Southern Hemisphere Observations of a $10^{18}$eV

arXiv:astro-ph/0009039v26Sep2000SouthernHemisphereObservationsofa1018eVCosmicRaySourceNeartheDirectionoftheGalacticCentreJ.A.Bellido,R.W.Clay,B.R.DawsonandM.Johnston-HollittDepartmentofPhysicsTheUniversityofAdelaide,Australia5005February1,2008AbstractWereportonananalysisofdatafromthesouthernhemisphereSUGARcosmicraydetector.Weconfirmtheexistenceofanexcessof1018eVcosmicraysfromadirectionclosetotheGalacticCentre,firstreportedbytheAGASAgroup.Wefindthatthesignalisconsistentwiththatfromapointsource,andwefindnoevidenceforanexcessofcosmicrayscomingfromthedirectionoftheGalacticCentreitself.SubmittedtoAstroparticlePhysics,June52000RevisedJuly112000AcceptedJuly29200001IntroductionItisgenerallyacceptedthatthemajorityofcosmicraysobservedonEarthareacceleratedwithinourgalaxy.However,apartfromourSunatlowenergies,nospecificaccelerationsitehaspreviouslybeenconfidentlyidentifiedforhadroniccosmicrays.Variousmechanismshavebeenproposedfortheiracceleration,includingtheveryefficientshockaccelerationinsuper-novaexplosions.Allproposedgalacticmechanismshavedifficultyexplainingthehighestenergycosmicrays,andthelackofanylikelycandidateshasledtoasearchforextragalacticsourcesfortheseparticles.Theenergyatwhichgalacticsourcesmightbeovertakenbyextragalacticsourcesisnotatallclearbutisofconsiderableinterest.Thesituationiscomplicatedbythegalacticmagneticfieldwhichisthoughttobestrongenough(oforder1μG)toscramblethedirectionsofchargedcosmicraysatenergiesuptoatleast1018eV[1].Itisonlyat1019eV,whentheLarmorradiusofacosmicrayprotonbecomescomparablewiththegalacticscale,thatonecanconfidentlyexpectacleargalacticanisotropyifgalacticprotonsexistattheseenergies.Nostrongcosmicrayanisotropyhadbeendetectedfromgalacticdirectionsuntilveryrecently.In1999theAGASAgroupreportedastudyofdatacollectedwiththeir20km2and100km2extensiveairshowerarraysovertheperiodfrom1984tomid-1995[2].Withsuchalargedatasettheywereabletosystematicallyperformharmonicanalysesinrightascensionasafunctionofenergy,from1017eVto1020eV.Theydiscoveredalargefirstharmonicanisotropy(amplitude4%)overthenarrowenergyrangefrom1017.9to1018.3eV.AskymapofthecosmicraydensityshowedthatthisanisotropywasapparentlycausedbyanexcessinadirectionclosetotheGalacticCentre,withapossiblesmallerexcessfromthedirectionoftheCygnusregionofthegalacticplane.Whenproducingtheskymap,theAGASAgrouptried4differentassumptionsforthesignalbeamsize,andfoundthatthenear-GalacticCentreexcesswasmostsignificantwhenabeamsizeofradius20◦wasassumed.Thatexcesswasa4.1σdeviationabovetheexpectedisotropicflux.Thisveryimportantanalysiswascomplicatedbythenumberoftrialsinenergyandbeamsizeusedtoproducethefinalresult.MorerecentlythegrouphascompletedananalysisofAGASAdatauptoApril1999[3].Thisextratimehasresultedinalmostadoublinginthenumberofeventsintheenergyregionaround1018eV.Anewanalysiswasperformedonthe1entiredatasetfrom1984to1999,andtheresultsoftheoriginalstudywereconfirmed.Thegroupselectedaslightlydifferentenergyrange,from1018.0to1018.4eV,andproducedaskymapassumingabeamsizeofradius20◦.Theyfounda4.5σexcessneartheGalacticCentre(506eventswithabackgroundof413.6events),withasmaller3.9σexcess(3401eventswithabackgroundof3148)seenintheCygnusregion.SomeevidenceforagalacticplaneexcesswasalsoseenindatafromtheFly’sEyeexperimentoverasimilarenergyrange[4].Thiswasabroad-scalestudy,andnoattemptwasmadetoiden-tifywhetheranyparticulargalacticlongitudeswereresponsiblefortheexcess.AspointedoutbytheAGASAgroup,otherexperimentssuchasHaverahParkandYakutskaretoonortherlytoseetheexcessregionidentifiedbyAGASAneartheGalacticCentre.Theenergydependenceoftheanisotropymayofferacluetothenatureofthesignalparticles.Atanenergyof1018eVaneutronwillhaveadecaylengthofapproximately10kpc,roughlythedistancetotheGalacticCentre[5].Atenergieslowerthanthis,neutronsfromthedistanceoftheGalacticCentrewoulddecaybeforereachingEarth.Theappearanceoftheanisotropyataround1018eVcouldbeexplainedbythiseffect.Itsdisappearanceat3×1018mightthenbeexplainedbythesourcereachingitsenergylimit.Othercluestothenatureofthesignalparticleswouldincludethescaleoftheanisotropyonthesky(pointsource-likeorbroad)andtheresponseofdifferentdetectorarraystothesignalevents(apossiblediscriminantbetweenhadronicandnon-hadronicprimarycosmicrays).2TheSUGARAirShowerDetectorBecauseofthedetectorlatitudeandashowerzenithanglecut(60◦),theAGASAskymapcutsoffatadeclinationof−24.2◦,some4.7◦northoftheGalacticCentre(α,δ)=(266.4◦,−28.9◦)(J2000.0).Theirmostsignificantexcess,neartheGalacticCentre,isontheedgeoftheskymap.Inordertostudythisregionandmoresoutherlydeclinations,wehaveuseddatacollectedbyalargesouthernairshowerarray,usingtheAGASAresultstodefineaprioricutsandthusavoidingstatisticalpenalties.TheSUGARarraywaslocatedinthePilligaStateForestinNewSouthWales,Australia(arraylatitude30.53◦S,longitude149.60◦E)andwasoperatedbetween1968and1979.It2enclosedanareaofupto70km2,andwasdesignedtodetectshowersproducedbythehighestenergycosmicrays[6].Thearrayhadanenergythresholdofabout2×1017eV,andinitsfinalconfigurationconsistedof47independentstations.Eachstationcontainedtwoburiedliquidscintillatorcounte