Neutralino Gamma-ray Signals from Accreting Halo D

arXiv:astro-ph/0012346v115Dec2000NeutralinoGamma-raySignalsfromAccretingHaloDarkMatterLarsBergstr¨om∗,JoakimEdsj¨o†andChristoferGunnarsson‡DepartmentofPhysics,StockholmUniversity,Box6730,SE-11385Stockholm,SwedenThereismountingevidencethataself-consistentmodelforparticlecolddarkmatterhastotakeintoconsiderationspatialinhomogeneitiesonsub-galacticscalesseen,forinstance,inhigh-resolutionN-bodysimulationsofstructureformation.Alsoinmoreidealized,analyticmodels,thereappeardensityenhancementsincertainregionsofthehalo.WeusetheresultsfromarecentN-bodysimulationoftheMilkyWayhaloandinvestigatethegamma-rayfluxwhichwouldbeproducedwhenaspecificdarkmattercandidate,theneutralino,annihilatesinregionsofenhanceddensity.Theclumpinessfoundonallscalesinthesimulationresultsinverystronggamma-raysignalswhichseemtoalreadyruleoutsomeregionsofthesupersymmetricparameterspace,andwouldbefurtherprobedbyupcomingexperiments,suchastheGLASTgamma-raysatellite.Asanorthogonalmodelofstructureformation,wealsoconsiderSikivie’ssimpleinfallmodelofdarkmatterwhichpredictsthatthereshouldexistcontinuousregionsofenhanceddensity,causticrings,inthedarkmatterhalooftheMilkyWay.Wefind,however,thatthegamma-raysignalfromcausticringsisgenerallytoosmalltobedetectable.I.INTRODUCTIONRecentdeterminationsofcosmologicalparametershavesingledoutaregionofthematterdensityΩM∼0.3clearlylargerthanallowedbybigbangnucleosynthesis.Thiscoupledwithmanyotherpiecesofevidencemakestheexistenceofnon-baryonicdarkmattercompelling[1].However,westillhavenoclueastothenatureofthedarkmatterotherthanthatitplausiblyexistsintheformofnon-relativistic(cold)particles.Iftheparticleismassiveandhasweak-interactioncouplingtoordinarymatter,i.e.,itisaWIMP(weaklyinteractingmassiveparticle),therearegoodprospectsforitseventualexperimentaldetection.Thelightestsupersymmetricparticle,usuallyaneutralino,isoneoftheprimecandidates.Todetectorruleoutparticledarkmattersuchastheneutralinoisobviouslyanimportantexperimentalundertaking.However,mostdetectionmethodsdependquitesensitivelynotonlyontheproperties(theexactvaluesofthemassandcrosssections)ofthecandidateparticleitself,butalsoonthedistributionofdarkmatterinourGalactichalo.Thisisstartingtobeprobedincomputersimulationsandtosomeextentalsothroughanalyticalmodellingoftheformationhistoryofdarkmatterhalos.Thecurrentlymostfashionablemodelofstructureformationisthatofprimordialfluctuation-seededhierarchicalclustering,whereN-bodysimulationsarebeginningtohavehighenoughresolutiontogiveinformationonsub-galacticscales[2].Inthisclassofmodels,galactichalosusuallyhaveaverycomplicatedmerginghistoryleading,intheinfallpicture,toextensiveirregularfoldingsoftheinitiallythinphasesheetsonwhichthedarkmatterparticleswerelyingatthetimeofkineticdecouplingfromtheprimordialplasma.AsshowninarecentworkbyCalc´aneo-Rold´anandMoore[3],theGalactichalointhisscenariocontainsalotofsubstructureleadingtosignificantpossibleenhancementsoftheannihilationrateintheoverdenseregions.SincetheannihilationrateisproportionaltothesquareoftheWIMPdensity,thegamma-raysignalinthedirectionofthesegalactichaloclumpsshouldbeconsiderablyenhancedcomparedtothecaseofasmoothhaloprofile,whichhasmostfrequentlybeenconsideredinpreviousanalyses.Inanother,highlyidealizedmodel,havingthevirtueofbeinganalyticallytreatable,proposedbySikivie[4–6],continuousinfallofdarkmatteronourgalaxyshouldgiverisetoringshapedcausticsofdarkmatter.Ifthevelocitydispersionoftheinfallingparticlesissufficientlysmall,thecausticscouldcontainsignificantoverdensities,againwithapossibledetectablegamma-rayfluxasaresult.Somegeneralresultsontheincreasedindirectdetectionsignalsofsupersymmetricdarkmatterinaclumpyhalowereobtainedin[7].Withthemodelsmentionedwenowhavetwospecificscenarioswithwhichtomakemorequantitativeestimatesofthepossibleenhancements.Inthispaperwefirstinvestigatethemagnitudeoftheenhancementsfromthehierarchicalclusteringmodel.Weadopttheresultsfromthenumericalsimulationsperformedin[3],butsupplementthatanalysiswithactualvaluesfortheannihilationcrosssectionswhichwecompute.Wefocusontheneutralino,∗E-mail:[e-mail]†E-mail:[e-mail]‡E-mail:[e-mail]1since,asmentioned,itarisesnaturallyinsupersymmetricextensionsofthestandardmodelasagooddarkmattercandidate,butourresultsshouldbeapplicabletothemoregeneralclassofWIMPs.Weareprimarilyinterestedinthegammarayflux(bothcontinuousandmonochromaticlines)sincethisisnotsmearedbypropagationuncertainties.Thenwealsoconsiderthefluxofgammarays(inthiscasemostlythecontinuousgammarays)fromannihilationsintheclosestcausticringinthemodelofSikivie.Thishasnotbeenstudiedbefore,unlikethecaseofdirectdetection,wherethecausticflowshavebeenshowntoleadtosomeinterestingpossibleeffects,suchasareversaloftheannualmodulationpatterncausedbythemotionoftheSolarsysteminthehalo[8].WeworkintheMinimalSupersymmetricStandardModel(MSSM)(see[1,9]forreviewsofsupersymmetricdarkmatter).WealsoestimatetheincreasedfluxofantiprotonswhichiscorrelatedtothecontinuousgammarayfluxandcompareittotheBESS97measurements[10]onantiprotons.InthenextSectionwebrieflyreviewthesignalpatternsandflu