The X-ray diffuse emission from the Galactic Center

(willbeinsertedbyhandlater)

arXiv:astro-ph/9908164v1 16 Aug 19991.Introduction

ThediffuseX–rayemissionfromtheGalacticCenter(GC)regionwasimagedforthefirsttimewiththeEinsteinObser-vatoryinthe0.5–4keVband(Watsonetal.1981)andlaterstudiedinmoredetailwithseveralsatellites(Kawaietal.1988,Skinneretal.1987,Koyamaetal.1989,1996).Therealna-tureofthisemissionisstillanopenissue.Thoughatleastpartofitcanbeduetotheintegratedemissionfromweakun-resolvedsources(Watsonetal.1981;Zane,Turolla&Treves1996),thereisevidencethatahotplasma,responsiblefortheobservedemissionlines,permeatestheGCregion.ThiswasindicatedbytheGingadiscoveryofastrongironlineat6.7keVwithanequivalentwidthofEW=600±70eV(Koyamaetal.1989).Morerecently,theASCAsatelliteconfirmedthepresenceofahotplasma(kT∼10keV)anddiscoveredanotherdiffusecomponentinthe6.4keVFe-linewithanasymmetricspatialdistributionwithrespecttotheGCandwellcorrelatedwiththedistributionofthegiantmolecularclouds(Koyamaetal.1996).ThedataonthediffuseX–rayemissionreportedherewereobtainedduringasurveyoftheGCperformedwiththeBeppoSAXNarrowFieldInstruments(Sidolietal.1998).Theresultsonthepointsourceshavebeenreportedelsewhere(Sidolietal.1999a,1999b).

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theinnermostandionizedpartofthe“CircumnuclearDisk”,astructureofmoleculargasextendingfrom1.7pcuptoabout7pcfromtheGCandrotatingaroundit(Marshall,Lasenby&Harris1995).

Proceedingoutward,wefindthe2′−3′shellofSgrAEast(Eckersetal.1983),whichischaracterizedbyanon–thermalradioemissionwithspectralindexα=−0.64.Itiscen-teredabout50′′offsetofSgrA*andisprobablylocatedbe-hindSgrAWest(Daviesetal1976,Pedlaretal.1989).Itssynchrotronradioemissionanditsshellmorphologywereex-plainedintermsofasupernovaremnant.Mezgeretal.(1989)discoveredadustringcompressedbytheSgrAEastshell,pos-siblyindicatingthatthisshelloriginatedfromanextremelyen-ergeticexplosion(4×1052ergs),maybeassociatedwithSgrA*,occurringinsideadensemolecularenvironment(104cm−3).Otheralternativeexplanationsarethesimultaneousexplo-sionofabout40supernovae,orasingleSNexplosioninsideamediumwithamuchlowerdensity.Inthiscasethedustshellcouldbeduetothestellarwindfromthenormalsuper-novaprogenitor(Mezgeretal.1989).Anotherpossibilityisthetidaldisruptionofastarbythecentralsupermassiveblackhole(Khokhlov&Melia1996).

Atriangularshapednon–thermalhalo(∼20pcindiame-ter)surroundsinprojectiontheSgrAEastshell(Yusef-Zadeh&Morris1987),butitisprobablynotphysicallyrelatedwithit(seePedlaretal.1989foradetaileddiscussion).Itsori-ginisunknown.ItcouldbecenteredattheSgrA*positionandrelatedwiththehighenergyactivityattheGC(Yusef-Zadehetal.1997,Meliaetal.1998).Thetotalenergyinrel-ativisticparticlesestimatedfromradioobservationsisabout5×1050ergss−1.Thisvalue,togetherwithitsnon–thermalra-diospectrumanditssize,suggestthepossibilitythatitisanevolvedsupernovaremnant(Pedlaretal.1989).3.ObservationsandDataAnalysis

OurresultswereobtainedwiththeMediumEnergyConcen-tratorSpectrometer(MECS,Boellaetal.1997)thatprovidesimagesinthe∼1.3–10keVenergyrangeoveranearlycircularfieldofviewwith∼28′radius.TheMECShasagoodangu-larresolution(50%powerradiusof∼75′′at6keV,󰀁on-axis)andamoderateenergyresolution(FWHM∼8.5

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ysedtheoverallspectrumbyextractingallthecountsfromthislargercircularcorona.Westartedbyfittingathermalbremsstrahlungplusthreegaussianlinesat∼1.8,2.4and6.7keV(Si,SandFerespectively).Theirestimatedequivalentwidthsareabout120,190and1000eV.Thebestfitvalueforthebremsstrahlungtemperatureis∼13keV.Thistempera-tureistoohightobeconsistentwiththepresenceofthelowenergyemissionlines(Kanedaetal.1997,Fig.2b).Apossi-bleexplanationisamulti-temperatureplasma.Thuswefit-tedthespectrumwithtwothermalemissionplasmamodels(two“MEKAL”inXSPEC).368d.o.f)areNH=7.93+0.09Ourbestfitparameters(χ2=1.29,

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.02−0.36×10cm−2,T1=0.57+0−0.05keV

andT2=8.69+0.26

−0.41keV.Agaussianaddedat6.4keVgivesanEW∼120eV.Thetotalunabsorbedflux(2–10keV)is

1.71+0.05−10

−0.08×10

ergscm−2s−1,aboutonethirdofwhichiscontributedbythesoftcomponent.

Tostudythespatialdistributionofthediffuseemission,weextractedimagesindifferentenergybands(Fig.3).Theim-agescorrespondingtothe2–5keVand7–10keVrangesshowsignificantlydifferentdistributionsofthediffuseemission.

BoththesoftandhardemissionsarepeakedattheGCposition,buttheyhavedifferentspatialextents.Inparticular,thesofteremissionhasanearlytriangularshape,witharathersharpdecreaseinthesouth-westernside,whichisabsentinthehardX–raymap.Tostudythespatialdistributionoftheironlinewehavealsoextractedanimageinthe5.5-7.5keVbandandsubtractedfromthismapthecontinuumemissioninter-polatedfromthecontiguousenergies.Theresultingironlineimageshowsaspatialdistributionelongatedalongthegalacticplane,verysimilartothatofthehardX–raymap.5.Discussion

OurobservationoftheSgrAcomplexconfirmsthepresenceofahotplasmawithmultipletemperaturesand/orinnon-equilibriumionization,asalreadyfoundwiththeASCAin-struments(Koyamaetal.1996).Theincreaseinthe6.4keVlineequivalentwidththatwefindintheNorth-Eastsector,isinagreementwiththemoredetailedmapsofthislineobtainedwithASCAthatshowacorrelationwiththemolecularclouds.ThelimitedspectralresolutionoftheMECS,comparedwiththatoftheASCAsolidstatedetectors,doesnotallowtostudyinmoredetailtheenergyprofileandspatialdistributionoftheindividuallines.Ontheotherhand,theregularpointspreadfunctionprovidedbytheBeppoSAXmirrorshasallowedustoproduceunbiasedmapsinwideenergybandsthatdemonstrateacleardifferenceinthespatialdistributionofthesofterandharderX–rayemission.SincealsoourspectraldatacouldbewelldescribedbythesumoftwothermalmodelswithkT∼0.6and8keV,itistemptingtogiveaninterpretationintermsoftwoplasmacomponentsatdifferenttemperaturesandwithdifferentspatialdistributions(althoughinrealitythesituationiscertainlymuchmorecomplex,with,e.g.,adistributionoftemperatures).Inthisinterpretation,itisremarkablethatthelowertemperatureplasmaiswellcorrelatedwiththeSgrAEasttriangularradiohalo(Fig.4).

Thepresenceofunresolvedpointsourcescouldaffecttheapparentdistributionofthediffuseemission.Wenotehow-everthatpreviousobservations,e.g.withEinsteinandROSAT(Watsonetal.1981,Predehl&Tr¨umper1994)donotshowthepresenceofstrongsourcesdistributedinsuchawaytore-producethetriangularshapevisibleinourlowenergymap.

Inparticular,thestraightcontoursofthesoftemissioncor-respondingtotheSWsideofthetriangularradiohaloseemhardlyexplainablebyadistributionofsources.AlthoughwecannotexcludethattheapparentshapeofthesofterX–raysisduetoanabsorptioneffect,wefavourtheinterpretationofthelowertemperatureplasmaasacomponentphysicallyrelatedtothehaloofSgrAEast.Inthefollowingwewilladoptthisworkinghypothesisandassumeadistanceof8.5kpc.

WederiveforthesoftcomponentanemissionmeasureEM=(1.2±0.4)×1014cm−5,whichassumingemissionfromasphericalregionwithradius10pcandanelectronfillingfac-torf,correspondstone∼(3×f−1/2)cm−3andtoatotalmassMg∼250M⊙f1/2.

TheaveragethermalpressureintheSgrAEasthalo,P∼3×10−9ergscm−3,isconsistentwiththepressurePSedov=0.106×(ESN/R3)ergscm−3derivedforaSNRinaSedovphase,whereESNistheexplosionenergyoftheSNRandRistheshellradiusincm.Indeed,ifweassumeESN=1051ergss−1andR=10pc,wefindP−9Sedov∼4×10ergscm−3.

TheX–rayluminosity(2–10keV)ofthesoftcomponentis∼4.5×1035ergss−1.IfweassumethatthisthermalemissionismostlyproducedbytheSgrAEasthalo,itsX–rayluminosity,pressure,density,temperatureoftheemittinggas(0.6keV)andsize(∼20pc),matchwellwithasupernovaremnantorigininwhichthermallineemissionisproducedwhentheexpandingsupernovaejectaheatstheISMtoX–raytemperatures.

AnothertestoftheSNRhypothesiscanbedonebycom-paringtheX–rayandradiosurfacestrongcorrelationΣR∝Σ0.69±0.08

brightnesses:indeeda

betweentheradioΣ(at1GHz)andtheX–rayX

RsurfacebrightnessΣX(0.15–4.5keV)wasfoundbyBerkhuijsen(1986).Fromthera-diospectrumreportedbyPedlaretal.(1989)wederivedΣR∼5×10−19Wm−2Hz−1sr−1.TheΣXmeasuredfromourX–raydataandconvertedtothe0.15–4.5energybandisΣX∼1034ergss−1pc−2.ThesevaluesfallwellinsidetheregiondefinedbyothertypicalSNRs.

Inconclusion,allthephysicalquantitiesderivedfromtheanalysisoftheMECSareconsistentwithaSNRoriginfortheSgrAEasthalo.

Fig.4.Contourlevelsofthe2–5keVemissionoverlayedonthe90cmradiocontours(dashedline,adaptedfromAnantharamaiahetal.1991).

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Fig.3.2–5keV(left)and5–10keV(right)emissionfromtheSgrAComplex.BothimageshavebeensmoothedwithagaussianwithFWHM=1arcmin.ThestrongsourceintheNEcorneris1E1743.1–2843(Cremonesietal.1999).Thelowsurfacebrightnessincorrespon-denceofthecircleisaninstrumentaleffectduetotheabsorptioninthedetectorwindowsupportstructure.

6.Conclusions

TheanalysisofthediffuseemissionfromtheGCregionrevealsthepresenceofatleasttwodistinctcomponents:asoftone(Tmekal∼0.6keV),spatiallycorrelatedwiththeSgrAEastradiohaloandahardcomponent(Tmekal∼8–9keV).WhilethehardcomponentiselongatedalongtheGalacticPlaneanddoesnotspatiallycorrelatewithphysicalstructuresobservedatotherwavelengths,thespatialdistributionofthesoftcom-ponentisremarkablywellcorrelatedwiththetriangularradiohaloofSgrAEast.Theparametersderivedforthesoftercom-ponentmatchextremelywellwiththeideathatthisthermalX–rayemissionisduetoaSNRoriginfortheSgrAEasthalo.Acknowledgements.

WethankL.Chiappetti,G.Matt,S.MolendiandA.Trevesforusefuldiscussionsandhelpwiththedataanaly-sis.

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