntolowtemperaturesin offtemperatureisentirelyconsistentwiththedata,withinthe1s
thissystem,thentherewillbegaspresentatacontinuousrangeof limits. The associated mass-deposition rate, M˙, is little changed.
temperatures,anditisnotobviouswhatthepreferredsinglelow-
FreeingN broadenstheallowedconfidenceregions,butdoesnot
H
temperature value will be in a model with only two temperature
greatlyaffecttheresults.
components, nor how it will vary between different instruments.
TheuppertemperaturesforthetwoMOSinstrumentsalsoseemto
besystematicallydifferent,albeitatalowerlevelthanforthelow
3.1 ComparisonwithChandra
temperatures.Intermsofthefit,thehighervalueforT inMOS2
2
seemstobeduetoaslightlybroaderironLhumpat1keVinthis WepresentabriefcomparisonoftheresultsofXMM-Newtonand
detector. Chandra observations of Abell 2597 through a re-analysis of the
Forcompleteness,wenotethatthepnresultsarestableagainst Chandra observation of McNamaraetal. (2001). We use a con-
alternative forms of light-curve filtering and background correc- servative flare screening of the Chandra data that results in 7ks
tion.Forexample,ifweapplythesamecount-ratecut(0.45cts−1) of good time. The agreement between the Chandra and XMM-
asusedintheblank-sky fieldsof Lumb(2002),and thenemploy Newton pointings for the location of the X-ray emission peak is
adoublebackgroundsubtractiontechnique(asusedfortheMOS), excellent. The largest complete circle centred on this point that
thenthepnfitsareessentiallyunchanged. canbeextractedfromtheACIS-S3chiphasaradiusof1arcmin.
Usingacoolingflowmodel(inwhichgascoolsfromthetem- This encompasses most of the cooling region (see Section 4.2).
perature of the bulkhot gas), insteadof asingle temperature, for A background spectrum was extracted fromthe appropriate stan-
(cid:13)c 0000RAS,MNRAS000,000–000
XMM-Newton Observationof A2597 7
4 RADIALPROPERTIES
Given the symmetric nature of the emission visible in Fig. 2, we
haveperformedanalysesusingcircularannulicentredontheclus-
teremissionpeak.
4.1 Surfacebrightnessprofile
Fig.5showsthesurfacebrightnessprofileofthecentral4arcmin
inradius of the cluster inthe 0.5–2.0keV band, from the sum of
theMOS1andMOS2data.Theprofilewasextractedfrom1arcsec
pixelimagesandvignetting-correctedexposuremaps,usingcircu-
lar annuli centred on the emission peak. A background was sub-
tractedusingtheappropriatescaledblank-fielddatasets.Thewidth
ofeachannuluswassuchthatthesignificanceofthebackground-
subtractedcountsitcontainedwasatleast5s .Pointsourceswere
excluded.
Also shown is the result of a b profile fit to the surface
Figure3.ConfidencecontoursintheM˙–TminplanefortheMOS1+MOS2 brightness,S (r)=S 0[1+(r/rc)2]−3b +0.5;withb =0.585+−00..000021,
cpoeorlpinangefllofiwxefidtsN(Hm,olodwelesrFpaanndelGfrieneTNaHb.leT2h,ebcurtowssitshhTomwinstfhreeebteosvt-afirtyv).aUlupe-, rc=T23h.e0s−+u00r..00fa22caercbsreicg.htnessprofileisaffectedbytheXMM-Newton
andthecontoursareplottedforD c 2=2.3,4.61,9.21;correspondingtothe
PSF,whichcanitselfbewelldescribed byab model (Ghizzardi
68,90,and99percentconfidencelimitsrespectivelyontwofreeparame-
ters.Theleft-handboundaryisduetothelowestusablemodeltemperature, 2001,2002),withacoreradiusofaround5arcsec.Whenanintrin-
0.081keV. sicbrightnessprofilea(~r)ismodifiedbyaninstrumentalPSFb(~r),
theobservedprofileistheconvolutionofthetwo,
¥
c(~r)= d2~ua(~u)b(~r−~u)=2p dkkJ0(kr)a˜(k)b˜(k), (1)
Z Z0
wherethesecondformisforthecaseofradialsymmetryandmakes
use of the convolution theorem. a˜(k) in this expression is the ra-
diallysymmetricFouriertransform(Birkinshaw1994),orHankel
transformoforderzero,
¥
f˜(k)= drJ0(kr)rf(r), (2)
Z0
withJ aBesselfunctionofthefirstkind.Evaluatingtheeffectof
0
instrumentPSFonasurfacebrightnessprofilethereforereducesto
performingthreeHankeltransforms.Thegeneralb functionhasan
analytic Hankel transform (Gradshteyn&Ryzhik 2000, equation
6.565.4),butthespecificcaseofb =2/3,whichisagoodapproxi-
mationforboththeXMM-NewtonPSFandmanyclusterbrightness
profiles,isespeciallysimple:
Finigruardeiu4s.C(thoenficdoeonlcinegcornetgoiuorns)infrothmeMt˙h–eTmCihnapnldanraefAoCrtIhSe-Sc3endtraatla1,awrcitmhina f(r)=(a2+r2)−3/2 ⇔ f˜(k)= e−ak (3)
a
GalacticNH.PlotdetailsareasperFig3.
Asaresult,if
S(r)=S0[1+(r/rs)2]−3/2 and P(r)=P0[1+(r/rp)2]−3/2 (4)
describe the source and PSF profiles respectively, with the latter
dardblank-skyfield.Thespectrumwasfittedoverthe0.6–7.0keV normalizedsothatP0=(2p rp)−1,thenitisstraightforwardtoshow
thattheconvolutionc(r)isgivenby
range.
Fig.4istheChandracounterpartoftheupperpanel(i.e.fora rs 2 r 2 −3/2
cfioxnesdtrNaHin)tsofwFhiegn.3thfeorabthseorMbiOngS.coWluemwnerweausnaalblolewteodotbotavianrysefnrseiebllye. c(r)=S0(cid:16)rs+rp(cid:17) (cid:20)1+(cid:16)rs+rp(cid:17) (cid:21) . (5)
Thepreferredlow-temperaturecut-offforthecoolingflowcompo- Thisisnothingmorethananotherb profile,withareducedcentral
nent,thoughnottightlyconstrained,isaslowaspossible.Theas- amplitudeS′ andincreasedcoreradiusr′:
0 s
swreoisctuhialtttshedearXmeMaalsMsso-dNienepawogstroitenieomEnPerInaCttewrieissthu∼ltths7o0fsoeMrft⊙rhoeymrc−etn1h,tereaRnlGt4irSaer,lcyamscisonhn.osTwishnteesinnet S0′ =S0(cid:16)1+rrps(cid:17)−2∼S0(cid:16)1−2rrps(cid:17) and rs′ =rs+rp. (6)
Fig.17.Fig4maybecontrastedwiththemorecommonChandra The simplest PSF correction we can make to the surface
resultinsuchcases,whereamuchhigherT ispreferred(e.g.in brightness profile is therefore to subtract the radius of the PSF,
min
Abell3581;Johnstoneetal.2005,fig.10). 5arcsec, from the core fitted to the convolved profile, 23arcsec,
(cid:13)c 0000RAS,MNRAS000,000–000
8 R.G. Morrisand A.C. Fabian
Figure5.Surfacebrightnessprofileofthecentral240arcsecinradius,0.5–
2.0keV band, from summed MOS1 and MOS2profiles. The dotted line
showsab profilefittothedata–seetextfordetails.Theprofilehasbeen
correctedforbackgroundandvignetting.
resultinginanestimateof18arcsecfortheintrinsicsurfacebright-
nesscoreradius.
4.2 Spectralprofiles
Inorder toobtain reliablespectral fits,annular radii werechosen
soastoencompassslightlymorethan10000(net,i.e.background-
subtracted) counts in each MOS, and thus over 20000 counts in
thepn. Itwas found that annuli withfewer counts (e.g. 7500 per
MOS)tendedtogivepoor(unphysical)resultsinthedeprojection Figure6.Projectedtemperature(upperpanel)andmetallicity(lowerpanel)
and PSF-correction analyses. In such cases, the lower signal-to- profiles.TheopentrianglesaresimultaneousMOS1+MOS2fits,thefilled
squaresarethepn.Themodelwasphabs×mekalwithacommon,free
noiseallowsphysicallyunrealisticsolutions(inwhichfittedquan-
tities tend to oscillate about the smoother profiles obtained from absorbingcolumnNHsharedbyallannuli.TheNHvalueswere(inunitsof
annuli with more counts) to become mathematically valid. Both 1020cm−2):1.52−+00..0088(MOS);0.19−+00..0066(pn).WithNHfreetovarybetween
annuli,theTandZprofilesarenotmuchchanged.TheMOSNHshowsno
correctionalgorithms(seee.g.Johnstoneetal.2005fortestingof
significantvariationwithradius;whereasthatofthepnisconsistentwith
thedeprojectionprocess)involveredistributionofcountsbetween theMOSinthecentraltwobins,butmuchloweroutsidethisregion.
annuli,potentially(dependingonthedetailsofthebrightnesspro-
file) leaving small net numbers of counts in any one annulus. In
practiceitisfoundtobebeneficialnottolettheuncorrectedcount
In Fig. 7, we show the radial dependence of the cumula-
numbersfalltoolow.Theoutermostannuluswastruncatedatara- tivemassdeposition rate, M˙(<r),withinthecooling flow radius
diusof5arcmin(duetotheincreasingrelativebackground)andso
(130kpc; see Section 4.2.2). There is a discrepancy between the
contains somewhat fewer counts. By the timethislast annulus is
MOS and pn detectors, with the former preferring smaller mass
reached, the background is contributing about 30 per cent of the
depositionrates.Owingtothewaythisfigurewasconstructed(fit-
totalcounts.
tingeachannulusindependentlywithasingletemperaturecompo-
nentandacoolingflowcomponent,thensummingtheM˙ valuesso
obtained from the centre outwards), the discrepancy between the
4.2.1 Projectedprofiles
two instruments builds with increasing radius. Also shown is the
Fig. 6 shows the results of fitting a single-temperature MEKAL M˙ value inferred by Oegerleetal. (2001) from their FUSE data;
(Meweetal. 1995) model to each spectral annulus, with an uni- namely40M yr−1withinaradiusof40kpc.Thefigureillustrates
⊙
form(butfreetovary)absorbingcolumnappliedtoallannuli.The thatthereisreasonableagreementbetweentheX-rayandUVmass
central temperature isa littleover 2.5keV (afunction of thesize depositionrateswithinaradiusof40kpc.
ofthecentralannulus).Overthecentral100kpcorsothetempera-
tureincreases,reachingamaximumof∼3.6keV.Outsidethecen-
tralcoolingregion,thereisperhapsevidenceforagradualdecline
4.2.2 Deprojectedprofiles
intemperaturebeyond∼150kpcorso.Themetallicityexhibitsa
fairlysmoothdeclinewithincreasingradiusinthecentralcooling The results of Fig. 6 are subject to projection effects, in which
region,droppingfromaround0.5Z atthecentreto0.2Z atthe thespectralpropertiesatanypointintheclusteraretheemission-
⊙ ⊙
outside. weightedsuperpositionofradiationoriginatingatallpointsalong
ResultsareshownforthesimultaneousfitofbothMOSinstru- thelineofsightthroughthecluster.InFig.8weshowtheresultsof
ments,andalsoforthepn.Thetwoinstrumentsagreereasonably adeprojectionoftheMOSandpnspectrausingtheXSPECprojct
well interms of T and Z,but have very different preferences for model.Undertheassumptionofintrinsicspherical(moregenerally,
N . ellipsoidal) shells of emission, this model calculates the geomet-
H
(cid:13)c 0000RAS,MNRAS000,000–000
XMM-Newton Observationof A2597 9
Figure7.Cumulativeprojectedmassdepositionrate,M˙(<r),forthecool-
ingflowregion.TheopentrianglesaresimultaneousMOS1+MOS2fits,
thefilledsquaresarethepn.Themodelwasphabs×(mekal+mkcflow)
withafixedGalacticabsorption.ThestarshowstheFUSEOVIdatapoint
ofOegerleetal.(2001),withanerrorbarappropriatefortheaccuracyof
theFUSEfluxmeasurement,(1.3±0.35)×10−15ergcm−2s−1.
ricweightingfactorsaccordingtowhichemissionisredistributed
amongsttheprojectedannuli.
As is to be expected, the temperature in the outer regions
wheretheprofileisfairlyflatisseentoberelativelylittleinfluenced
by projection. Deprojection recovers a lower central temperature
thanbefore,sinceintheprojectedfitsthespectrumofthecentral
annulusiscontaminatedbyoverlyinghotteremission.Thereisalso
Figure 8. Deprojected temperature (upper panel) and metallicity (lower
some evidence for a higher central abundance in the deprojected
panel)profiles.TheopentrianglesaresimultaneousMOS1+MOS2fits,the
metallicityprofile.
filledsquaresarethepn(asperFig.6).Themodelwasprojct×phabs×
Thedeprojectedpntemperatureprofileshowssomesignsof mekalwithacommon,freeabsorbingcolumnNHsharedbyallannuli.The
instabilityinthecentre,inwhichneighbouringbins‘oscillate’with NH valueswere(inunitsof1020cm−2):1.53−+00..0088 (MOS);0.23+−00..0066 (pn);
highandlowtemperatures.Theinterpolatedvalues,though,agree littlechangedfromtheprojectedresults.
wellwiththeMOSresults.Recently,Johnstoneetal.(2005)have
shownthattheprojctapproachworkswellatreproducingvarious
simplesyntheticclustertemperatureanddensitystructures.Possi-
(inperfectagreementwiththeROSATresultofSarazinetal.1995),
blytheslightinstabilityofthepnfitreflectsamorecomplexunder-
andlessthan5Gyrinside85kpc.
lyingtemperaturedistribution(e.g.morethanonephase);butmore
AlsoshowninFig.9aretheresultsofsimplemodelfitstothe
likely it is simply a quirk of geometry that renders certain phys-
densityandentropyprofiles.Thedensityprofileisrepresentedby
iacbaulnlydaunncreesalaitsttihcesporloujteicotnesdmvaaltuheemmaatkiecsallliyttlpeldauifsfiebrleen.cFer,ebeuztinthgetehfe- ab model,n(r)=n0[1+(r/rc)2]−1.5b ;withb =0.57+−00..0011,n0=
fectcanberemovedbyusinglargerannuli(althoughthepnannuli 0.0354−+00..00000066cm−3,rc=49.0−+00..99kpc.Theseresultsfortheden-
sityprofileareinreasonable agreement withthoseobtainedfrom
shownalreadycontainsimilarnumbersofcountstothecombined
fittingthesurfacebrightnessprofile(Section4.1).Theabsenceofa
MOS annuli), but weretainthesameannuli here for bothinstru-
significantexcessinthedeprojecteddensityinthelastannulusin-
mentsforillustration.
dicatesthatwereachasufficientlylargeradiusthatprojectionfrom
InFig.9weshowvariousquantitiesderivedfromthedepro-
more distant material is not significant (compare with the Chan-
jected spectral fits. The electron density ne is obtained from the
1/2 dradeprojectionresultsofJohnstoneetal.2005).Theentropypro-
mekalno−rm1/a2lization,andscalesasH0 .Thecoolingtime(scal- fileisparametrizedasaconstantpluspower-law,S(r)=S0+krh ;
ing as H0 ) is an approximate isobaric one, calculated as the withS0=15.1−+11..55keVcm2,k=0.28−+00..0055keVcm2kpc−1.19,h =
ctiomoelitnagkerantefoarttahneygatesmtoperaradtiuartee,itsenthalpyusingtheinstantaneous 1.19−+00..0043.ThefitwiththeS0 termissubstantiallybetterthanthat
without,butbearinmindthatthePSFhasnotbeencorrectedfor
H g kT here. A logarithmic slope for the entropy profileof ≈1.1 ispro-
tcool≈ nenHL (T) = g −1m XHneL (T), (7) duced in simulations involving gravitational collapse and shock
heating(Tozzi&Norman2001).
where:g =5/3istheadiabaticindex;m ≈0.61(forafully-ionized
Thevirialradiusbecomesaquantityofinterestatthispoint.
0.3Z⊙ plasma) isthemolecular weight; XH≈0.71 isthe hydro- It is known that the calibrations of the scaling relations obtained
genmassfraction;andL (T)isthecoolingfunction.The‘entropy’
fromsimulations havenormalizations quitedifferent tothose ob-
−2/3
is the standard astronomer’s entropy, S=Tne , and scales as tained from observation. Allenetal. (2001) studied the cluster
−1/3
H . virialscalingrelationsusingChandra,butdonotquoteanexplicit
0
Thecoolingtimeislessthan10Gyrinsidearadiusof130kpc radius–temperature relation. Using their data for an SCDM cos-
(cid:13)c 0000RAS,MNRAS000,000–000
10 R.G. Morrisand A.C. Fabian
Figure10.xmmpsfmixingfactorscalculatedat1.0keV.Eachcurveshows,
foroneoftheannularregions,thefractionofthefluxinthatregionorigi-
natingfromeachoftheotherregions.Onlycontributionsgreaterthan1per
centareshown.Thepeakfluxisalwaysfromtheregionitself.Theactual
mixingfactorsusedinternallybythemodelrelatetothefractionoftheflux
fromeachregiongoingtootherregions.Thefactorsshownherearecalcu-
latedfromtheseusingthemodelnormalizationsfittedtoeachannulus.By
construction,thepointswiththesamex-coordinatesumto1.0.
icallyverylargeareemployed;butinsuchcasesthePSFredistri-
bution becomes less significant anyway, and we also obtain very
limitedresolutionofthecentralcoolingregion,whichisthemain
areaofinteresthere.
Foragivensetofspectralannuli,andagivenbrightnesspro-
Figure9.Derived quantities fromthe deprojected profiles ofFig.8:up- file,thexmmpsfmodelcalculatesthemixingfactorsthroughwhich
perpanelelectrondensity;middlepanelapproximateisobariccoolingtime; fluxfromanygivenannulusisredistributedtoalltheothers.The
lowerpanelentropy.Thedottedlinesshowthefitsofsomesimplemodels factorsarecalculatedattenenergiesintherange0.5–6.0keV,al-
tothedata:abetaprofileforthedensity;andaconstantpluspower-lawfor
though the energy dependence of the XMM-Newton PSF is rela-
theentropy.Seetextfordetails.
tivelysmall.Theinputbrightnessprofilecanbespecifiedusingei-
theranimage,oramodelparametrization.Wefindthatthereisno
significant difference between using a MOS1 (say) image or the
mology,wecalibratether–T relationasr2500=0.31Mpch5−01(1+ PSF-correctedb profilefromSection4.1toprovidethebrightness
z)−3/2(T2500/keV)0.5.Theconversionfromanoverdensityof2500 information.Bywayofexample, weshow themixingfactorsfor
tooneof200(appropriateforthevirialradius)dependsuponthe
oursetofannuliat1.0keVinFig.10.Atthetenpercentlevel,each
formassumedforthedensityprofile.TakinganNFWprofilewith
annulusisonlyaffectedbyitsimmediateneighbours.Nevertheless,
atypicalclusterconcentrationc=5,wefindthatr200≈2.75r2500. themixingispotentiallyquitesignificant,sinceformanyregions
Usingatemperatureof3.5keV,wethereforeestimateavirialra-
only50percentoftheirfluxoriginatesfromthatsameregion.
dius of 1.4Mpc for Abell 2597. The radius 0.1R therefore lies
V InFig.11,weshowtheT andZprofilesobtainedfromaPSF-
withinthesixthannulusfromthecentre(abinwhosemid-pointis
correctedfitoftheMOS1data.Comparedtotheuncorrectedpro-
at 150kpc). Themeasured entropy in thisbin isS0.1RV =(137± files(Fig.6),asteepercentraltemperaturegradientisrecovered,as
6)h−1/3keVcm2. Thisisa relativelylow value (see e.g. fig. 4of wellasalargercentralabundancepeak.
50
Ponmanetal.2003),butrecallPSFeffectsarestillpresentinthis Fig.12displaysthedensity,coolingtime,andentropyderived
result. fromthePSF-correctedprofiles.ComparedtotheresultsofFig.9,a
highercentraldensity,andalowercentralcoolingtimeandentropy
areobtained.Alsoshownisab modelfitforthedensity,with:b =
4.2.3 PSF-correctedprofiles 0.579−+00..000044, n0 =0.068−+00..000011cm−3, rc =31.6−+00..66kpc. The PSF
correctedcoreradiusof15arcseciscomparabletotheestimateof
ThebroadeningeffectoftheXMM-NewtonPSFactstoredistribute
18arcsecmadeinSection4.1usingthesurfacebrightnessprofile.
countsbetweenspectralannuliinamannerconceptuallysimilarto Theentropyprofileiswell-fit–reduced c 2=7.49/(9 − 3)
thatofprojection.Consequently,itcanbecorrectedforinthesame
– by a constant plus power-law model of the same form
way,Tnahmeemlyosbtyreucseinngt vaenrsXiSoPnEaCvamiliaxbinlegamtothdeelt,imxmemopfsfw.ritingwas used in Section 4.2.2, with: S0 = 7.9−+11..53keVcm2, k =
0.40+0.11keVcm2kpc−1.15,h =1.15+0.05.
unable to fit observations from different instruments simultane- −0.08 −0.04
ously,sowepresenthereonlytheresultsforMOS1,inordertoil-
lustratetheeffectsofPSFonthespectralfits.Wehavebeenunable
4.3 ComparisonwithChandratemperatureprofile
toachievephysicallyrealisticsolutionswhenusingthexmmpsfand
projctmodelsincombination,tocorrectforbothprojectionand AsperSection3.1,wehavecomparedtheXMM-NewtonandChan-
PSFatthesametime.Theexceptioniswhenannulithatarephys- draresults.Chandraspectrawereextractedfromthesameannuli
(cid:13)c 0000RAS,MNRAS000,000–000