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Astronomy&Astrophysicsmanuscriptno.15770 (cid:13)c ESO20111 January21,2011 Constraints on high-energy neutrino emission from SN 2008D IceCubeCollaboration:R.Abbasi1,Y.Abdou2,T.Abu-Zayyad3,J.Adams4,J.A.Aguilar1,M.Ahlers5,K.Andeen1, J.Auffenberg6,X.Bai7,M.Baker1,S.W.Barwick8,R.Bay9,J.L.BazoAlba10,K.Beattie11,J.J.Beatty12,13, S.Bechet14,J.K.Becker15,K.-H.Becker6,M.L.Benabderrahmane10,S.BenZvi1,J.Berdermann10,P.Berghaus1, D.Berley16,E.Bernardini10,D.Bertrand14,D.Z.Besson17,M.Bissok18,E.Blaufuss16,J.Blumenthal18, D.J.Boersma18,C.Bohm19,D.Bose20,S.Bo¨ser21,O.Botner22,J.Braun1,S.Buitink11,M.Carson2,D.Chirkin1, B.Christy16,J.Clem7,F.Clevermann23,S.Cohen24,C.Colnard25,D.F.Cowen26,27,M.V.D’Agostino9, M.Danninger19,J.C.Davis12,C.DeClercq20,L.Demiro¨rs24,O.Depaepe20,F.Descamps2,P.Desiati1, G.deVries-Uiterweerd2,T.DeYoung26,J.C.D´ıaz-Ve´lez1,M.Dierckxsens14,J.Dreyer15,J.P.Dumm1, M.R.Duvoort28,R.Ehrlich16,J.Eisch1,R.W.Ellsworth16,O.Engdegård22,S.Euler18,P.A.Evenson7,O.Fadiran29, 1 A.R.Fazely30,A.Fedynitch15,T.Feusels2,K.Filimonov9,C.Finley19,M.M.Foerster26,B.D.Fox26, 1 A.Franckowiak21,R.Franke10,T.K.Gaisser7,J.Gallagher31,M.Geisler18,L.Gerhardt11,9,L.Gladstone1, 0 T.Glu¨senkamp18,A.Goldschmidt11,J.A.Goodman16,D.Grant32,T.Griesel33,A.Groß4,25,S.Grullon1,M.Gurtner6, 2 C.Ha26,A.Hallgren22,F.Halzen1,K.Han4,K.Hanson14,1,K.Helbing6,P.Herquet34,S.Hickford4,G.C.Hill1, n K.D.Hoffman16,A.Homeier21,K.Hoshina1,D.Hubert20,W.Huelsnitz16,J.-P.Hu¨lß18,P.O.Hulth19,K.Hultqvist19, a J S.Hussain7,A.Ishihara35,J.Jacobsen1,G.S.Japaridze29,H.Johansson19,J.M.Joseph11,K.-H.Kampert6, 0 A.Kappes1,43,T.Karg6,A.Karle1,J.L.Kelley1,N.Kemming36,P.Kenny17,J.Kiryluk11,9,F.Kislat10,S.R.Klein11,9, 2 J.-H.Ko¨hne23,G.Kohnen34,H.Kolanoski36,L.Ko¨pke33,D.J.Koskinen26,M.Kowalski21,T.Kowarik33, M.Krasberg1,T.Krings18,G.Kroll33,K.Kuehn12,T.Kuwabara7,M.Labare20,S.Lafebre26,K.Laihem18, ] E H.Landsman1,M.J.Larson26,R.Lauer10,R.Lehmann36,J.Lu¨nemann33,J.Madsen3,P.Majumdar10,A.Marotta14, H R.Maruyama1,K.Mase35,H.S.Matis11,M.Matusik6,K.Meagher16,M.Merck1,P.Me´sza´ros27,26,T.Meures18, . E.Middell10,N.Milke23,J.Miller22,T.Montaruli1,37,R.Morse1,S.M.Movit27,R.Nahnhauer10,J.W.Nam8, h U.Naumann6,P.Nießen7,D.R.Nygren11,S.Odrowski25,A.Olivas16,M.Olivo22,15,A.O’Murchadha1,M.Ono35, p - S.Panknin21,L.Paul18,C.Pe´rezdelosHeros22,J.Petrovic14,A.Piegsa33,D.Pieloth23,R.Porrata9,J.Posselt6, o P.B.Price9,M.Prikockis26,G.T.Przybylski11,K.Rawlins38,P.Redl16,E.Resconi25,W.Rhode23,M.Ribordy24, r t A.Rizzo20,J.P.Rodrigues1,P.Roth16,F.Rothmaier33,C.Rott12,T.Ruhe23,D.Rutledge26,B.Ruzybayev7, s a D.Ryckbosch2,H.-G.Sander33,M.Santander1,S.Sarkar5,K.Schatto33,S.Schlenstedt10,T.Schmidt16, [ A.Schukraft18,A.Schultes6,O.Schulz25,M.Schunck18,D.Seckel7,B.Semburg6,S.H.Seo19,Y.Sestayo25, 1 S.Seunarine39,A.Silvestri8,K.Singh20,A.Slipak26,G.M.Spiczak3,C.Spiering10,M.Stamatikos12,40,T.Stanev7, v G.Stephens26,T.Stezelberger11,R.G.Stokstad11,S.Stoyanov7,E.A.Strahler20,T.Straszheim16,G.W.Sullivan16, 2 Q.Swillens14,H.Taavola22,I.Taboada41,A.Tamburro3,O.Tarasova10,A.Tepe41,S.Ter-Antonyan30,S.Tilav7, 4 9 P.A.Toale26,S.Toscano1,D.Tosi10,D.Turcˇan16,N.vanEijndhoven20,J.Vandenbroucke9,A.VanOverloop2, 3 J.vanSanten1,M.Voge25,B.Voigt10,C.Walck19,T.Waldenmaier36,M.Wallraff18,M.Walter10,Ch.Weaver1, 1. C.Wendt1,S.Westerhoff1,N.Whitehorn1,K.Wiebe33,C.H.Wiebusch18,G.Wikstro¨m19,D.R.Williams42, 0 R.Wischnewski10,H.Wissing16,M.Wolf25,K.Woschnagg9,C.Xu7,X.W.Xu30,G.Yodh8,S.Yoshida35,and 1 P.Zarzhitsky42 1 : (Affiliationscanbefoundafterthereferences) v i ReceivedSeptember16,2010;acceptedDecember9,2010 X r a ABSTRACT Context.SN2008D,acorecollapsesupernovaatadistanceof27Mpc,wasserendipitouslydiscoveredbytheSwiftsatellitethroughanassociated X-rayflash.Corecollapsesupernovaehavebeenobservedinassociationwithlonggamma-rayburstsandX-rayflashesandaphysicalconnection iswidelyassumed.Thisconnectioncouldimplythatsomecorecollapsesupernovaepossessmildlyrelativisticjetsinwhichhigh-energyneutrinos areproducedthroughproton-protoncollisions.ThepredictedneutrinospectrawouldbedetectablebyCherenkovneutrinodetectorslikeIceCube. Aims.AsearchforaneutrinosignalintemporalandspatialcorrelationwiththeobservedX-rayflashofSN2008Dwasconductedusingdata takenin2007-2008with22stringsoftheIceCubedetector. Methods.Eventswereselectedbasedonaboosteddecisiontreeclassifiertrainedwithsimulatedsignalandexperimentalbackgrounddata.The classifierwasoptimizedtothepositionanda“softjet”neutrinospectrumassumedforSN2008D.Usingthreesearchwindowsplacedaroundthe X-raypeak,emissiontimescalesfrom100−10000swereprobed. Results.Noeventspassingthecutswereobservedinagreementwiththesignalexpectationof0.13events.Upperlimitsonthemuonneutrinoflux fromcorecollapsesupernovaewerederivedfordifferentemissiontimescalesandtheprincipalmodelparameterswereconstrained. Conclusions.Whilenomeaningfullimitscanbegiveninthecaseofanisotropicneutrinoemission,theparameterspaceforajettedemissioncan beconstrained.Futureanalyseswiththefull86stringIceCubedetectorcoulddetectupto∼100eventsforacore-collapsesupernovaat10Mpc accordingtothesoftjetmodel. Keywords.corecollapsesupernovae–SN2008D–cosmicneutrinos–SN-GRBconnection–high-energyneutrinos 1. Introduction generation,andaspherical,pressure-resistantglasshousing.The DOMsdetectCherenkovphotonsemittedbyrelativisticcharged Observationsinrecentyearshavegivenrisetotheideathatcore particlespassingthroughtheice.Inparticular,thedirectionsof collapsesupernovae(SNe)andlongdurationgamma-raybursts muons (either from cosmic ray showers above the surface or (GRB)haveacommonoriginormayevenbetwodifferentas- neutrino interactions within the ice or bedrock) can be well re- pects of the same physical phenomenon, the death of a mas- constructedfromthetrack-likepatternandtimingofhitDOMs. sive star with M > 8M(cid:12) (for a review, see Woosley, Bloom Identification of neutrino-induced muon events in IceCube has 2006). Like GRBs, SNe could produce jets, though less ener- beendemonstratedinAchterbergetal.(2006)usingatmospheric getic and collimated and possibly “choked” within the stellar neutrinosasacalibrationtool.Sourcesinthenorthernsky,like envelope.ObservedassociationsofsupernovaewithXRFs,short SN 2008D, can be observed with very little background since X-rayflasheswithsimilarcharacteristicstolongGRBs,suggest contaminationbyatmosphericmuontracksiseliminatedbythe including XRFs in the SN-GRB connection as well. Although shielding effect of the Earth. When SN 2008D was discovered, XRFs are considered a separate observational category from theinstallationofIceCubewasaboutonequartercompletedand GRBs,acommonoriginandacontinuoussequenceconnecting thedetectorwastakingdatawith22strings. them have been suggested (Lamb et al. 2004, Yamazakia et al. As shown above, a search for cosmic neutrinos from core 2004).XRFcouldbelongGRBswithveryweakjetsorsimply longGRBsobservedoff-axis.SeveralXRFsororlongduration, collapse SNe is motivated by both observational evidence and theoretical predictions. While analyses using catalogs of soft-spectrum GRBs have been observed in coincidence with SNe/GRBswithtiminguncertainties∼1dasthesignalhypoth- core collapse SNe thus far: SN 1998bw (Galama et al. 1998), esishavebeenperformedonarchivedAMANDA/IceCubedata SN 2003lw (Malesani et al. 2004), SN 2003dh (Hjorth et al. (seeLennarz2009forSNeandAbbasietal.2010bforGRBs), 2003), SN 2006aj (Pian et al. 2006), and of course SN 2008D theunprecedentedlyprecisetiminginformationavailableforSN (Soderbergetal.2008,Modjazetal.2009,Mazzalietal.2008). 2008Dsuggestsadesignatedstudyofthisevent.Whileelectro- For SN 2007gr (Paragi et al. 2007) and SN 2009bb (Soderberg magnetic observations provide no conclusive evidence for the etal.2010),twocorecollapseSNenotassociatedwithanXRF existenceofhighlyrelativisticjets,soft,hiddenjetscouldbere- or GRB, recent radio observations provide strong evidence for jets with bulk Lorentz factors of Γ > 1. If some core collapse vealedbyhighenergyneutrinos,assumingsufficientalignment withthelineofsight. SNeindeedformsuch”soft”jets,protonsacceleratedwithinthe jetcouldproduceTeVneutrinosincollisionswithprotonsofthe The paper is organized as follows: Section 2 discusses the stellarenvelope(Razzaqueetal.2005,Ando&Beacom2005). assumedmodelforneutrinoproduction.Section3describesthe The soft jet scenario for core collapse SNe can be probed with experimental and simulated data used for the analyis. The se- high-energy neutrinos even if the predicted jets stall within the lection criteria used to separate signal events from background stellar envelope and are undetectable in electromagnetic obser- are detailed in Section 4. Section 5 presents the results of the vations. searchandconstraintsderivedtherefrom.Finally,theanalysisis OnJanuary9,2008,theX-raytelescopeaboardtheSWIFT summarizedinSection6. satellite serendipitously discovered a bright X-ray flash during a pre-scheduled observation of NGC 2770. Optical follow-up observations were immediately triggered and recorded the optical signature of SN 2008D, a core collapse supernova of type Ib at right ascension α = 09h 09m 30.70s and declination 2. Modelneutrinospectrum δ=33◦08(cid:48)19.1” (Soderbergetal.2008).SN2008Doffersare- Amodelfortheemissionofhigh-energyneutrinosinjetsformed alisticchancetodetecthigh-energysupernovaneutrinosforthe by core collapse supernovae has been proposed by Razzaque, firsttimesincetheobservedX-raypeakprovidesthemostpre- Meszaros,andWaxman(2005)andfurtherelaboratedbyAndo cisetiminginformationeveravailabletosuchasearch.Whether and Beacom (2005). This model will be referred to as “soft jet ornottheexistenceofjetsinasphericalexplosionsisevidenced model”inthefollowing.Abriefsummaryofthephysicalmoti- inthespectroscopicdataforSN2008Dremainshighlydebated. vationandaderivationofitsanalyticalformshallbepresented. WhileSoderbergetal.(2008)“firmlyruleout”anyasphericity and Chevalier and Fransson (2008) speak of a purely spherical The soft jet model assumes the collapse of a massive star shock-breakoutemission,Mazzalietal.(2008)andTanakaetal. M (cid:38)8M withsubsequentformationofaneutronstarorblack (cid:63) (cid:12) (2009)findevidencethatSN2008Dpossessedjetswhichhave hole,rotatingsufficientlytopowerjetswithbulkLorentzfactors beenobservedsignificantlyoff-axis. ofΓ ∼ 1−10andopeninganglesθ ≈ 1/Γ = 5◦−50◦.Such b j b The IceCube neutrino detector, currently under construc- “soft”jets,tooweaktopenetratethestellarenvelope,wouldnot tion at the South Pole and scheduled for completion in 2011, beobservableintheelectromagneticspectrum.Therebounding is capable of detecting high-energy neutrinos (Eν (cid:38) 100GeV) core collapse is assumed to deposit Ej ∼ 3 × 1051erg of ki- of cosmic origin by measuring the Cherenkov light emitted neticenergyinthematerialejectedinthejets–valuesofupto by secondary muons with an array of Digital Optical Modules Ej =6×1051erghavebeensuggestedforSN2008DbyMazzali (DOMs) positioned in the transparent deep ice along vertical et al. (2008). Protons are Fermi accelerated to a E−2-spectrum p strings (J. Ahrens et al. 2004). The full detector will comprise and produce muon neutrinos through the decay of charged pi- 4,800 DOMs deployed on 80 strings between 1.5 and 2.5 km onsandkaonsformedinproton-protoncollisions.Theneutrino deep within the ice, a surface array (IceTop) for observing ex- spectrum, shown in Fig. 1, follows the primary proton spec- tensiveairshowersofcosmicrays,andanadditionaldensesub- trum at low energies and steepens at four break energies above array(DeepCore)inthedetectorcenterforenhancedlow-energy which pions (kaons) lose a significant fraction of their energy sensitivity.EachDOMconsistsofa25cmdiameterHamamatsu inhadronicandradiativecoolingreactions,beforedecayinginto photo-multipliertube(PMT,seeAbbasietal.2010a),electron- neutrinos.Thesebreakenergiesaredistinctforpionsandkaons icsforwaveformdigitization(Abbasietal.2009),highvoltage and exihibit a sensitive dependence on the jet parameters (see N.KemmingfortheIceCubeCollaboration:Constraintsonhigh-energyneutrinoemissionfromSN2008D 3 Table1.Parametersofthesoftjetmodelusedinthisanalysis. Fig.1. Assumed E2-weighted muon neutrino and antineutrino spectrum of SN 2008D according to the soft jet model. For comparison, the atmospheric muon neutrino flux is shown for Parameter Description Default Parameter Value Dependence a100stimewindowandacircularaperturewithopeningangle ω=10◦. E Total kinetic energy of 1051.5erg - j Eνπ, c(1b) Eνπ, c(2b) EνK, c(1b) EνK, c(2b) ejectedmaterial 102 Γ Bulk Lorentz factor of 3 - −2]m Kaon Decays b thejet c eV 10 Pion Decays d DistanceofSN2008D 27Mpc - G Total Flux dN2 [ EdE 1 Atm. Neutrinos Bπ(BK) Bπ±ra→nchµi±nνgµr(aKti±o→forµ±νµ) 1(0.63) - (cid:104)n(cid:105) Pion (kaon) multiplicity 1 (0.1) - π ((cid:104)n(cid:105) ) inppcollisions K 10−1 E mininmum proton en- 10GeV - p,min ergy 10−2 Ep,max maximumprotonenergy 7×104GeV - Eπ(1) hadronic cooling break 30GeV ∝ 102 103 104 E [GeV] (cid:16)Eν,νKc,bc(b1)(cid:17) energyforpions(kaons) (200GeV) ≈E−jE1−Γ17bΓθ52j j b Eπ(2) radiative cooling break 100GeV ∝Γ (cid:16)Eν,Kcb(2)(cid:17) energyforpions(kaons) (20000GeV) b ν,cb Table1).UsingthenotationofAndoandBeacom,thespectrum Eπ maximum neutrino en- 10500GeV ∝Γ canbewrittenas: (cid:16)ν,max (cid:17) b EK ergy from pion (kaon) (21000GeV) Φ (E )= (cid:88) η × EEν−−23Ei(1) EEip(,1m)in≤≤EEν<<EEi(νi2,()c1b) (1) ν,max decay ν ν i=π,K i  Eνν−4Eννi,,(cc1bb)Eνi,(c2b) Eννi,,(cc2bb) ≤ Eνν ≤ Eννi,,cmbax 3. DataandSimulation where The analysis uses experimental data to determine the expected number of background events for a particular search window. (cid:104)n(cid:105) B E ηi = (cid:16) (cid:17) i (cid:16)i j (cid:17) (2) Thesignalexpectationsaswellasthecharactericticsofthesig- 8 2πθ2d2 ln E /E nalarederivedfromsimulations.Rawdataconsistsoftimese- j p,max p,min ries of photon detections (henceforth “hits”) for each triggered Withtheexceptionofthedistanced,weassumethesameparam- DOM. From these hit patterns, track reconstruction algorithms etersforSN2008DthatarequotedinAndo&Beacom(2005). derive the muon’s direction, measured in zenith θ and azimuth AsummaryisgiveninTable1. φinafixeddetectorcoordinatesystemwheremuonstravelling AnoptimisticextensionofthismodelproposedbyKoersand upwards in the ice have θ > 90◦ and downgoing tracks have Wijers(2007)predictsthatmesonsareagainFermi-accelerated θ < 90◦.TheabsolutetimeofaneventisdeterminedbyaGPS afterproduction.Thisre-accelerationgivesrisetoasimple E−γ clockwithaprecisionofbetterthan200ns,whichismorethan neutrinospectrumwithγ = 2.0,...,2.6extendingtomaximum sufficientforthisanalysis. energiesofE ∼ 10PeVwhereradiativecoolingprocesseslead ν toasteepeningandeventualcutoffoftheneutrinospectrum.The 3.1. Backgrounddata details of this high-energy cutoff are negligible in the context ofthisanalysis,whereneutrinoswithenergiesof100GeV-10 At trigger level (detailed in Sec. 3.3 below), IceCube data is TeVareexpectedtoyieldthedominantcontributionofthesignal dominatedbythereduciblebackgroundofatmosphericmuons, expectation. falselyreconstructedasupgoing,i.e.havingpassedthroughthe Neutrinos are expected to be emitted in alignment with the Earth.Acomparisonofexperimentaldataandsimulatedmuons jets. Their energy range is set by the maximum proton energy fromcosmicrayshowersshowsgoodagreement(seeFig.2).In andreachesfarintothesensitiverangeoftheIceCubedetector addition, background data contains an irreducible background (E (cid:38) 100GeV).Inordertodetecttheseneutrinos,thejetmust of muons produced by atmospheric neutrinos from the north- ν bepointingtowardsEarth(e.g.5%chanceforajetwithanopen- ern hemisphere, at a rate lower by a factor of 105. At the final inghalfangleof17◦).Duetotheunknownjetpointing,however, cut levels of this analysis (see Tab. 2), data consists of approx- noconstraintscanbeplacedonthemodelinthecaseofanon- imately equal contributions of reducible and irreducible back- detection. To do so with a confidence level of e.g. 90% would groundevents. requirealargesampleof∼200nearbysupernovae.Incontrast,a The data sample used to measure and characterize back- positivedetectionwouldnotonlyindicatethejet’sdirection,but groundwastakenbyIceCubeinthe22stringconfigurationover alsoyieldconstraintsonthesoftjetmodel–constraintsentirely 275.72daysofdetectorlivetimebetweenMay2007andMarch independentofobservationsintheelectromagneticspectrum.If, 2008.ThesampleisidenticaltotheoneusedinthefirstIceCube inaddition,aresolvedneutrinospectrumcouldberecordedwith search for neutrino point sources (R. Abbasi et al. 2009). On futureneutrinodetectors,theobservationofspectralbreaksand the day of SN 2008D, IceCube was taking data continuously a spectral cutoff would place strong constraints on the physical in a time range of [−9.5h, +1.8h] around the observed X-ray parametersofthesupernovajet. flash. To prevent a bias in the cut optimization, this data was 4 N.KemmingfortheIceCubeCollaboration:Constraintsonhigh-energyneutrinoemissionfromSN2008D kept“blind”,i.e.excludedfromthedevelopmentandtestingof Ndir,E Number of direct hits, i.e. photon hits detected selection criteria, and only “unblinded” in the final step of the within a [−15ns, +250ns] time window of the analysis. arrival time predicted for unscattered Cherenkov emissionunderthetrackhypothesis S Smoothnessofhitdistribution.S = 0indicatesa all all 3.2. SignalSimulation homogeneousenergydepositionalongthetrack Toquantifyandcharacterizetheexpectedsignal,extensivesim- θmin Minimum zenith when the 1st and 2nd half of the ulations of the complex Earth–ice–detector system were con- photon hits (ordered in time) are reconstructed as ducted. IceCube simulation generates primary neutrinos at the separatetracks surface of the Earth and propagates them through the Earth, σ Estimator for the uncertainty of the reconstructed p tracking charged and neutral current interactions, and record- trackdirection(quadraticaverageoftheminorand ing all secondary particles which can reach the detector (see majoraxisofthe1σerrorellipse) Kowalski et al. 2005). All secondary muons are then passed L Value of the negative log-likelihood for the recon- R tothemuonpropagationsoftware(seeChirkin&Rhode2008) structedtrackdividedbythenumberofdegreesof which simulates their random energy loss and the emission of freedom in the fit (number of hit optical modules Cherenkovphotons.Finally,thepropagationofphotonsissim- minusnumberoffitparameters) ulated accounting for absorption and scattering according to a R Ratio of the log-likelihoods with and without a depth dependent ice model (see Lundberg et al. 2007). In the B Bayesian prior that favors a downgoing track hy- laststep,thephotomultiplierresponse,readout,andlocalaswell pothesis as global triggers are simulated yielding time series of photon hitswhicharesubsequentlypassedthroughthesameprocessing RU Ratioofthelog-likelihoodswithandwithoutseed- pipelineasexperimentaldata. ingthereconstructionwiththeinversetrackdirec- tion Inconjunctionwiththeselectionofupgoingtracks,thereduced 3.3. Triggeringanddataprocessing log-likelihood L hasproventobeanefficientvariableforsep- R TheIceCubetriggersystemonlyreadsoutaphotonhitataspe- arating upgoing atmospheric neutrinos from misreconstructed cificopticalmoduleifaneighboringmoduleonthesamestring downgoing atmospheric muons. It exploits the fact that for a is also hit within 1µs (local coincidence). To initiate the event light pattern originating from a downgoing muon the incorrect read-out,theglobaltriggerofIceCube22required8suchlocal upgoing track hypothesis yields rather low absolute likelihood coincidenceswithina5µstimewindow.Thisrequirementlead values. In addition, the likelihood ratios RU and RB allow for a to trigger rates of ∼550 Hz, dominated by atmospheric muon vetooneventsforwhichinvertingthetrackhypothesisleadsto events. Data contamination was immediately reduced to ∼25 asignificantrelativeenhancementinthelikelihoodvalue. Hz by first-guess reconstructions running online at the South Histograms of all selection parameters are shown in Fig. 2 Pole,whichfitasimpletrackhypothesistoeacheventandreject forbackgrounddata,backgroundsimulation,andsimulatedsig- downgoingtracksinrealtime(Ahrensetal.2004).Eventspass- nalevents.Tocombinealleightparametersefficiently,theywere ing this online muon filter are transferred to the North, where incorporated into a boosted decision tree (BDT) classifier (see extensive likelihood track reconstructions are performed. For a e.g.Yangetal.2005andreferencestherein).TheBDTmethod givenhitpatternandafirstguesstrackhypothesis,thelikelihood classifies an event by passing it through a tree structure of bi- functioniscalculatedastheproductoftheprobabilitiesforeach narysplitswhicheffectivelybreaksuptheparameterspaceinto hittimetooccurunderthegiventrackhypothesis.Thelikelihood a number of signal or background-like hypercubes. The classi- reconstruction algorithm then iteratively searches for the track fier is first trained with background data and simulated signal which maximizes the value of this likelihood function (Ahrens andthenevaluatedwithindependentdatasets.Theresultingdis- etal.2004).Forthefinalfitresult,theoptimizationsofwarecom- tribution of classifier scores K for experimental data and sim- putesqualityparameterswhichcanbeusedforeventselection. ulated signal is shown at the bottom of Fig. 2. The classifier allows for a simple one-dimensional cut on the classification score.Extensivetestswereconductedtoassureastableresponse andtoestimatetheuncertaintyoftheclassification.Thisuncer- 4. Eventselection taintywasestimatedbycomparingtheclassificationefficiencies The background event rate is further diminished to ∼3 Hz for several independent experimental data and simulated signal throughanothercutonthemoreprecisetrackdirectionfromthe samples.Variationsintheclassifierresponseprovedtobenegli- likelihood track reconstruction selecting events with θ > 80◦. giblecomparedtostatisticaluncertainties. For this analysis, events outside a circular signal region (10◦ openingangle)aroundthepositionofSN2008Dwereremoved 4.2. SearchWindows from the dataset to obtain a manageably sized sample. At this filtering level, the background rate is 0.03 Hz and 0.26 sig- The search for neutrinos in the on-time data from January 9, nal events are expected for SN 2008D according to the soft jet 2008wasconductedusingthreesearchwindowsofdifferentdu- model. rations, apertures, and selection cuts. A circular aperture was used in all cases. Since the soft jet model does not explicitly predict the time profile of the neutrino emission, search win- 4.1. QualityCuts dows with durations of 100s, 1000s, and 10000s were chosen Specific cuts tailored to the simulated properties of SN 2008D tocoveralargerangeofemisssiontimescales.Thecorrespond- werebasedonthefollowingeightqualityparameters: ingopeningangleandqualitycutsforeachsearchwindowwere determinedbyoptimizingthemodeldiscoveryfactorMaccord- N.KemmingfortheIceCubeCollaboration:Constraintsonhigh-energyneutrinoemissionfromSN2008D 5 Table 2. Windows used to search for neutrinos in correlation Fig.2.Normalizedhistogramsofcutparametersusedforevent with SN 2008D. For each time scale, quality and angular cuts selection(top)andresultingdistributionofboosteddecisiontree wereoptimizedtoyieldamaximummodeldiscoverypotential. classifier values (bottom). Dashed line indicates experimental backgrounddata,dottedlinemarksbackgroundsimulation,solid linerepresentssimulatedsignal. Duration Centering Aperture BDTcut ∆t wrt.X-raypeak ω K 10−1 10−1 Window1 100s −70s, +30s 6.2◦ 0.390 Window2 1000s −500s, +500s 2.6◦ 0.464 Window3 10000s −7000s,+3000s 1.5◦ 0.580 10−2 10−2 10−3 Fig.3. Effective areas for a neutrino spectrum obeying the soft jetmodel.Eachlinerespresentsoneofthefinalsearchwindows 10−3 0 10 R20 30 40 0 1θ [rad]2 3 usedinthisanalysis.Inset:Cumulativepointspreadfunctionfor U min thedirectioninwhichSN2008Dwasobserved. 10−1 10−1 10−2 10−2 10−3 10−3 10−40 0.5 1 1.5 10−40 20 40 60 σ [rad] R p B 10−1 10−1 10−2 10−2 10−3 10−3 gorithm(Feldman&Cousins1998).Thesignalexpectationisin- 10−40 200 400 600 800 1000 10−40 10 20 30 40 creaseds→ s(cid:63)until50%ofthetrialsyieldadiscovery,thatis,a L [m] N dir,E dir,E lowerlimitonthesignalsgreaterthanzero.Whenthiscriterion 10−1 10−1 ismet,themodeldiscoveryfactorisgivenby s(cid:63) 10−2 10−2 M = (3) s 10−3 10−3 Foreachwindow,theBDTcutK andtheopeningangleωyield- ingtheminimalvalueofMweredeterminednumerically.Lower 10−4 8 10 12 10−4−1 −0.5 0 0.5 1 limitsaccordingtotheFeldman&Cousinsorderingschemewere L S R all requiredtohaveasignificanceof5σ.Thechoicesofcutsforthe three search windows which yielded minimal model discovery factorsaresummarizedinTable2.Theresultingeffectiveareas 10−2 foraneutrinospectrumobeyingthesoftjetmodelareshownin Fig.3. 10−3 Withthesechoices,twoobservedeventswouldconstitutea 5σdiscoveryinanyofthewindowstakenbyitself.Thesignifi- 10−4 cancesforthecompletemeasurementconsistingofthreesearch Signal Monte Carlo windowsweredeterminedinasimulationstudywith1010trials. 10−5 Foreachpossibleobservationofn , n , n eventsinwindow1, Data 1 2 3 2,3,thep-valuewascalculatedasthefractionofequallyorless 10−6 Atm. Muon Monte Carlo likelyobservations. −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 BDT score 5. Results 5.1. Unblinding ingtoHilletal.(2006).Forthispurpose,aPoissondistribution with mean b + s is randomly sampled, where b and s repre- Noeventspassingthecutswerefoundintheexperimentaldata. senttheexpectedbackgroundandsignal,respectively.Foreach AsshowninTable3,thisresultisconsistentwithexpectations, drawn number of observed events n the lower limit on the evenmoresoifweaccountforthe∼5%probabilityofajetwith obs signalcontributioniscomputedusingtheFeldman&Cousinsal- openinghalfangle∼17◦pointingtowardsEarth. 6 N.KemmingfortheIceCubeCollaboration:Constraintsonhigh-energyneutrinoemissionfromSN2008D Table 3. Summary of the unblinding results and comparison Fig.4.SpectrumofSN2008Daccordingtothesoftjetmodelfor withexpectations. different assumed jet Lorentz factors and under the assumption thatthejetispointingtowardsEarth. Window1 Window2 Window3 ObservedEvents n =0 n =0 n =0 1 2 3 ExpectedEvents Signals 0.13 0.060 0.020 Backgroundb 3.67×10−4 5.52×10−4 5.55×10−4 5.2. LimitsontheSoftJetModel Intheabsenceofmoreprecisetheoreticalpredictionsonthetime profileoftheemission,quotinglimitsforparticulartimescales is the only viable way to constrain the soft jet model. Since n = n = n = 0 and b ≈ b ≈ b , the signal upper limits 1 2 3 1 2 3 s¯ areidenticalforallthreesearchwindowstothefourthsignif- i icant digit: s¯ = s¯ = s¯ = s¯ = 2.44 (at 90% CL). The upper 1 2 3 limit Φ¯(90) on the neutrino flux in terms of the expected flux ν Φ isgivenbytheratioofthesignalupperlimit s¯tothesignal ν expectations: Φ¯(90) s¯ Fig.5.Expectednumberofeventsasafunctionoftheassumed Φνν = s (4) jetLorentzfactorΓbundertheassumptionthatthejetispointing towardsEarth.Theplottednumberscorrespondtoa10◦-signal- Due to the different signal expectations in each window, the regionandcutlevel3atwhichthebackgroundrateis0.03Hz. fluxupperlimitsdependontheassumedemissiontimescaleτ . e Therefore,wequotethelimitsonthesoftjetmodelforcanoni- cτaelapnadraamtaetreerfser(eTnacbe.e1n)esregpyaoraftEelνy=fo1r0e0aGcheVem: ission time scale al µs 103 Φ¯(νG90e)V(1−010cmG−e2V)=(cid:34)10Md pc(cid:35)2×000...00135758 τττeee ===11100000000s0ss (5) ected Sign 11002 p x Eachlimitisonlyvalidundertheassumptionthattheentireneu- E trino signal is contained in the corresponding time window. In 1 E = 1052 j otherwords,SN2008Dcouldhaveemittedatmost19(41,122) E = 1051.5 twimithesdmefoaureltnpeauratrmineotsersthΓanb =ass3umaneddEunjd=er1t0h5e1.5soerfgt.jeAt hmigohdeerl 10−1 Ejj = 1051 fluxwouldhavebeenobservedbyIceCubewithaprobabilityof 90%. 10−2 2 3 4 5 6 7 8 9 10 Γ The primary systematic uncertainty in these limits stems b from a possible bias in signal simulation, i.e. the value of s. Systematics for IceCube 22 have been studied by Abbasi et al. (2009) and lead to a ∼15% uncertainty in s, corresponding to a +17 percent shift in the limits. Incorporating the uncertainty −13 oftheBDTclassificationresponse,thatisdecreasingthesignal predictionandincreasingthebackgroundexpectationbythecor- wellasstrongerbeaminginmorerelativisticjetsleadstoadras- respondinguncertaintyresultedinanegligibleshiftof∼0.5%in ticincreaseinthesignalexpectation.IncreasingΓ placesmore b thelimits. neutrinosathighenergies(cid:38)1TeVwhereIceCubeismoresensi- Next,wewishtoconstrainthemainparametersofthemodel, tive,thoughthecorrespondingreductioninthejetopeningangle thekineticenergyreleaseEjandtheLorentzfactorofthejetΓb. leads to smaller probability of jet detection. The measured sig- Due tothe significant Γb dependence ofthe hadronicbreak en- nalupperlimits¯=2.44andthesignalpredictionss (cid:16)Γ , E (cid:17)for i b j eErνπg,/cyKb(E2)νπ,/∝cKb(Γ1)b,∝theEn−j1umΓ5bbearndantdhespreacdtiraatlivdeisctroiobluitnigonborefapkroednuecrgeyd eΓabcthhrwouingdhowsi(cid:16)cΓabn,Ebej(cid:17)u<sesd¯i.toVaclounesstroafinΓbthaendjetEpjanroatmfeutlefirlsliEngj tahnids neutrinosdependsstronglyonΓb(seeFig.4).Moreover,theflux relationareruledoutat90%CL.Theselimitsareillustratedin is scaled with EjΓ2b which accounts for the energy release and Fig.6. thebeamingoftheneutrinoemission.Athighboostfactors,ra- Finally, the scenario proposed by Koers and Wijers (2007) diativecoolingofmesonssetsinatlowerenergiesthanhadronic (cid:16) (cid:17) shallbeexaminedbriefly.Assumingthatmesonre-acceleration cooling,i.e.Eνπ,c(1b) > Eνπ,c(2b) EνK,c(b1) > EνK,c(b2) forΓb (cid:38)4(Γb (cid:38)9). leads to a simple power law neutrino spectrum in the relevant To derive constraints on Γb and Ej, we calculated the sig- energyrange(roughly100GeV-10PeV)thesourcespectrum nal expectations in the intervals Γb = 1.5 − 10 and Ej = canbeapproximatedbyanE−γ-lawwithahigh-energycutoffat 1051 − 1052erg. As Fig. 5 shows, the less efficient cooling as 10PeV.Forthethreevaluesofthespectralindexγdiscussedby N.KemmingfortheIceCubeCollaboration:Constraintsonhigh-energyneutrinoemissionfromSN2008D 7 Fig.6.ConstraintsonthejetparametersE andΓ whereE = Sweden; German Ministry for Education and Research (BMBF), Deutsche j b 51.5 Forschungsgemeinschaft (DFG), Germany; Fund for Scientific Research 1051.5erg.Foreachassumedemissiontimescaleτe,thecolored (FNRSFWO), Flanders Institute to encourage scientific and technological re- regionsareruledoutat90%confidencelevel. searchinindustry(IWT),BelgianFederalSciencePolicyOffice(Belspo);the NetherlandsOrganisationforScientificResearch(NWO);M.Ribordyacknowl- edgesthesupportoftheSNF(Switzerland);A.KappesandA.Groacknowledge supportbytheEUMarieCurieOIFProgram;J.P.Rodriguesacknowledgesup- portbytheCapesFoundation,MinistryofEducationofBrazil. 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A two 9000Gent,Belgium dimensional parameter scan in Γ and E shows that the jet 3 Dept.ofPhysics,UniversityofWisconsin,RiverFalls,WI54022, LorentzfactorisgenerallyconstraibnedtoΓj <4forjetenergies USA b 4 Dept.ofPhysicsandAstronomy,UniversityofCanterbury,Private E > 1051erg.Asmentionedabove,theconstraintsquotedhere j Bag4800,Christchurch,NewZealand onlyholdiftheassumedjetofSN2008Dwaspointingtowards 5 Dept.ofPhysics,UniversityofOxford,1KebleRoad,OxfordOX1 Earth. 3NP,UK IceCube is now operating in an additional mode, scanning 6 Dept. of Physics, University of Wuppertal, D-42119 Wuppertal, online data for neutrino bursts, i.e. two nearly collinear neutri- Germany nos within 100 s, in real time. If a burst is detected, IceCube 7 Bartol Research Institute and Department of Physics and triggersopticalfollow-upobservationssearchingforaSNinthe Astronomy,UniversityofDelaware,Newark,DE19716,USA corresponding direction (Franckowiak et al. 2009). Constantly 8 Dept. of Physics and Astronomy, University of California, Irvine, CA92697,USA monitoringtheentirenorthernsky,thisapproachhasthepoten- 9 Dept. of Physics, University of California, Berkeley, CA 94720, tialtogeneralizetheconstraintsobtainedfromstudyingindivid- USA ualobjects. 10 DESY,D-15735Zeuthen,Germany 11 LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA Acknowledgements. We acknowledge the support from the following agen- 12 Dept. of Physics and Center for Cosmology and Astro-Particle cies: U.S. National Science Foundation-Office of Polar Program, U.S. Physics,OhioStateUniversity,Columbus,OH43210,USA National Science Foundation-Physics Division, University of Wisconsin 13 Dept.ofAstronomy,OhioStateUniversity,Columbus,OH43210, Alumni Research Foundation, U.S. Department of Energy, and National EnergyResearchScientificComputingCenter,theLouisianaOpticalNetwork USA Initiative (LONI) grid computing resources; Swedish Research Council, 14 Universite´ Libre de Bruxelles, Science Faculty CP230, B-1050 SwedishPolarResearchSecretariat,andKnutandAliceWallenbergFoundation, Brussels,Belgium 8 N.KemmingfortheIceCubeCollaboration:Constraintsonhigh-energyneutrinoemissionfromSN2008D 15 Fakulta¨t fu¨r Physik & Astronomie, Ruhr-Universita¨t Bochum, D- 44780Bochum,Germany 16 Dept.ofPhysics,UniversityofMaryland,CollegePark,MD20742, USA 17 Dept.ofPhysicsandAstronomy,UniversityofKansas,Lawrence, KS66045,USA 18 III. Physikalisches Institut, RWTH Aachen University, D-52056 Aachen,Germany 19 OskarKleinCentreandDept.ofPhysics,StockholmUniversity,SE- 10691Stockholm,Sweden 20 VrijeUniversiteitBrussel,DienstELEM,B-1050Brussels,Belgium 21 Physikalisches Institut, Universita¨t Bonn, Nussallee 12, D-53115 Bonn,Germany 22 Dept.ofPhysicsandAstronomy,UppsalaUniversity,Box516,S- 75120Uppsala,Sweden 23 Dept. of Physics, TU Dortmund University, D-44221 Dortmund, Germany 24 LaboratoryforHighEnergyPhysics,EćolePolytechniqueFe´de´rale, CH-1015Lausanne,Switzerland 25 Max-Planck-Institutfu¨rKernphysik,D-69177Heidelberg,Germany 26 Dept.ofPhysics,PennsylvaniaStateUniversity,UniversityPark,PA 16802,USA 27 Dept. of Astronomy and Astrophysics, Pennsylvania State University,UniversityPark,PA16802,USA 28 Dept. of Physics and Astronomy, Utrecht University/SRON, NL- 3584CCUtrecht,TheNetherlands 29 CTSPS,Clark-AtlantaUniversity,Atlanta,GA30314,USA 30 Dept. of Physics, Southern University, Baton Rouge, LA 70813, USA 31 Dept.ofAstronomy,UniversityofWisconsin,Madison,WI53706, USA 32 Dept.ofPhysics,UniversityofAlberta,Edmonton,Alberta,Canada T6G2G7 33 Institute of Physics, University of Mainz, Staudinger Weg 7, D- 55099Mainz,Germany 34 Universite´deMons,7000Mons,Belgium 35 Dept.ofPhysics,ChibaUniversity,Chiba263-8522,Japan 36 Institut fu¨r Physik, Humboldt-Universita¨t zu Berlin, D-12489 Berlin,Germany 37 alsoUniversita` diBariandSezioneINFN,DipartimentodiFisica, I-70126,Bari,Italy 38 Dept.ofPhysicsandAstronomy,UniversityofAlaskaAnchorage, 3211ProvidenceDr.,Anchorage,AK99508,USA 39 Dept.ofPhysics,UniversityoftheWestIndies,CaveHillCampus, BridgetownBB11000,Barbados 40 NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA 41 SchoolofPhysicsandCenterforRelativisticAstrophysics,Georgia InstituteofTechnology,Atlanta,GA30332,USA 42 Dept. of Physics and Astronomy, University of Alabama, Tuscaloosa,AL35487,USA 43 affiliated with Universita¨t Erlangen-Nu¨rnberg, Physikalisches Institut,D-91058Erlangen,Germany