A deep Chandra observation of the cluster environment of the z=1.786 radio galaxy 3C294

10 Pages·0.45 MB·English

by  A. C. Fabian

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed5February2008 (MNLATEXstylefilev2.2)
A deep Chandra observation of the cluster environment of the
z = 1.786 radio galaxy 3C294
A.C. Fabian1, J.S. Sanders1, C.S. Crawford1 and S. Ettori2
1InstituteofAstronomy,MadingleyRoad,Cambridge.CB30HA
2ESO,Karl-Schwarzschild-Str.2,D-85748Garchingb.Munchen,Germany
5February2008
3
0 ABSTRACT
0 Wereporttheresultsfroma200ksChandraobservationofthez=1.786radiogalaxy3C294
2 anditscluster environment,increasingby tenfoldourearlierobservation.Thediffuseemis-
n sion,extendingabout100kpcaroundthenucleus,hasaroughlyhourglassshapeintheN-S
a directionwithsurprisinglysharpedgestotheNandS.Thespectrumofthediffuseemission
J iswellfittedbyeitherathermalmodeloftemperature3.5keVandabundance<0.9Z⊙(2s ),

3 orapower-lawwithphotonindex2.3.Iftheemissionisduetohotgasthenthesharpedges
2 meanthatitisprobablynotinhydrostaticequilibrium.Muchoftheemissionisplausiblydue
toinverseComptonscatteringoftheCosmicMicrowaveBackground(CMB)bynonthermal

1 electronsproducedearlierbytheradiosource.Therequiredrelativisticelectronswouldbeof
v much lower energyand older than those responsiblefor the presentradio lobes. This could
8
accountforthelackofdetailedspatialcorrespondencebetweentheX-raysandtheradioemis-
6 sion,theaxisofwhichisatapositionangleofabout45◦.Hotgaswouldstillberequiredto
4
confine the relativistic plasma; the situation could parallel that of the radio bubbles seen as
1
holesin nearbyclusters, exceptthatin 3C294the bubblesarebrightinX-raysowingto the
0
extremepowerinthesourceandthesixtyfoldincreaseintheenergydensityoftheCMB.The
3
0 X-rayspectrumoftheradionucleusishard,showingareflectionspectrumandironline.The
/ sourceisthereforeanobscuredradio-loudquasar.
h
p Keywords: X-rays:galaxies–galaxies:active:clusters:individual(3C294)–intergalactic
- medium
o
r
t
s
a
:
v 1 INTRODUCTION ies.Hardcastle&Worrall(1999)laterexaminedalongROSATob-
i servationtakenwithitsHighResolutionImager(HRI),concluding
X 3C294isapowerfulFanaroff-RileyIIradiosource(FRII)atared-
thatthesourcewaspossiblyspatiallyextended.
shiftof1.786.Ahighresolutionimageoftheradioemissionshows
r
a it to have a Z-shaped morphology, which may be explained by a We first observed 3C294 with Chandra in 2000, for a total
precessingortorquedjet(McCarthyetal.1990).ExtendedLyman of 20 ks with the ACIS-S detector (Fabian et al. 2001). The X-
a emissionwasfoundtobeassociatedwiththesource,withalu- ray emission was spatially extended about the central core X-ray
minosityof7.6×1044ergs−1 (McCarthyetal.1990).Thesource sourceinadistinctivehourglassshape.Thehourglasswasaligned
waslaterobservedbyStocktonetal.(1999) intheK′ bandusing inthenorth-south(N-S)directionandextendedtoradiiofatleast
anadaptiveopticssystemontheCanada-France-HawaiiTelescope, 100kpc.Aspectrumofthediffuseemissionwasfittedusingather-
resolvingitintoanumberofknotsarrangedoveranapproximately mal model, giving a best-fittingtemperature of 5 keV. The emis-
triangular region. They interpreted this pattern as a set of dusty sioncouldhavebeenduetoinverseComptonscattering,butsince
dwarf galaxy-like clumps illuminated in a cone by an obscured therewaslittleexcessemissionassociatedwiththeradiojetofthe
nucleus. Quirrenbach et al. (2001) observed the system at higher source, except on the southern hotspot, this emission mechanism
resolutionwithanadaptiveopticssystemontheKecktelescopein was considered unlikely. The most likely explanation for the ob-
K′andHbands,findingemissionfromtwocomponentswithasep- serveddiffuseemissionwasathermalone,generatedbytheintra-
arationof∼1arcsec.Onecomponentwasidentifiedastheactive clustermedium(ICM)oftheinnerpartsofacluster.Theexistence
nucleus,theotherasthecoreofacollidinggalaxy. ofsuchahotclusteratthisredshiftisconsistentwithalow-density
X-ray emissionwas detected fromtheposition of 3C294 by universe.
Crawford & Fabian (1996), using ROSAT proportional counter
data. The data quality was insufficient to conclude whether the HerewepresenttheresultsofatentimesdeeperChandraex-
emission was spatially extended or thermal in origin. Their posure of 3C294, for almost 200 ks. Luminosities and distances
favouredmodelwasforX-rayemissionfromarichclusterofgalax- werecalculatedinthispaperbyassumingH0=70kms−1Mpc−1,
(cid:13)c 0000RAS
2 A.C. Fabian,J.S. Sanders,C.S. Crawford andS. Ettori
W m=0.3andW L =0.7.Relativeabundanceswerecalculatedas-
sumingtheresultsofAnders&Grevesse(1989).
2 DATAANALYSIS
Theobservationwassplitintotwosections:oneperiodbeginning
on 2002 Feb 25 with an exposure of 69.8 ks, and the second on
2002 Feb 27 with an exposure of 122.0 ks. Visual inspection of
thelightcurves for thetwoobservations intheband 0.3 to5keV
showednoevidenceforcontaminationbyflares,thereforethetotal
effectivelengthofobservationswas191.8ks.
BothofthedatasetsweretakenusingtheACIS-S3detector
inVFAINTmode.Thismodeofobservationgradeseventsusinga
5×5matrixof detectorpixelsaround thecentroidofeachevent,
rather than the standard 3×3 matrix for the FAINT observation
mode,andyieldsadatasetwithasubstantiallylowerbackground.
InordertotakeadvantageoftheVFAINTobservationmode,
thedatawerereprocessedusingtheCAIOACIS PROCESS EVENTS
toolwiththecheck vf phaoptionswitchedon.Thegainfileapplied Figure1. Energy-mapped image ofthe emission. Events between 0.3to
to the reprocessed dataset was acisD2000-08-12gainN0003.fits 1keVarecolouredred,1and1.5keVgreen,and1.5to5keVblue.The
fromCALDBversion2.12. dataareplottedwith0.49arcsecpixels,smoothedwithaGaussianofwidth
Sincethetwodatasetsweretakenoveronlyafewdays,the 0.25arcsec.Thesizeoftheimageis47×48arcsec2,correspondingtoan
roll angles of the spacecraft were within a couple of degrees of areaofroughly400×400kpc2 attheredshiftoftheradiogalaxy.Inthis
andallourimages,northistothetopandeastistotheleft.
eachother.Thereforewemergedthetwoeventsfilestogetherinto
asinglefile,reprojectingthepositionoftheevents(inskyaligned
detectorcoordinates)ofthefirstdatasettomatchthatofthesecond.
Region Fit-type Best-fittingparametersandreducedc 2
Totestwhetherthisstepwasvalidwegeneratedspatially-weighted
responseandancillarymatricesforthediffuseemissionforthetwo Core Power-law G =−0.65,cn2=2.7
data sets individually and compared the differences in fitting the PEXRAV∗ rel.refl=5.1−+12..50,
spectra using the two responses on both data sets. There was no cover.NH=8.4−+01..91×1023cm−2,
significantdifferenceineitherthequalityofthefitsortheirbest- cover.fract.=97.6−+31..00%,cn2=0.55
fittingparameters.
The weighted responses for each region were created from NE Power-law G =0.7,cn2=0.77
theappropriateFEFcalibrationfile(acisD2000-01-29fef piN0002) Par.Cover.† cover.NH=2.5−+11..74×1022cm−2,
andtheCIAOMKWARFandMKRMFtools,weightingtheresponse cover.fract.=75+−1546%,G =1.6−+11..01,
usingthenumberofcountsinthe0.3to7.5keVband. cn2=0.82
Creatingasuitablebackgroundspectrumisanimportantpart Diffuse MEKAL kT=5.9−+11..38keV,Z=0.07−+00..0276,
of the analysis of low surface brightness objects such as the dif- NH=3.8−+12..81×1020cm−2,cn2=1.0
fuseemissionhere.Giventhattheemissionissmallinextentand Power-law G =2.3+0.3,
−0.1
theobservationislong,itispreferabletouseabackground spec- NH=1.1±0.3×1021cm−2,cn2=0.91
trumgeneratedfromtheobservationitselfratherthandifferentob- MEKAL‡ kT=3.5−+00..56keV,Z=0.30−+00..2350Z⊙,
servations. A blank-sky background dataset would have a differ- cn2=0.98
entinstrumentresponseandgalacticabsorptiontoourobservation. Power-law‡ G =2.29±0.10,cn2=0.64
WethereforeusedblankregionsoftheS3detectorinthemerged
events file to provide the background. Too small a region gives Table1.Summaryofresultsofspectralfits.Uncertainties shownare1s .
uncertain background subtraction, and toolargerisksintroducing ∗PEXRAVreflection discmodel plus emission from neutral iron line, ab-
systematicsfromthevariationofthebackgroundoverthechip.We sorbedwithpartialcoveringmodel.†Power-lawmodelabsorbedwithapar-
usedabackgroundfield4.97×1.67arcmin2 alongtheCCDnode tialcoverer.‡MEKALorpower-lawmodelabsorbedwithACISABSmodel
whichharboursthediffuseemission,subtractingacircleofradius plusPHABSmodelsettoGalacticabsorption.
0.41arcmincentredonthecentralsource.
theresultofwhichshowninFig.2(top).Weremovedthosepho-
2.1 Thediffuseemission
tonswhichdidnothave3ormoreneighbouringphotonswithina
ThediffuseX-rayemissionhasthedistinctivehourglassshapeseen radiusof1arcsecfromthem.Theremovedphotonswereinaflat
in our earlier observation of this object (Fabian et al. 2001). In distributionaroundthediffuseemission.
Fig.1weshowimagesoftheemission,smoothedbyaGaussian, Tohighlightthemainfeaturesofthethree-colourimage(Fig.
inthreeobservedenergybands(0.3-1.0,1.0-1.5and1.5-5.0keV), 1)weshow asmoothed versionofitinFig.2(bottom). Weused
superimposedasred,greenandblue. thebinaccretionalgorithmofCappellari&Copin(2002)tocreate
Inordertoseethediffuseemissionabovethebackgroundeas- atessellatedimageineachofthethreebandswitharatioofsignal
ilywehaveappliedabackground-reduction technique(analogous toPoissonnoiseof0.8.Thecentroidsofthecellsformedbytessel-
to applying a cut in brightness to an image with many photons), lationweretheninterpolatedwiththeNATGRIDnaturalneighbour
(cid:13)c 0000RAS,MNRAS000,000–000
A deep Chandraobservationoftheclusterenvironmentof thez=1.786radiogalaxy3C294 3
Figure3.Imagesoftheemissioninthe0.3-1.0(left),1.0-1.5(middle)and1.5-5.0keV(right)bands.Dataareshownwith0.49arcsecpixels,smoothedwith
aGaussianofwidth0.25arcsec.
interpolationlibrary1toformimages,whichweresuperimposedas
red,greenandbluelayers.
Therawimages areshown individually inFig.3,emphasiz-
ingthat theemission isextended ineach of thethree bands. The
diffuseemissionisstillvisibleina3-5keVimage,corresponding
to8.4-13.9keVintherestframe.Binningthedatainto16arcsec
binsshowsthereisnoevidenceforanylowintensityemissionsur-
rounding the central diffuse emission (wecalculate limitsfor the
outlyinggasinSection4.1.1).
The images also show several point sources: a central hard
sourcecorrespondingtothepositionoftheradiocore,anotherhard
source 14arcsectothenorth-east (NE),asoft source 9arcsecto
thewest-south-west(WSW),andtwofairlysoftsourcesalongthe
directionoftheradioaxis(6arcsectotheNEand8arcsectothe
SW)corresponding totheposition of theradio hotspots. Wewill
discussthesepointsourcesinlatersections.
The surface brightness declines sharply at the edge of the
hourglassshape.Fig.4showsasurfacebrightnesscutinanorth-
southdirection,measuredbymovinga∼21×1arcmin2 boxalong
thataxis,excludingthepointsourceinthecentre,theNEandthe
WSW.Weshowthenumberofcountsinthethreeobservedbands:
0.3-1.0,1.0-1.5and1.5-5.0keV.Theprofileappearsbox-likewith
thenumberofcountsdroppingfromabout60to10inaround5arc-
secattheNandSedges.
Theprofileintheeast-westdirectiondeclinesmoresmoothly
thanthatinthenorth-southdirection.Inthesouthernmostlobethe
diffuse emission declines slowly to the east. In the northernmost
lobeitdeclinesslowlytothewest.
Wefittedthe0.3-6keVspectrumofthediffuseemissionfrom
theregionsshown inFig.5by a MEKAL model (Mewe, Gronen-
schild&vandenOord1985;Liedahl,Osterheld&Goldstein1995)
with a PHABS absorbing screen (Balucinska-Church & McCam-
mon1992),givingbest-fittingparametersofkT=5.9+1.8keV,Z=
−1.3
0.07−+00..0276Z⊙,NH =3.8−+12..81×1020cm−2 (with1s uncertainties).
At2s ,theuncertaintyonthetemperaturewaskT =5.9+3.4 keV.
−2.0
The data were binned into spectral bins containing at least 20
Figure2.Greyscaleimageofthediffuseemissionbetween0.3and5keV counts. The spectrum is shown in Fig. 6, with the contributions
usingabackgroundremoval technique andsmoothedwithaGaussianof to c 2 from each spectral bin. The reduced c 2 of the fitwas1.00
width0.5arcsec(top).Energymappedimageofthediffuseemission(asin
Fig.1)smoothedusingatessellationtechnique(bottom).
1 http://ngwww.ucar.edu/ngdoc/ng/ngmath/natgrid/nnhome.html
(cid:13)c 0000RAS,MNRAS000,000–000
4 A.C. Fabian,J.S. Sanders,C.S. Crawford andS. Ettori
100
0.3-1.0 keV
1.0-1.5 keV
1.5-5.0 keV
Total
80 A2199
60
nts
u
o
C
40
20
0 -20 -10 0 10 20 Figure6.Spectrumofthediffuseemissionbetween0.3and6keV(from
Relative declination (arcsec) theboxedregioninFig.5,alsoshowingc contributionfromeachspectral
binwhenfittedwithasingle-componentMEKALmodel.
Figure4.Surfacebrightnessprofilealongthenorth-southdirection(nega-
tivevaluesaresouth),excludingthecore,NEsourceandtheWSWsource.
Valuesshownarethenumberofcountsina0.98×20.6arcsec2box,inthe
bands0.3-1,1-1.5,1.5-5.0and0.3-5keV.Thebackgroundlevelisaround5 there is a known degradation of the ACIS soft X-ray response2
countsinthe0.3-5keVband(i.e.beyond15-20arcsecisjustbackground). which may account for this. We have refitted the spectra making
Thecontinuouspeakedlineshowsasimilarnorth-southprofileofA2199, useofthepreliminaryresponsemodelACISABSofChartas&Get-
onthesamespatialscaleas3C294,inabandequivalentto0.3-1.0keV.The man(2002).Fittingthespectrumfrom0.4-6keV byapower-law
countsinthecentral3binsareunderestimated sincethecoresourcewas model(increasingthelowerenergyboundofthespectralfitassug-
removedbyexcludinga1.6arcsecradiuscircle. gestedbythedocumentation ofthismodel)andabsorbingitwith
the ACISABS model plus a PHABS model (fixed to the Galactic
valueof1.2×1020cm−2),thereducedc 2ofthebest-fitwas0.64=
20.6/32. Thebest-fittingphotonindexwas2.29±0.10(1s ),and
the intrinsic luminosity of the power-law was 2.5×1044ergs−1
between 1 and 10 keV in the restframe. There was no indication
ofanironlineintheresidualsofthepower-lawfit.Wenote,how-
ever,thattheenergyofanironlinewouldlieclosetotheiridium
M-edgeofthemirror,whichcouldcauseaproblemifthespectral
calibrationisnotperfect.
We also fitted a MEKAL model absorbed by the ACISABS
and PHABS models as above, allowing the abundance and tem-
peraturetobefree.Inthatcasethebest-fittingtemperatureofthe
gas was 3.5+0.6 keV (1s ) or 3.5+1.1 keV (2s ). The best-fitting
−0.5 −0.7
abundance was 0.30−+00..2350Z⊙ (1s ) or 0.30−+00..3507Z⊙ (2s ). A plot
of c 2 spaceshowstheerrorsintemperatureandabundance tobe
largelyorthogonal. The reduced c 2 of the best-fittingparameters
was 0.98=30.3/31. Separate spectra of the northern and south-
ern lobe were made, and fitted simultaneously with the absorbed
Figure5.Regionsusedforspectralextraction,overlayedontherawX-ray MEKALmodelabove.Therewasnoevidenceintheresidualsofthe
imageofthediffuseemissioninthe0.3-5.0keVband.Thetwoboxeswere fitindicatingthatthespectraofthetwolobesweredifferent.
theregionsusedfortheanalysisofthediffuseemission.Thecircletothe Asafinalcheck,werefittedthespectrumofthediffuseemis-
NEwastheNEsourceregion,andthecircleinthecentrewastheregion sionbyminimizingamodifiedversionoftheC-statistic(allowing
usedforthecore.ThediamondontherightmarkstheUSNO-A2.0position backgroundsubtraction;Arnaud2002)ratherthanthec 2-statistic.
ofthestar. Withthe MEKALmodel,wefoundtemperatureandabundance of
kT =3.9+−00..65 andZ=0.2−+00..22Z⊙ (1s ),fittingthespectrumfrom
0.4-6 keV. Fitting from 1-6 keV, we obtained a temperature of
(33.0/33). Theregionswerechosen toexclude theemissionfrom 7.4+2.7 keV, confirmingtheresultaboveusingthe c 2-statistic.A
−1.6
alongtheradioaxisofthesource,anareawhichcouldbecontam- power-lawmodelgavethebest-fittingspectralindex2.26+0.09.The
−0.10
inatedbyinverseComptonemission.Wesummarisetheresultsof C-statisticdoesnotprovideagoodness-of-fit.
spectralfitsinTable1. In summary, providing that we assume that the ACISABS
We also fitted the spectrum of the diffuse emission using a model is appropriate for correcting the ACIS-S low energy re-
power-lawabsorbedbyaPHABSmodel.Thismodelfittedthespec- sponse,bothMEKALandpower-lawmodelsgiveacceptablefitsto
trum well (reduced c 2 =0.91=31.0/34) with a photon index thespectrumofthediffuseemission.Thebest-fittingMEKALtem-
of2.3+−00..31,howeveritrequiredstrongabsorptionof(1.1±0.3)× perature is ∼3.5 keV and the abundance is <0.9Z⊙ (2s ), with
1021cm−2 whichwouldbeunreasonableifitweredistributedover
thewholevolumeofthediffuseemission.
Althoughbothmodelsfittedsofarrequireexcessabsorption, 2 http://cxc.harvard.edu/cal/Links/Acis/acis/Cal prods/qeDeg/
(cid:13)c 0000RAS,MNRAS000,000–000
A deep Chandraobservationoftheclusterenvironmentof thez=1.786radiogalaxy3C294 5
Figure8.SpectrumofthesourcetotheNEoftheradiosource.Thespectral
fitshownisapowerlawwithapartialcoveringmodel.Dataarebinnedinto
spectralbinscontainingatleast20counts.
2.3 TheNorth-East(NE)source
TheX-rayspectrumofthesourcetotheNEofthecoreisshown
in Fig. 8. It is well fitted by an absorbed (N ∼ 1020cm−2)
H
Figure7.SmoothedHSTimageoftheareaofthediffuseX-rayemission, power-law model, but the best-fitting photon index is unphysical
overlayedbycontoursfromasmoothedX-rayimage,adjustedsothatthe at 0.7. A partial covering model fitsbetter, withanabsorption of
opticalpositionofthestariscoincidentwiththepositionofitsX-raysource. 2.5+1.4×1022cm−2,apartialcoveringfractionof75+14percent,
−1.7 −56
Northistothetopinthisimage.Thearrowshowsthepositionofavery apowerlawindexof1.6+1.1(1s errors).Thereducedc 2forthefit
faintopticalpointsourcecoincidentwiththeNEX-raysource.Thereisan −1.0
was0.82=3.3/4.Themodelwasfittedtothedatainspectralbins
opticalelongatedstructureatthepositionoftheX-raynucleusof3C294.
ofatleast20counts,andbetweentheenergiesof0.5and7keV.We
added Galactic absorption and corrected the data with the ACIS-
areduced c 2 of0.98.Thebest-fittingpower-lawspectralindexis ABS model, although the correction makes no difference to the
2.3,withareducedc 2of0.64. best-fittingparameters.Thesourcecontainsveryfewcountsbelow
0.8keV,suggestingitisheavilyabsorbed.Ifthissourceliesatthe
sameredshiftas3C294itsintrinsicluminosityis2.8×1044ergs−1
2.2 ThesourcetotheWest-South-West(WSW) between1and10keV.
TheHSTimageof3C294inFig.7containsavery-faintpoint
Weextractedarchival HubbleSpace Telescope(HST)WideField
sourceatthelocationoftheNEsource.Weestimatedaphotometric
PlanetaryCamera2(WFPC2)dataoffourobservationsof3C294
redshiftforthissourceusingthepubliclyavailablecode HYPERZ
(datasetsU27LFC0{1,2,3,4}T).Theobservationsweretakenusing
(Bolzonella,Miralles&Pello´2000),andcalibrated,archivalimag-
afilterwithacentralwavelengthof6895A˚,andeachhadanexpo-
ing data from the HST (F702 filter), INT Wide-Field Camera (i´
sure time of 140 s. The data were combined using the IRAF task Sloanfilter)andUKIRT(Kfilter).Thebestsolutionisconsistent
CRREJ. withz=z3C294, though weemphasize that thisisbased on three
The HST field in Fig. 7 is dominated by a star known as filtersonly.Thebest-fittingsolutionisz=1.93+0.14,andthereisa
U1200-07227692intheUSNO-A2.0catalogue(Monetetal.1998). −1.23
secondarysolutionz=2.97+0.41(90percentconfidenceintervals).
ThestarhasBandR-bandmagnitudesof13.0and11.5inthatcat- −0.97
ThefluxfromthissourceinX-raysvariedbetweentheorigi-
alogue,andaVbandmagnitudeof12.0(HubbleGuideStarCata-
nalChandraobservationandtheonewepresenthere.Intheorig-
logue1.2,Laskeretal.1990).Quirrenbachetal.(2001)discussthe
inal observation on 2000 October 29 the count rate between 0.5-
various disagreeingmeasurements of thepositionof thisstar,but
7keVwas(2.2±0.3)×10−3s−1.Thecombinedcountratefrom
statethattheUSNO-A2.0positionisthemostaccurate.Theposi-
themergeddatasetshereis(9.2±0.7)×10−4s−1.Thereforethe
tioninthiscatalogue(14:06:43.3,+34:11:23.5)matcheswithinan
fluxofthesourcedeclinedbyafactorof∼2.4in4.2×107s.
arcsecthepositionofthesoftX-raysourcetothesouth-east-eastof
thecentralnucleus(Figs.3&5).
ThisstarwasreportedtobeanF-typestarbyWyndham(1966;
2.4 Thecorespectrum
spectrumtakenbyF.Zwicky1965).Stockton,Canalizo&Ridgway
(1999)foundthestartobeadoublewithaseparationof0.13arc- A power law model is not a good fit to the spectrum of the core
sec, and an intensity ratio of 1.5:1 in K′. McCarthy et al. (1990) source,associatedwiththecentralactivenucleus.Thebest-fitting
from its spectrum find it to be a subgiant K star, with a spectral photonindexwasnotphysical at−0.65, andthereduced c 2 was
breakat4000A˚. 2.7.Thedatawerefitbetween0.5and7keV.
Thefluxofthissourceis∼6×10−16ergcm−2s−1 between Amuchbetterfitisfoundusingapartially-obscuredPEXRAV
0.3and5keV. Thisfluxislow(relativetoitsoptical luminosity) (Magdziarz&Zdziarski1995)reflectionmodelplusemissionfrom
whencomparedwiththefluxesofopticallybrightmain-sequence a neutral iron line at 6.4 keV (Fig. 9), with Galactic absorption.
stars(Hu¨nsch,Schmitt&Voges1998).Thelowfluxismoretypical ACISABScorrection had littleeffect on thebest fittingvalues, al-
ofsubgiantKstarsthanF-typestars. thoughtheuncertaintiesonthebest-fittingparameterswerelower
(cid:13)c 0000RAS,MNRAS000,000–000
6 A.C. Fabian,J.S. Sanders,C.S. Crawford andS. Ettori
Figure 9. Spectrum and PEXRAV plus iron line fit of the central X-ray
source.Dataarebinnedintospectralbinscontainingatleast20countsand
fittedbetween0.5and7keV.
Figure11.X-rayemissionbetween0.3and5keVshowningreyscale(asin
Fig.2[top]).Overlayedarethe6cmradioemissioncontours,andaLyman-
a contour(roughlytriangular)takenfromFig.2,McCarthyetal.(1990).
2.5 Radiohotspots
Fig.11 shows anoverlay of the X-rayemission between 0.3and
5 keV, overlayed with 6 cm radio and a Lyman-a contour taken
from Fig. 2 of McCarthy et al. (1990). To make this image, the
McCarthyetal.datawereconvertedtoJ2000coordinates,andthe
Figure10.Best-fittingmodelforthecorespectrum,plottedasEFE.The central radio source was aligned withthe Chandra X-ray source.
widthoftheironlineissethereto0.05keV.Modelincludespartialcoverer,
TheradiohotspotsareinasimilarpositiontothetwoX-raypoint
ACISABSandGalacticabsorptions.
sourcesalongtheradioaxis,buttheX-raysourcesarerotatedby
about6degreesclockwiseaboutthecorefromtheradiohotspots,
since we did not fit for Galactic absorption, instead fixing it. We andappeartobecloserin.Eachspotisdisplacedbyabout1.8arc-
quotethevalues correctedby ACISABSabsorption here. Thepar- sec.TheX-raysourcescontaintoofewcountstogenerateaspec-
tialcovererwasplacedattheredshiftofthesource. trum,butcaneasilybeseeninthe0.3-1and1-1.5keVimages,but
The relative ratio of the reflected to direct component was notfrom1.5-5keV,indicatingthattheyhavesoftspectra(G >2.3,
foundtobe5.1+−21..05(1s ).Thecoveringmodelhadacolumndensity thevalueofthediffuseflux,Section2.1).Ifweassumeaphoton
of8.4+1.1×1023cm−2 andacoveringfractionof97.6+1.0percent indexforthespectrumofthehotspotsof2.3,theunabsorbedflux
−0.9 −3.0
(1s ). Solar abundance was assumed, as was an inclination angle emitted in a 1.2 arcsec radius circle about the NE hotspot from
of 60◦, and the width of the iron line was set to zero. The re- 1-10keVis5.8×10−16ergcm−2s−1 (restframe1-10keVlumi-
duced c 2 of the fit was 0.55=5.0/9. The flux in the iron line nosityL1−10∼1.7×1043ergs−1).TheintrinsicfluxfortheSW
was2.0+−11..31×10−6photons−1 cm−2.UsinganF-test,weestimate hotspotis5.5×10−16ergcm−2s−1 (L1−10∼1.6×1043ergs−1).
thereisonlya3.5percentprobabilityofanintrinsicspectrumwith-
outtheaddedneutralironlinegivingasagoodafitasamodelwith
theline.
Fig.10showsthespectrumofthebest-fittingmodel,withthe
widthoftheironlinesetto0.05keV.Theintrinsicluminosityofthe 3 INTERPRETATIONOFTHEPOINTSOURCES
underlyingpowerlawis1.1×1045ergs−1 between1and10keV
3.1 TheNEsource
intherestframe,aftercorrectionforabsorption.Wenotethatthe
directemissionleakingtoourlineofsightbelow1.5keV(Fig.10) ThissourceisbrightintheX-raybandandhasalowopticalflux.
neednotbeduetopartialcovering,butjusttosomeoftheradia- TheX-rayspectrumindicatesthatitishighlyabsorbed.Thesource
tionbeingscatteredbyionizedgas.Inthiscasetheabovenucleus hasalsovariedinX-raybrightnessstronglyover18months. Itis
luminosityisalowerlimit. thereforelikelytobeaSeyfertIIgalaxy(ofanintrinsicluminosity
The HST image in Fig. 7 shows a faint elongated structure of2.5×1044ergs−1 between1and10keV),harbouringahighly
atthepositionoftheX-raynucleus,alignedroughlyinthenorth- obscuredAGN.Thegalaxymaybeamemberofthecluster,apos-
southdirection. sibilitysupportedbyitsphotometricredshift.
(cid:13)c 0000RAS,MNRAS000,000–000
A deep Chandraobservationoftheclusterenvironmentof thez=1.786radiogalaxy3C294 7
3.2 Radioaxisfeatures ∼1044ergs−1.Usingthislimitwecansaythatthemeandensityof
gasintheshellmustbelessthan1/5oftheaveragedensityofgas
Foursourcesappeartoliealongthedirectionoftheradioaxis:the
making the observed extended X-ray source. This is inconsistent
core(§2.4),twosourcescoincidentwiththeradiohotspots,andthe
withthecoldfronthypothesis.
NEsource(§2.3).Indeed,theNEsourceisonadirectlinejoining
Fabianetal.(2001)concludedthatthediffuseemissionfound
the lower hotspot and the core. It is tempting to directly connect
around the point source in 3C294 was most likely direct X-ray
the NE source with the radio axis, but it is difficult to suggest a
emissionfromthecoreofacluster.Unfortunatelyouranalysisof
mechanismdescribinghowtheradioaxiscouldtriggeractivityin
this longer observation presents some difficulty for this interpre-
theNEsource,whichappearstobeaSeyfertII.Thehotspots,the
tation. In particular, the sharp drops observed at the edges of the
core,andtheNEsourceareprobablyalignedbychance.
diffuseemissionareincontrasttotheexpectedprofilefromintra-
ThereisanenhancementoftheX-rayemissionalongtheSW
clustergas.WeplotasurfacebrightnesscutofAbell2199(taken
side of the jet, and there isalso ahole in emission along the NE
fromdatainJohnstoneetal.2002),aclusterataredshiftof0.0309,
side(Figs.11 and2). Thismay indicatethat thejetisrelativistic
onFig.4.Thiscutwasproducedbyenlargingthesizeofthemov-
andbeamed,withtheSWsidebeingbeamedtowardsus.
ingboxby afactor of 13.7, corresponding totherelativeangular
diameterdistanceof3C294comparedtoA2199,andexaminingan
energyrangeof0.82to2.71keV,correspondingtoanenergyrange
3.3 Thecentralsource
of 0.3to1keV attheredshift of 3C294. Thecut through A2199
The spectrum of the central source is fit well with a model of a has alargecentral peak, indicating gas withashort cooling time
highly obscured, reflection-dominated, AGN. The intrinsic lumi- in the centre, and no sharp drops in brightness. The cut through
nosityofthenucleusisoftheorderof1045ergs−1. 3C294isquiteunlikeA2199, withalargelyflatprofileandsharp
dropsattheedges.Note,however,thatthediffuseemissionhasa
lessabruptedgetotheeastandwest.ThesharpnessoftheNand
SedgestotheX-rayemissionmeanthat thegasisdisturbedand
4 INTERPRETATIONOFTHEDIFFUSEEMISSION
notincompletehydrostaticequilibrium.Transonicmotionmaybe
4.1 Thermal sufficienttoaccountforwhatisseen.
Unfortunately we cannot distinguish between a thermal and
4.1.1 Hydrostaticmodels
non-thermalsourceofthediffuseemissiononspectralgrounds,nor
If we approximate the emitting volumes as two spheres of ra- identifyanironline.
dius6.4arcsec,thenthetotalemittingvolumeis3.9×1070cm3.
Using the normalisation of the MEKAL model (with the ACIS-
4.1.2 Shocksandmergers
ABScorrection),wefindtheelectrondensitytobeapproximately
2.6×10−2cm−3.Theenergycontentofthehotgasisthereforeof Alternatively,ifthecentralengineunderwentamassiveexplosion
theorder(3/2)kBTnV ∼1.5×1061erg(assumingkT ∼3.5keV), inthepastwemaybeobservingtheshockedmaterialbehindtwo
where n is the total particle density. If this were delivered over shockfrontstravellingtothenorthandsouth.InthatcasetheAGN
108yr,thiswouldrequireapowerof5×1045ergs−1,notinclud- must be embedded in a dense environment such as a cluster, as
ingPdV work. is shown by the calculated electron density. A factor of 4 den-
Ifweareobservinggasinhydrostaticequilibrium,thesharp sityincreasebytheshockwouldimplyne∼7×10−3cm−3 inthe
edgesat100kpcmeanthatthescaleheightthereis∼20kpc.The preshockedgas,unlessadiabaticexpansionwasalreadystrong.The
virialtemperatureoftheclusterthenhastobe∼100/20=5times preshocktemperatureisunknownbutmustbesuchthatthegasout-
thetemperatureofthegas.Thereforeifthisisaclusterinhydro- sidetheshockisundetectable(e.g.kT <1.5keV).Thetotalther-
staticequilibriumitmustbeveryhot,extremelymassive,andmore malenergyinthegasis∼1.5×1061erg.Theshockvelocitymust
massivethanRXJ1347-1145(Allen,Schmidt&Fabian2002). be greater than 2000kms−1. Therefore the timesince theexplo-
SharpX-rayedgesareseeninmanynearbyclustersinaphe- sionis∼5×107yr.Thetotalenergyintheexpandinggas(thermal
nomenonknownasacoldfront(Markevitchetal.2000).Thegas plus kinetic) must be ∼2×1061erg, and the power greater than
temperaturedecreasessharplyacrossthesefeatures,withthecooler 1046ergs−1.
gas being closer to the centre; pressure is continuous across the Simpson&Rawlings(2002)makethehypothesisthatthedif-
front. If this is the explanation for the sharp edges to the diffuse fuseX-rayemissionin3C294isproducedbytwoshockspropagat-
X-ray emission of 3C294, there must be hotter surrounding gas. inginoppositedirectionsfromthesiteofacollisionoftwoclusters.
Assuming pressure equilibrium across the front, then the ratioof Inthismodelthepowerfulradiosourceinthecoreof3C294would
thetemperaturesofthegaseithersideofthefrontisapproximately have been triggered by the merger. If the collision velocity were
equal to the inverse of the ratio of their densities. If we estimate ∼1000−2000kms−1 thentherewouldbeenoughenergytoraise
thedensitydropasafactorof2.5(estimatedfromFig.4),thenthe thetemperatureoftheclusterfrom2to5keV.Oneproblemwith
outertemperatureshouldberoughly8keV.Thisagainmakesthe this model is that the surface brightness profile we observe does
clusterveryhotandrareatitsredshift(Fabianetal.2001).More- notmatchthatexpectedfromsimulationsofmerginggalaxies(e.g.
over,theinteriorsofnearbyclusterswithcoldfrontsdonotshow Ritchie&Thomas2002).Theregionofemissionisnotexpectedto
brightnessprofilesasflatasweseehere. haveaflatbrightnessprofileandtheedgesoftheheatedregionare
We can place a limit on the luminosity of any gas that lies notsharp.
outside of the observed diffuse emission. We calculated the flux Itisinterestingthatalthoughthediffuseemissionfrom3C294
limit in a shell between radii of 16 and 55 arcsec from the core, doesnotappeartohavethesamemorphologyassomeclustersof
assuming a temperature for the gas of 3.5 keV, an abundance of galaxiesthoughttobemerging(e.g.Markevitch&Vikhlinin2001),
0.3Z⊙, and Galactic absorption, and find the 3s upper limit on thereisaresemblancewithAbell2256whichhassubcomponents
the intrinsic luminosity of that gas between 1 and 10 keV to be of ∼4.5 keV and 7−8 keV in an early state of merger at red-
(cid:13)c 0000RAS,MNRAS000,000–000
8 A.C. Fabian,J.S. Sanders,C.S. Crawford andS. Ettori
andawayfromtheobserver, rotatedaboutthex-axis.Thecentral
source was switched on at t =0 for 104 yr. The emission along
eachlineofsightwasthenintegratedalongthex-axistosimulate
thesurfacebrightnesscutinFig.4.
WeshowinFig.12thecalculatedprofilesforthreedifferent
times.Althoughtheprofilesappearfairlyflat,inlineartermsthey
fallsteeplyfromthecentre.Thereisalsoagapinthecentre,which
iscausedbythegrowingshellofphotonsmovingalongthecones.
Thegapmaybeavoidedbymodifyingtheburstofradiationfrom
the core to have a longer-living tail. 3C294 has a dip in surface
brightness to the north of the core, but it is shallow. This model
has difficulty in reproducing the flat brightness profile across the
cluster.Ifthedensityprofileintheclusterwereinverted(asfora
bubble,seeSection4.3),oriftheradiationwasconfinedcloseto
theplaneoftheskyinafan-likeshape,thenthebrightnessprofile
couldbeflattened.
Figure12. Surface brightness profiles from the simplescattering model.
Threetimesareshown,7×104,1.7×105,and2.5×105yr,fromtheinner- Afurtherproblemwiththismodelisthatonlyasmallfraction
mostout. ofthephotonsemittedwillbescatteredbythecluster.Wecanes-
timatewhattherequiredpowerforthecentralsourceis.Thelumi-
nosityofthecones(excludingtheradioaxis)is∼3×1044ergs−1.
shift0.058(Sunetal.2002).ThecoreofA2256hastwosharpand Ifathirdoftheskyissubtendedbythecones(approximatelycor-
smoothedgeslike3C294.Howeverthephysicalscaleofthecoreis rectiftheopeninganglesoftheconesare30◦),and1percentof
abouttwiceaslargethanthatoftheemissionfrom3C294,andthe theradiationisscatteredbytheelectrons,thenthiswouldrequirean
profilenotasflat. X-rayluminositybetween1and10keVfromthecentralsourceof
Carilli et al. (2002) argue that diffuse X-ray emission seen ∼1047ergs−1.Thetotalbolometricpowerofthesourcewouldbe
around the narrow-line radio galaxy PKS 1138-262 is caused by significantlylarger.Thisluminosityismuchhigherthanthatseen
theradiosourceshockingacocoonofgas.Themorphologyofthe (Section2.4)butthesourcemaybevariable.Itwouldneedtobe
X-rayemissionfrom3C294suggeststhisisnotthecaseinthisob- poweredbyaccretionontoa1010M⊙blackhole.
ject,astheemissiondoesnothavetheellipsoidalshapeofaradio
Adifferentnon-thermalmodelisthatweareobservinginverse
cocoon(e.g.Cygnus-A,Smithetal.2002).
Comptonscatteredradiationfromrelativisticelectrons.Inthatcase
In summary, the evidence suggests that a thermal origin for
theelectronscouldeitherbeapopulationscatteringCMB(Cosmic
the diffuse emission is implausible unless the gas is in a dynam- Microwave Background) photons requiring energies of g ∼1000
ically active state, and requires significant gas extending beyond (inordertoscatterphotonsintothe1keVband),org ∼10−100to
thatseen.
scatterUV-IRradiationfromthecentralsource(e.g.Brunetti,Setti
&Comastri1997).TheenergydensityintheCMBatthatredshift
was ∼2×10−11ergcm−2, and the energy density from the nu-
4.2 Non-thermal cleuswouldbe∼3×10−12L d−2ergcm−2 (L istheluminos-
47 2 47
AdifferentinterpretationisthatweareobservingX-raysfromthe ity of the source in units of 1047ergs−1, and d2 is the distance
centralAGNscatteredbytheelectronsinasurroundingcluster(see fromthesourceinunitsof102kpc).TheinverseComptoncooling
Sazonov, Sunyaev &Cramphorn 2002). Inthismodel thecentral time of the CMB-scattering electrons would be ∼6×107yr and
sourceswitchedononlyrecently,andthefluxrisetimewasshort thecoolingtimeoftheUV-scatteringelectronslongerbyafactor
so the furthest edges are sharp. A toroidal dust cloud around the of∼1000.Wethereforerequirethetotalenergyinrelativisticelec-
nucleus could collimate the radiation into the two cones seen to tronsto be∼5×1059ergif theradiation was scattered fromthe
either sideof the nucleus. Thismodel hasdifficultyinproducing CMB,andafactor1000timesgreaterifitwasscatteredfromthe
theobservedsurfacebrightnessprofile.Sincethefluxdropsasthe UV radiation field. We now ignore the UV case given its energy
square of the radius, and the density of any plausible scattering requirements.Thetotalenergyinrelativisticparticlesmaybe10to
medium will decrease withdistance fromthe AGN, the brightest 100timeslargerthan5×1059ergifweaccountforotherparticles
partofthescatteredradiationshouldbeclosesttotheAGN,with suchaslowenergyelectronsandprotons.Toprovideanequivalent
thebrightnessdecliningquicklywithradius. energydensity totheCMB magneticallywould require afieldof
Thebrightness profilecanbeflattenedbyinclining theradi- 24m G,whichismuchmorethanthatfoundinlow-redshiftclusters
ation cone towards the observer. To test whether this is a viable (B∼fewm G;Clarke,Kronberg&Bo¨hringer2001).Providingthe
explanationfortheobservedsurfacebrightnessprofile,wemadea magneticfieldisofasimilarstrengthtothatinlowredshiftclusters,
simplenumericalscatteringmodel.Weassumedtheopticaldepth inverseComptonlossesareexpectedtodominateoversynchrotron
waslowenoughsothatonlysinglescatteringeventswereimpor- losses. g ∼1000 electrons will radiate at around 2MHz in a m G
tant.Theemissionalongalineofsightwascalculatedbyintegrat- field,butotherwisewouldbeundetectable.Suchan‘inverseComp-
ingalongthatlinetheproductofthedensityofthescatteringmate- toncloud’modelwouldbethemostenergeticallyefficientwayof
rial(takentobeproportionaltothereciprocalofthedistancefrom explainingthedata.Inordertoconfinethisrelativisticplasmathe
thecentre), and thefluxatthat position atthe timewhen theob- gaseousmediumofatleastagroupwouldberequiredinordernot
serverseesit(originallyemittedfromacentralsource,buttaking tolosetheenergybyadiabaticexpansion.
into account light travel time). The emission is integrated along AninverseComptonoriginforthediffuseX-rayssurrounding
positionswhich lieinsidetheradiationcones. Thecones havean 3C294thereforeappearstobeenergeticallyfeasible.Suchacom-
opening angleof30◦ andareinclinedatanangleof15◦ towards ponentinlowerredshiftclusterswillnotbedominant,owingtothe
(cid:13)c 0000RAS,MNRAS000,000–000
A deep Chandraobservationoftheclusterenvironmentof thez=1.786radiogalaxy3C294 9
sixty-foldlowerenergydensityoftheCMB.Itmayhoweverbede- 3C294isanexceptionalobject,andmayberevealinganen-
tectable(e.g.theComacluster,Fusco-Femianoetal.1999;seealso ergeticphaserelevanttomanyrichclustersofgalaxiesinwhicha
thetheoreticaldiscussionbySarazin1999).InverseComptonscat- powerful active nucleus transfers considerable energy, and a rel-
teringisalsoinvokedtoexplainthehotspotsinseveralotherradio ativistic particle population, into the ICM. This may be crucial
sources(Celotti,Ghisellini&Chiaberge2001;Hardcastle,Birkin- forexplainingthepropertiesofpresent-dayclusters(seee.g.Wu,
shaw & Worrall 2001), and may account for the excess emission Fabian&Nulsen2000).
from the radio hotspots in 3C294 (Section 2.5). The X-ray mor-
phology of the diffuse emission does not match that of the radio
source(Fig.11;McCarthyetal.1990) orof deeper radioimages
ACKNOWLEDGEMENTS
(K.Blundell,priv.comm.).Inparticular,theX-rayemissiontothe
NWandSEofthenucleushasnoradiocounterpart.Thatneednot ACFandCSCthanktheRoyal Societyfortheirsupport. Theau-
beamajorproblemthough,sincethehigherenergyelectronpopu- thors are also grateful to Poshak Gandhi for his analysis of the
lation(g &104)necessaryforradioemissionwouldhavealifetime availableopticaldatafortheNEsource.
of<106yrandsobeabsent.Wecouldsimplybeseeingtheolder The HST data presented in this paper were obtained from
electronpopulationinasourcewherethejetdirectionhaschanged theMultimissionArchiveattheSpaceTelescopeScienceInstitute
by∼60◦overthepast108yr. (MAST).STScIisoperatedbytheAssociationofUniversitiesfor
ResearchinAstronomy,Inc.,underNASAcontractNAS5-26555.
4.3 Ahybridmodel
Manyclustersseenatlowredshiftwithcentralradiosourceshave REFERENCES
bubbles of radio plasma displacing the X-ray emitting gas (e.g.
AllenS.W.,SchmidtR.W.,FabianA.C.,2002,MNRAS,335,256
Perseuscluster,Fabianetal.2000;HydraA,McNamaraetal.2000;
AndersE.,GrevesseN.,1989,GeochimicaetCosmochimicaActa,53,197
CygnusA,Wilson,Young&Shopbell2000).Perhapsthepowerful ArnaudK.A.,2002,ApJ,submitted
radiosource3C294hascreatedexceptionallylargebubblesinthe Balucinska-ChurchM.,McCammonD.,1992,ApJ,400,699
NSdirectioninthesurrounding ICMonatimescaleof∼108 yr. BolzonellaM.,MirallesJ.-M.,Pello´R.,2000,A&A,363,476
AsforthebubblesinHydraAandPerseus,theexpansionmaybe BrunettiG.,SettiG.,ComastriA.,1997,A&A,325,898
subsonic.Thegaswillfallbackwhentheradiosourcedropsinin- CappellariM.,CopinY.,2002,inRosadoM.,BinetteL.,AriasL.,eds,ASP
tensity.TheX-rayemissioncanthenbepartiallythermal,atlarge Conf.Ser.,Galaxies:theThirdDimension,astro-ph/0202379
CarilliC.L.,HarrisD.E.,PentericciL.,Ro¨ttgeringH.J.A.,MileyG.K.,Kurk
distancesfromthecentre,andnon-thermal nearerthecentre,par-
J.D.,vanBreugelW.,2002,ApJ,567,781
ticularlyalongtheradioaxis.
CelottiA.,GhiselliniG.,ChiabergeM.,2001,MNRAS,321,L1
Ifweareobservingbubbles,thepressureofnon-thermalpar-
ChartasG.,GetmanK.,2002,http://www.astro.psu.edu/users/chartas/xcontdir/xcont.html
ticlesinthebubbleswillbeclosetothepressureofthesurround-
ClarkeT.E.,KronbergP.P,Bo¨hringerH.,ApJ,547,L111
ingthermal gas.Ifweassumeacoolingtimefortheelectronsof CrawfordC.S.,FabianA.C.,1996,MNRAS,282,1483
∼6×107 yr,emittingapower of3×1044ergs−1,thenthetotal EttoriS.,FabianA.C.,AllenS.W.,JohnstoneR.M.,2002,MNRAS,331,
energyinthebubbleswouldbe5.7a×1057erg(whereaisafactor 635
to account for energy inparticles not emittedin the band we are FabianA.C.etal.,2000,MNRAS,318,L65
observing in). Assuming a volume of 4×1070cm3, the pressure FabianA.C.,CrawfordC.S.,EttoriS.,SandersJ.S.,2001,MNRAS,322,
insidethelobeswouldbe1.5a×10−11ergcm−3.Thispressureis L11
consistentwiththepressurefoundinmanyclusters. Fusco-FemianoR.,dalFiumeD.,FerettiL.,GiovanniniG.,GrandiP.,Matt
G.,MolendiS.,SantangeloA.,1999,ApJ,513,L21
Inthispicture,were3C294tobeatz=0,theinverseCompton
HardcastleM.J.,WorrallD.M.,1999,MNRAS,309,969
emissionwouldbe60timesweaker,andthermalemissionfromthe
HardcastleM.J.,BirkinshawM.,WorrallD.M.,2001,MNRAS,323,L17
surroundingclusterwoulddominatetheX-rayimage.Thebubbles
Johnstone R.M., Fabian A.C., Allen S.W., Sanders J.S., 2002, MNRAS,
couldappearasholesintheX-rayemission.
336,299
LaskerB.M.,SturchC.R.,McLeanB.J.,JenkerH.,SharaM.M.,1990,AJ,
99,2019
LiedahlD.A.,OsterheldA.L.,GoldsteinW.H.,1995,ApJ,438,L115
5 CONCLUSIONSANDDISCUSSION MagdziarzP.,ZdziarskiA.A.,1995,MNRAS,273,837
MarkevitchM.,etal.,2000,ApJ,541,542
3C294 is a powerful, type II, radio-loud quasar, surrounded by
MarkevitchM.,VikhlininA.,2001,ApJ,563,95
extensive diffuse X-ray emission. A diffuse ICM is likely to be McCarthyP.J.,SpinradH.,vanBreugelW.,LiebertJ.,DickinsonM.,Djor-
present,atleastastheworkingsurfacefortheradiolobesandpro- govskiS.,EisenhardtP.,1990,ApJ,365,487
viding at least some the X-ray emission via bremsstrahlung. The McNamaraB.R.etal.,2000,ApJ,534,L135
sharpNandSedgestotheX-rayemissionmean,forathermalori- MeweR.,GronenschildE.H.B.M.,vandenOordG.H.J.,1985,A&AS,62,
gin,thatthegasisnotincompletehydrostaticequilibrium.Itmay 197
havebeendisplacedwithintheinnerthirdbyabubbleofrelativistic MonetD.,et.al.,1998,USNO-A2.0(Washington:USNO)
QuirrenbachA.,RobertsJ.E.,FidkowskiK.,deVriesW.,vanBreugelW.,
plasma.InverseComptonemissionfromrelativisticelectronswith
g ∼103 probably accounts for emission from the radio hotspots, 2001,ApJ,556,108
RitchieB.W.,ThomasP.A.,2002,MNRAS,329,675
and possibly all the diffuse emission if such a population exists
SarazinC.L.,1999,ApJ,520,529
wellbeyondtheradiostructure.TheX-rayspectrumofthediffuse
SimpsonC.,RawlingsS.,2002,MNRAS,334,511
emission mildly favours a power-law of non-thermal origin, pro- SazonovS.Yu.,SunyaevR.A.,CramphornC.K.,2002,A&A,393,793
videdthatthepreliminaryACISABSmodelforthecorrectionofthe SmithD.A.,WilsonA.S.,ArnaudK.A.,TerashimaY.,YoungA.J.,2002,
softX-raydegradationofthedetectorisadequate. ApJ,565,195
(cid:13)c 0000RAS,MNRAS000,000–000
10 A.C. Fabian,J.S.Sanders,C.S. Crawford andS. Ettori
StocktonA.,CanalizoG.,RidgwayS.E.,1999,ApJ,519,L131
SunM.,MurrayS.S.,MarkevitchM.,VikhlininA.,2002,ApJ,565,867
WilsonA.S.,YoungA.J.,ShopbellP.L.,2000,ApJ,544,L27
WuK.K.S.FabianA.C.,NulsenP.E.J,2000,MNRAS,318,889
WyndhamJ.D.,1966,ApJ,144,459
(cid:13)c 0000RAS,MNRAS000,000–000