The Signature of Large Scale Structures on the Ver

arXiv:astro-ph/0612559v219Apr2007DSF-36/2006,FERMILAB-PUB-06-478-A,UCI-TR-2006-24TheSignatureofLargeScaleStructuresontheVeryHighEnergyGamma-RaySkyA.Cuoco1,S.Hannestad2,T.Haugbølle2,G.Miele1,P.D.Serpico3,H.Tu2,41DipartimentodiScienzeFisiche,Universit`adiNapoliFedericoIIandINFNSezionediNapoli,ComplessoUniversitariodiMonteS.Angelo,ViaCinthia,I-80126Napoli,Italy.2InstitutforFysikogAstronomi,AarhusUniversitetNyMunkegade,Bygn.15208000AarhusDenmark.3CenterforParticleAstrophysics,FermiNationalAcceleratorLaboratory,Batavia,IL60510-0500USA.4DepartmentofPhysicsandAstronomy,UniversityofCalifornia,Irvine,CA92697-4575USAAbstract.Ifthediffuseextragalacticgammarayemissiontracesthelargescalestructuresoftheuniverse,peculiaranisotropypatternsareexpectedinthegammaraysky.Inparticular,becauseofthecutoffdistanceintroducedbytheabsorptionof0.1-10TeVphotonsontheinfrared/opticalbackground,prominentcorrelationswiththelocalstructureswithinarangeoffewhundredsMpcshouldbepresent.WeprovidedetailedpredictionsofthesignalbasedonthePSCzmapofthelocaluniverse.WealsousemockN-bodycataloguescomplementedwiththehalomodelofstructurestostudysomestatisticalfeaturesoftheexpectedsignatures.Theresultsarelargelyindependentfromcosmologicaldetails,anddependmostlyontheindexofcorrelation(orbias)ofthesourceswithrespecttothelargescaledistributionofgalaxies.Forinstance,thepredictedsignalinthecaseofaquadraticcorrelation(asitmayhappenforadarkmatterannihilationcontributiontothediffusegammaflux)differssubstantiallyfromalinearcorrelationcase,providingacomplementarytooltounveilthenatureofthesourcesofthediffusegammarayemission.Thechancesofthepresentandfuturespaceandgroundbasedobservatoriestomeasurethesefeaturesarediscussed.PACSnumbers:95.85.Pw,98.70.Vc,95.35.+dTheSignatureofLSSontheVHEGamma-RaySky21.IntroductionGammarayastronomyisaflourishingfieldinastroparticlephysics.Resultshaverapidlyaccumulatedinthelastdecadeortwo:afterthebreak-throughresultsoftheEGRETsatellite,awholeseriesofEarth-basedobservatorieshavedeveloped,bothusingtheimagingairCherenkovtelescopes(ACTlikeWHIPPLE,HEGRA,CANGAROO,HESS,andMAGIC)orsurveyingtheskyviaextensiveairshowers(EASdetectors,likeTIBET,ARGO,andMILAGRO).Newprojectsusingallthesetechniques(theACTsVERITAS,HESSII,MAGICII,MACE,theEASdetectorHAWC,thesatellitesAGILEandGLAST)areontheway(forarecentreviewofthefield,see[1]).AtTeVenergies,afewdozensourceshavebeendetectedmainlybyairCherenkovexperiments,mostofwhicharehighenergycounterpartsofMeV-GeVsourcesintheEGRETcatalog.The0.1–10TeVrangerepresentsoneofthe“last”photonicwindowsyettobeexploredatlargedistances.Startingfromanenergyofabout100GeV(whichweshallrefertoasvery-highenergy,VHE),theabsorptionofhighenergyphotonsontotheextragalacticbackgroundlight(EBL)viapair-productionintroducesanenergy-losshorizonoftheorderofafewhundredsMpcorsmaller,wellbelowthesizeoftheobservableUniverse.Thishorizondropsdownto∼10kpcatPeVenergies,basicallyprecludingdeepspaceastronomywithphotonsaboveabout10TeV.Farfrombeingonlyalimitation,thisphenomenonalsoallowstheuseofγ-rayastronomytoprobetheEBL,whichisotherwisedifficulttostudydirectly.Besidessinglesources,widefieldofviewinstrumentsandsatellite-basedobservationsaresensitivetodiffuseγ-rayemissions.Aparticularlyinterestingemissionistheextragalacticdiffuseγ-raybackground(inthefollowing,cosmicgammabackground,orCGB).TheCGBisasuperpositionofallunresolvedsourcesemittingγ-raysintheUniverseandprovidesaninterestingsignatureofenergeticphenomenaovercosmologicaltime-scales.WhileacleardetectionofthisbackgroundhasbeenreportedbytheEGRETmission[2],itsoriginisstilluncertain,despitethefactthatmanymodelshavebeenproposed.Themostlikelycontributionistheonefromunresolvedblazars,i.e.beamedpopulationofactivegalacticnuclei[3],with(probablysub-leading)componentsfromordinarygalaxies[4],clustersofgalaxies[5],andgammaraybursts[6].However,exoticpossibilitieslikedarkmatterannihilationhavebeenproposed,thatarecompatiblewithexistingdataandconstraints[7,8,9,10].Itisextremelydifficulttotestsuchmodelsaslongastheonlyobservableistheenergyspectrum.Recently,itwasproposedtousethepeculiarsmall-scaleanisotropyencodedintheMeV-GeVgammaskytoprobedarkmatter[11]orastrophysical[12,13]contributionstotheCGB.Inthiswork,wefurtherstudythistopic,withparticularemphasisonthelargescaleanisotropyintheenergyrange0.1-10TeV.ThelowerpartofthisrangewillbeprobedbytheGLASTtelescope[14,15],whiletheenergywindowabovetheTeVisinprincipleaccessibletoEASdetectors.DifferentcandidatestoexplaintheCGBpredictdistinctivelargescalefeatures,evenwhensimilarenergyspectraareexpected.ThisisaconsequenceofthecombinedeffectofacutoffdistanceafterwhichVHEγcantravelTheSignatureofLSSontheVHEGamma-RaySky3undampedtous,andoftheanisotropicdistributionofmatterinthelocaluniverse(i.e.,withinafewhundredMpcfromus).Asimilaranisotropypatterninthetheultra-highenergycosmicrayskywasrecentlyanalyzedbysomeoftheauthors[16](seealso[17]).ThemaingoalofthispaperistocharacterizevariousfeaturesoftheCGBwhichmaybeusedfordiagnosticsatVHE.WedonotattemptheretoderivethepropertiesoftheCGBfromspecificastrophysicalmodels,suchasblaza