Spectral Gamma-ray Signatures of Cosmological Dark Matter An(4)

We propose a new signature for weakly interacting massive particle (WIMP) dark matter, a spectral feature in the diffuse extragalactic gamma-ray radiation. This feature, a sudden drop of the gamma-ray intensity at an energy corresponding to the WIMP mass,

appearwithhighsigni cance,forthelineitisoftheorderof10σfora5-yearexposurewithGLASTassumingthebackgroundextrapolation～E 2.1givenin[26],whichbecomes20σif,inanalogywiththespectralslopeofothercosmicrays,thebackgroundabove10GeVdropslikeE 2.7instead.Evenifhaloshavelesssingulardensitypro les,suchasthecoredisothermalsphere,adetectablesignaturemayappear.Alternatively,fortheMooreandNFWpro les,modelswithsmallergamma-raybranchingratiosand/orhighermassesmaybeprobed.

(f)Therelativelybroadspectralfeaturecausedbytheredshiftedgamma-raylinealleviatesthedemandforveryhighenergyresolutionofthedetectinginstrumentputbytheprevioussuggestionsofsearchingforthelineinthelocalGalacticneighbourhood.We ndthatanenergyresolutionof10–20%isinfactadequate.ForGLAST,thismeansthatthee ectiveareacaninfactbelargerthanwehaveassumed(sinceasmallerrequirementontheenergyresolutionmeansthateventsfromalargerpartofthe eldofviewofthedetectorcanbeused).(g)Ithastoberemindedthatthestrengthofthegamma-raylinecanbemuchlowerthantheexampleswechoseforFig.1,inwhichcaseadetectionwouldbecor-respondinglymoredi cult.However,inmanyofthosecasesthecontinuumsignalmaybelarge.Inparticular,we ndthatmodelscompatiblewiththerecentresultsonthemuong 2[30]accordingtothesuggestedcontri-butionsof“newphysics”(suchassupersymmetry)of[31]allgivehighvaluesforthecontinuumgamma-ray ux.(h)Althoughtheclumpystructureisinthedarkmat-terdistribution,andmaynothaveopticalcounterparts,therateoftheannihilationshouldscalewiththeoverallmatterdistributioninthenearbyUniverse.Thus,oncemassmapsfrom,e.g.,theSloanDigitalSkySurveyareavailable,across-correlationanalysiswiththegamma-raymapsshouldenableafurtherreductionofGalacticgamma-rayforegrounds.

Toconclude,wehavesuggestedanewpossiblesigna-turefordarkmatterdetection,whichemploystheclus-teringpropertiesofColdDarkMatterastheyemergefromN-bodysimulations.TheupcomingGLASTsatel-lite,andpossiblygroundbaseddetectors,mayhaveagoodchanceatdetectingthecharacteristicfeatures,inparticulartheriseandsuddendropingamma-ray uxaroundtheWIMPmassproducedbyannihilationintotwophotons.

ACKNOWLEDGEMENTS

L.B.andJ.E.wishtothanktheSwedishResearchCouncilfor nancialsupport.P.U.wassupportedbytheRTNprojectundergrantHPRN-CT-2000-00152.L.B.wantstothanktheInstituteforAdvancedStudy,Prince-ton,forhospitalitywhilethisworkwascompleted,andD.Spergelforusefuldiscussions.