Very high-energy gamma-ray observations of the Cra

arXiv:astro-ph/0108270v116Aug2001Veryhigh-energyγ-rayobservationsoftheCrabnebulaandotherpotentialsourceswiththeGRAALexperimentF.Arqueros1,J.Ballestrin2,M.Berenguel3,D.M.Borque1,E.F.Camacho4,M.Diaz5,H.-J.Gebauer5,R.Enriquez1,R.Plaga51FacultaddeCienciasFisicas,UniversidadComplutense,E-28040Madrid,Spain2CIEMAT-DepartamentoEnergiasRenovables,PlataformaSolardeAlmeria,E-04080Almeria,Spain3DepartamentodeLenguajesyComputaci´on,UniversidaddeAlmeria,04120Almeria,Spain4EscuelaSuperiordeIngenieros,UniversidaddeSevilla,E-41012Sevilla,Spain5Max-Planck-Institutf¨urPhysik,80805M¨unchen,GermanyFebruary1,2008AbstractThe“GammaRayAstronomyatALmeria”(GRAAL)experimentuses63heliostat-mirrorswithatotalmirrorareaof≈2500m2fromtheCESA-1fieldatthe“PlataformaSolardeAlmeria”(PSA)tocollectCherenkovlightfromairshowers.Thedetectorislocatedinacentralsolartoweranddetectsphoton-inducedshowerswithanenergythresholdof250±110GeVandanasymptoticeffectivedetectionareaofabout15000m2.AcomparisonbetweentheresultsofdetailedMonte-Carlosimulationsanddataispresented.DatasetstakenintheperiodSeptember1999-September2000inthedirectionoftheCrabpulsar,theactivegalaxy3C454.3,theunidentifiedγ-raysource3EG1835+35anda“pseudosource”wereanalyzedforhighenergyγ-rayemission.Evidenceforaγ-rayfluxfromtheCrabpulsarwithanintegralfluxof2.2±0.4(stat)+1.7−1.3(syst)×10−9cm−2sec−1abovethresholdandasignificanceof4.5σinatotalmeasuringtimeof7hoursand10minutesonsourcewasfound.Noevidenceforemissionfromtheothersourceswasfound.Somedifficultieswiththeuseofheliostatfieldsforγ-rayastronomyarepointedout.Inparticulartheeffectoffield-of-viewrestrictedtothecentralpartofadetectedairshoweronthelateraldistributionandtimingpropertiesofCherenkovlightarediscussed.Uponrestrictionthespreadofthetimingfrontofprotoninducedshowerssharplydecreasesandthereconstructeddirectionbecomesbiasedtowardsthepointingdirection.Thisisshowntomakeefficientγ-hadronseparationdifficult.11Introduction-aimsandplanofthepaperMeasuringatmosphericCherenkovradiationispresentlythemosteffectivewaytodetectcosmicγ-rayswithprimaryenergiesbetweenabout100GeVand1TeV[1].InordertoreachlowenergythresholdswithtechniquesbasedonCherenkovlight,largemirrorcollectionareasareneeded.GRAALisanexperimentthatemploysthelargemirrorareaofanexistingtowersolar-powerplantforthispurpose.ThispaperbrieflydescribestheGRAALdetectorandreportsresultsaboutthedetectionofγ-raysfromcosmicsources.Inadditionsomegenerallessonswelearntabouttheheliostat-fieldapproachtoγ-rayastronomyarereported.Insection2theGRAALdetectorisdescribedandcomparedtootherheliostat-fielddetectorsforCherenkovlight.Section3describestheeventreconstructionbasedmainlyonthearrivaltimeofsignalsatthecentraldetector.Section4treatstheMonteCarlosimulationoftheexperiment.Section5explainshowthedatasetusedfortheanalysisofthispaperwaschosenfromthetotalsetofalltakendata.Thedatareductionprocedures—andthefundamentalproblemsbesettingit—areexplainedinsection6andtheresultsarepresentedinsection7.Finallysomeconcludingremarksareofferedinsection8.Amoredetailedreportabouttheseresultswillbeavailableintwotheses[2,3].2TheGRAALdetector2.1TheCESA-1heliostatfieldatthePSACESA-1isaheliostatfieldcomprisingof300steerablemirrorstothenorthofacentraltowerlocatedwithinthe“PlataformaSolardeAlmeria”(PSA)asolarthermal-energyresearchcentreoperatedbytheSpanishCIEMAT.ThePSAislocatedinthedesertofTabernas(37◦.095N,2◦.360W)about30kmfromthecityofAlmeriaandthesea,atthefoothillsoftheSierra-Nevadamountains(heighta.s.l.of505m).The63heliostatsusedforGRAALhaveamirrorareaof39.7m2eachandconsistof12rectangular“facets”(submirrors)withasphericalcurvaturethatare“canted”(adjustedrelativetotheoverallframe)toaroughlysphericaloverallheliostatshape.ThebeamspreadfunctionoftheheliostatshasaRMSofabout0.25◦.EachheliostatisindividuallysteerablewithsteppingmotorsviaacentralPC.ForthepurposeofGRAALacontrolprogramwasdevelopedthatallowedtoperformthespecialtrackingneededfortheuseofthefieldforCherenkovastronomy.TheheliostatfocustheCherenkovlightofairshowersfromthedirectionofpotentialgamma-raysourcestosoftwareadjustable“aimingpoints”inthecentraltower(seefig.1).Theso-called“convergentview”[4]—thepointingoftheheliostatstowardsapointintheatmospherecorrespondingtoanatmosphericdepthof230g/cm2inthegeneraldirectionofthepotentialsourceofgammarays—wasalwaysapplied.Therelativelythinglassusedfortheheliostatmirrors(4mmthickness)—leadingtoalow2overallheatcapacity—andtheproximityoftheoceanleadtofrequentdewformationonthemirrorsinthewinter.Topreventmicrodropformation,allmirrorsweresprayedeveryseconddaywithatensidsolutionintheeveningusingaspeciallyconstructedspraycart.Thisprocedurewasfoundtoworkwellafterallmirrorshadbeencleandwithsulfonicacidfromtracesofsilicongel—acommoncontaminantinglassproduction.2.2Detectorsetup2.2.1SecondaryopticsCherenkovlightfromfourgroupsofheliostats(with13,14,18,18members,respectively)isdirectedontofoursinglenon-imaging“coneconcentrators”eachcontainingasinglelarge-areaphotomultipliertube(PMT).ThelightcollectorshavetheformoftruncatedWinstonconeswithanopeningangleof10◦.Eachconehasafrontdiameter