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EPJWebofConferenceswillbesetbythepublisher DOI:willbesetbythepublisher (cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2017 7 1 0 2 HAWC High Energy Upgrade with a Sparse Array n a J 3 V.Joshi1,a fortheHAWCcollaboration2 2 1MaxPlanckInstitutfürKernphysik,Heidelberg,Germany 2Foracompleteauthorlist,seewww.hawc-observatory.org/collaboration/ ] M I Abstract. The High Altitude Water Cherenkov (HAWC) gamma-ray observatory has . h been fully operational since March 2015. To improve its sensitivity at the highest en- p ergies,itisbeingupgradedwithanadditionalsparsearraycalledoutriggerarray.Wewill - discussinthiscontribution,thedifferentoutriggerarraycomponents,andthesimulation o resultstooptimizeit. r t s a [ 1 HAWC and the Motivation for Outriggers 1 v HAWCissituatedincentralMexicoatanaltitudeof4100mabovethesealevel. Ithasawidefield 6 7 of view of 2 sr and operational energy range of 0.1-100 TeV. It consists of 300 Water Cherenkov 3 Detectors (WCDs) in the main array encompassing a surface area of 20000 m2. The main array 6 WCDs comprised of cylindrical steel water tanks of diameter 7.3 m and height 4.5 m with 4 Photo 0 MultiplierTubes(PMTs)(three8”andone10”)ineachoneofthem. HAWCdetectstheCherenkov . 1 lightproducedinthewaterbyparticlesgeneratedinanatmosphericairshower. 0 WhentheenergyoftheprimaryparticleisoftheorderoftensofTeV,thefootprintoftheshower 7 becomescomparabletothesizeofthemainarray. Therefore, mostoftherecordedshowersarenot 1 containedinthearray,whichcauseschallengestoconstraintheshowerproperties. Toaddressthese : v challengestheconstructionoftheoutriggerarrayaroundthemainarrayhasstarted. Itwillincrease i X the fraction of well-reconstructed showers above multi-TeV energies. The outrigger array will help r in determining the position of the core of the shower falling outside the main array and it will also a improvethedeterminationoftheprimaryparticle’sdirectionandenergy. 2 Outrigger Array The outrigger array [1] consists of 350 cylindrical tanks of diameter 1.55 m and height 1.65 m (see Figure1a). EachtankhasoneHamamatsuR59128"PMTatthebottomofthetank. Theoutrigger array will be deployed in a circular symmetric way around the main HAWC array with a mutual separationof12mto18m(seeFigure1b). To trigger and readout, the system electronics developed for the FlashCAM [2] will be used. FlashCAM is a readout electronics, which has been developed for the cameras of the medium-size telescopesoftheCherenkovTelescopeArray.ThereasonforusingtheFlashCamreadoutforoutrigger ae-mail:[email protected] EPJWebofConferences Figure1. a. Outriggertankandmainarraytanks. b. OutriggerarraysurroundingthemainHAWCarray. The redlinesshowsthedifferentsectionsoftheoutriggerarray. array is that each PMT of the outrigger array is equivalent to a pixel of an Imaging Atmospheric Cherenkov Telescopes (IACTs) camera. The outrigger array is divided into five sections with 70 outriggers in each of them. One such section will contain a readout and trigger electronics, which wenamedasthe: FlashAdcelectronicsfortheCherenkovOutriggerNode(FALCON).Anodewill contain 3 Flash-ADC boards, each of them can digitize 24 channels with a sampling speed of 250 MHzwitha12-bitaccuracy. Italsoallowsaflexibledigitalmultiplicitytriggeraswellasthereadout offullwaveforms,withsettablelength(typically40samplesi.e.160ns)whichcanbeusedforcharge extractionandsignaltiminginformation. 3 Simulations We performed extensive simulations in order to optimize the outrigger array. This can be further dividedintotwoparts: 1.SimulationstostudytheeffectofdifferentPMToptionsandtankcolors. 2.Simulationstodevelopalikelihoodfitmethodinordertofittheshowercoreandtoconstrainthe showerenergyandthedepthoftheshowermaximum. 3.1 SimulationsforPMTOptionsandTankColors InordertochoosethesizeofthePMT,differentPMTsizeshavebeensimulatedincombinationwith different tank wall colors. Here we present the results for the 3" and 8" PMT with tank wall colors blackandwhite. Wehavefocusedonthefollowingfiguresofmerit: 1.AveragenumberofPhoto-Electrons(PEs)observedatagivendistancefromtheshowercore. 2.RMSofthedistributionofthetimedifferencebetweenneighboringtankpairsforthearrivaltime ofthefirstPE. ItcanbeseenfromtheFigure2thatonegets10timesmorePEswiththe8"PMTincomparison tothe3"PMTandtheeffectofthewhitewallcolorinthecontrastofblackwallcoloris20%increase in the number of PEs observed. Furthermore, the white wall color is more diffusive than the black Givetheexacttitleoftheconference wall color, and the loss of the timing information by using the white wall color it can be more than 20%(seeFigure3)incomparisontotheblackwallcolor.Itcanbeconcludedthatwedon’tgainmuch intheaveragenumberofPEsobservedbyusingthewhitewallcolorandweloseconsiderablyinour timinginformation. Wedecidedthatblackwallcolortanks(lessdiffusive)with8"PMTseemstobe theappropriatechoice. pe Black, Energy = 3 TeV pe Black, Energy = 3 TeV White, Energy = 3 TeV White, Energy = 3 TeV m103 Black, Energy = 10 TeV m104 Black, Energy = 10 TeV White, Energy = 10 TeV White, Energy = 10 TeV Black, Energy = 30 TeV Black, Energy = 30 TeV White, Energy = 30 TeV White, Energy = 30 TeV 102 Black, Energy = 50 TeV 103 Black, Energy = 50 TeV White, Energy = 50 TeV White, Energy = 50 TeV 10 102 0 20 40 60 80 100 0 20 40 60 80 100 Distance from the shower core [m] Distance from the shower core [m] Figure2. AveragenumberofPE(µ )observedfor3”PMT(left)and8"PMT(right)withblackandwhite pe tanksasafunctionofdistancefromtheshowercorefordifferentenergies. k) n2.2 RMS ratio for E=3TeV a k t 2 RMS ratio for E=10TeV c a RMS ratio for E=30TeV Bl1.8 nk/ 1.6 RMS ratio for E=50TeV a e t1.4 Whit1.2 S ( 1 M R0.8 0 20 40 60 80 100 Distance from the shower core [m] Figure3. RatiooftheRMSs(seeSection3.1)forwhite/blacktanksfor8”PMTasafunctionofdistancefrom theshowercorefordifferentenergies(E). 3.2 SimulationsforLikelihoodCoreFitMethod Toconstrainthecorelocationofthemulti-TeVγ-rayshowersfallingoutsidethemainHAWCarray alikelihoodcorefitterisbeingdeveloped. InFigure4wecanseethatacoreresolutionof<10mis achievedbyjustusingtheoutriggersforenergies>10TeVandforzenithangleupto30◦.Inaddition, thislikelihoodmethodalsoconstrainstheshowerenergyanddepthoftheshowermaximum. Inthe nextstep,thislikelihoodfitmethodfortheoutriggerswillbemergedwiththeoneforthemainarray toultimatelyimprovethecoreresolutionformulti-TeVshowers. EPJWebofConferences 50 ] m Center of Gravity, Zang = 0 deg 45 [ Likelihood Fit, Zang = 0 deg n o 40 Center of Gravity, Zang = 15 deg ti Likelihood Fit, Zang = 15 deg u 35 l Center of Gravity, Zang = 30 deg o s 30 Likelihood Fit, Zang = 30 deg Re Center of Gravity, Zang = 45 deg 25 Likelihood Fit, Zang = 45 deg e or 20 PRELIMINARY C 15 10 5 0 10 102 Energy [TeV] Figure4.Thecoreresolutionobtainedwithalikelihoodfitincomparisonwiththecenterofgravityofthesignal fordifferentzenithangles(Zang).Theverticaldashedlinesrepresentthebinningintheenergyrange.Thepoints ineachoftheseenergybinscorrespondtothe68%containmentofthecoreresolutiondistribution. 4 Current Status of the Outrigger Array The deployment of the outrigger array has already started. FALCON electronics is being used to takethedatafromthefirstsetofoutriggersinstalledattheHAWCsite. IntegrationoftheFALCON readoutwiththecentralDAQisongoingandwillbefinishedsoon. Acompleteoutriggerarraywill befullyoperationalbytheendofthenextyear. Acknowledgements Weacknowledgethesupportfrom:theUSNationalScienceFoundation(NSF);theUSDepartmentof EnergyOfficeofHigh-EnergyPhysics;theLaboratoryDirectedResearchandDevelopment(LDRD) programofLosAlamosNationalLaboratory;ConsejoNacionaldeCienciayTecnologa(CONACyT), Mexico (grants 260378, 55155, 105666, 122331, 132197, 167281); Red de Fsica de Altas Energas, Mexico;DGAPA-UNAM(grantsIG100414-3,IN108713,IN121309,IN115409,IN113612);VIEP- BUAP(grant161-EXC-2011);theUniversityofWisconsinAlumniResearchFoundation;theInstitute ofGeophysics,PlanetaryPhysics,andSignaturesatLosAlamosNationalLaboratory;theLucBinette FoundationUNAMPostdoctoralFellowshipprogram. References [1] A. Sandoval, Proc. of the 34rd ICRC, The Hague, The Netherlands, September (2015), astro- ph.IM:1509.04269. [2] G. Pühlhofer. C. Bauer, F. Eisenkolb, D. Florin, C. Föhr, A. Gadola, G. Hermann, C. Kalkuhl, J. Kasperek, T. Kihm, J. Koziol, A. Manalaysay, A. Marszalek, P. J. Rajda, W. Romaszkan, M. Rupinski,T.Schanz,S.Steiner,U.Straumann,C.Tenzer,A.Vollhardt,Q.Weitzel,K.Winiarski, K.Zietara,andf.t.CTAconsortium,Proc.ofthe33rdICRC,RiodeJaneiro,Brazil,July(2013), astro-ph.IM:1307.3677.