The large scale structure of the soft X-ray backgr

astro-ph/95110092Nov1995A&Amanuscriptno.(willbeinsertedbyhandlater)Yourthesauruscodesare:(12.03.3;12.04.2;13.25.2;13.25.3)ASTRONOMYANDASTROPHYSICS2.11.1995ThelargescalestructureofthesoftX-raybackground.I:ClustersofgalaxiesAndrzejM.Soltan1;2,GuntherHasinger3,RolandEgger1,SteveSnowden1;4andJoachimTrumper11MaxPlanckInstitutfurextraterrestrischePhysik,D-85748GarchingbeiMunchen,Germany2CopernicusAstronomicalCenter,Bartycka18,00-716Warsaw,Poland3AstrophysikalischesInstitut,D-14482Potsdam,Germany4NASA/GSFC,LHEACode666,Greenbelt,MD20771,USAReceivedDec1994;acceptedMarch1995Abstract.Large-scaleuctuationsoftheX-rayback-groundareinvestigatedusingtheROSATAll-SkySurvey.Theautocorrelationfunctionoftheextragalacticback-groundat1keVisdeterminedatseparations0:140.Wedetectasignicantsignalfor6,whichislargerthanpreviousupperlimits.Weanalysethecross-correlationbetweentheangularpositionsofAbellclus-tersofgalaxiesandtheintensityoftheX-raybackground.Thecross-correlationextendstoseveraldegreesforclus-tersofdistanceclass4.WeconcludethatAbellclustersareassociatedwithextended,lowsurfacebrightnessX-rayhaloes.Themeanluminosityofthediusesourcesinthe0:52:0keVbandisL2:51043h2ergs1andtheircharacteristicradiusa10h1Mpc(H0=100hkms1Mpc1).ItisshownthatthehigherdensityofnormalgalaxiesandAGNsaroundclusterscannotbere-sponsiblefortheincreasedX-rayemission.ThisisbecauseitwouldproduceasignalintheautocorrelationfunctionmeasurablebytheGingaLACexperiment.Apparentex-tendedemissioncouldbeproducedeitherbyanumberofdiscretesourcesassociatedwithgroupsofgalaxiesconcen-tratedaroundrichclustersorbyahotdiusegas.ItisshownthatsourcesassociatedwithAbellclustersaccountfor30%oftheX-raybackgrounductuationsonscalesabove1.Keywords:Cosmology:observations,diuseradiation,X-rays:galaxies,X-rays:general1.IntroductionThestatisticalcharacteristicsoftheX-raybackground(XRB)reectpropertiesofdierentclassesofobjectsSendoprintrequeststo:A.M.Soltan,CopernicusCenteraddresswhichcontributetothisbackground.Atsmallscalespoint-likesourcesproduceuctuationswithanangularextentdenedbythepointspreadfunctionofthein-strument.Fluctuationsatlargerscalesarecreatedbyex-tendedsourcesaswellasbygroupsofdiscretesources.Inthe1-2keVbandmost(ifnotall)oftheextragalac-ticXRBisproducedbydiscretesources(Hasingeretal.1993).SourcescontributingtotheXRBconstituteahighlyheterogeneousgroupofobjects.PracticallyeachclassofstrongX-rayemittersexhibitsitsuniquechar-acteristicswithregardtospaceandluminositydistribu-tions,spectrum,evolution,clusteringproperties,etc.Thisimpliesthateachclassofobjectsproducesdistinctvaria-tionsoftheXRBandtheexpectedXRBstructureisfairlycomplex.Theproblemofbackgroundanisotropyhasbeenin-vestigatedusingseveralsetsofobservationsbymanyau-thors(seeFabian&Barcons1992andreferencestherein).ThemainobjectiveofnumerousanalyseswastomeasuretheamplitudeoftheXRBuctuationsandtouseitasaconstraintonclusteringpropertiesofobjectscreatingtheXRB(e.g.,Persicetal.1989;Martin-Mironesetal.1991;Barcons&Fabian1990;Carreraetal.1991,1993;Chenetal.1994;Soltan&Hasinger1994).ItwaspointedoutbyHasinger(1992)andSoltan&Hasinger,thatex-tendedlowsurfacebrightnessobjectscanmimicclustersofunresolvedsourcesandanyscrupulousstudyofsourceclusteringshouldtakeintoaccounteectsproducedbyextendedsources.InthepresentpaperweinvestigatetheanisotropyoftheextragalacticsoftXRBonscalesbetween0:5and40usingtheROSATAll-SkySurvey(RASS)asadatabase.ThisistherstpaperofaserieswhichisdevotedtotheanalysisofthelargescaleeectsintheXRB.Theob-jectiveoftheseriesistodeterminerelationshipsbetweenvariousclassesofsourcesandthesoftXRB.Inthispa-perwecalculatetheautocorrelationfunctionoftheXRB2A.M.Soltanetal.:ThelargescalestructureofthesoftX-raybackground.IandanalysethecontributionofAbellclusterstotheXRBanisotropyinawiderangeofangularscales.TheAbellclustersasaclassarestrongX-rayemitters(Burgetal.1994andreferencestherein)andanumberoftheirX-raypropertiesareestablished.ThismakesAbellclustersasuitablesampleofobjectstobeusedintheinvestiga-tionofdistinctcomponentsoftheXRB.Nearby,smallergroupsofgalaxiesconstituteanotherpotentiallyimpor-tantclassofextendedX-raysourceswhichcanproducelargescaleXRBuctuationsandwillbeinvestigatedinafuturepaperofthisseries.TheROSATAll-SkySurveyprovidesexcellentmate-rialtostudyisotropyoftheXRBonangularscalesabove100(forthedescriptionoftheROSATobservatoryseeTrumper1983,1990).ThesurveycombineshighangularresolutiontypicaltotheimagingX-rayopticswithgoodqualitystatisticaldatawhichcovervirtuallythewholesky.ExcellentperformanceoftheimagingproportionalcounterPSPC(Pfeermannetal.1986)characterizedbyaverylowintrinsicbackground(Snowdenetal.1992;Plu-cinskyetal.1993)allowstheexaminationofextremelylowsurfacebrightnessfeaturesoftheXRBinawiderangeofangularscales.TheobservationaldataarebrieydescribedbelowinSect.2.ThecalculationsoftheautocorrelationfunctionarepresentedinSect.3andthecrosscorrelationoftheX-raydatawiththeAbellcatalogueisinvestigatedinSect.4.PhysicalmodelsoftheextendedemissionaroundclustersarediscussedinSect.5,and