ANTARES constraints on a Galactic component of the IceCube cosmic neutrino flux

0.34 MB·English

by  Maurizio Spurio

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(cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2015
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ANTARES constraints on a Galactic component of the IceCube
b
e cosmic neutrino flux
F

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MaurizioSpurio1,2,a

]
E 1DipartimentodiFisicaeAstronomiadell’UniversitàdiBologna-VialeBertiPichat6/2,40127Bologna(Italy)
H 2IstitutoNazionalediFisicaNucleare-SezionediBologna-VialeBertiPichat6/2,40127Bologna(Italy)
.
h
p Abstract.
- The IceCube evidence for cosmic neutrinos has inspired a large number of hypothesis
o ontheirorigin,mainlyduetothepoorprecisiononthemeasurementofthedirectionof
tr showeringevents. ANorth/Southasymmetryinthepresentdatasetsuggeststhepres-
s enceofapossibleGalacticcomponent. Thiscouldbeoriginatedeitherbysinglepoint-
a
like sources or from an extended Galactic region. Expected fluxes derived from these
[
hypothesesarepresented.Somevalueshavebeenconstrainedfromthepresentavailable

2 upperlimitsfromtheANTARESneutrinotelescope.
v
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1 The IceCube cosmic neutrinos and a possible Galactic component
1
0
The recent IceCube (IC) evidence for extraterrestrial high-energy neutrinos [1, 2] opened new win-
.
1 dows in the field of astroparticle physics [3]. With the present statistics, the High Energy Starting
0 Events (HESE) flux observed by IC is compatible with flavor ratios ν : ν : ν = 1 : 1 : 1, as ex-
e µ τ
5
pectedfromchargedmesondecaysincosmicray(CR)acceleratorsandneutrinooscillationontheir
1
waytotheEarth. Thenon-observationofeventsbeyond2PeVsuggestsaneutrinofluxwithapower
:
v law Φ(E) ∝ E−Γ with hard spectral index, e.g. Γ (cid:39) 2.0, and an exponential cutoff, or an unbroken
i
X powerlawwithasofterspectrum,e.g. Γ(cid:39)2.3.
r ThemajorityofHESEaredowngoing;astheICdetectorisattheSouthPole,thiscorrespondstoa
a largerfluxfromtheSouthernsky,wheremostoftheGalacticplaneispresent. Table1(columnsfrom
2to5)reportsforHESEwithdepositedenergyE >60TeV[2]:thenumberofevents;theestimated
dep
background;thenumberofcosmicneutrinos(i.e. the signal);andthenumberofexpectedcosmicνs
assumingthebest-fithypothesis. ValuesaregivenseparatelyfortheNorth/Southskyregions.
Recently,ICpresentedanewsearchforneutrinosinteractingintheinstrumentedvolumeandwith
energy between 1 TeV and 1 PeV, using 641 days of livetime [4]. Table 1 (columns from 6 to 10)
reports,fortheeventsinthisnewsamplehavingE >25TeV,thesamequantitiesdefinedabovefor
dep
the HESE. No hypothesis test on HESE as reported in [2] yielded at present statistically significant
evidence of clustering or correlations, in particular from the Galactic Center or the Galactic Plane.
Thesamefortheneutrinosamplestudiedin[4].
InthedatapresentedinTable1,anexcessofdowngoing(Down)eventswithrespecttoexpectation
seems however be present. The Northern sky, inducing upgoing (Up) events in IceCube, contains
ae-mail:[email protected]
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HESE[2] Data Bck n N Newνsample[4] Data Bck n N
IC IC IC IC
E >60TeV E−2 E >25TeV E−2.46
dep dep
Up(North) 5 1.4 3.6 6.7 Up(sinδ>0.06) 11 5.3 5.7 12.1
Down(South) 15 1.3 13.7 11.5 Down(sinδ<−0.06) 29 4.8 24.2 15.0
All 20 2.7 17.3 18.2 All 43 11.7 31.3 29.1
Table1.Column2to5:numberofHESEwithE >60TeV(Data);numberofbackgroundevents(Bck);
dep
numbern ofsignalevents(Data-Bck);expectednumberN ofsignaleventsfromthebest-fit.Column7to
IC IC
10:thesamequantitiesforsamplewithE >25TeVin[4].Quantitiesaregivenforupgoing(fromthe
dep
Northernhemisphere)anddowngoing(fromtheSouthernhemisphere)events,andforthewholesky(All).
onlyasmallfractionoftheGalaxy. Forthisreason,letusassumethatthe3.6estimatedHESEfrom
cosmic neutrinos (n ) arising from the Northern sky are all of extragalactic origin. Assuming a
IC
E−2.0 spectrum,asymmetriccontributionfromtheNorth/Southextragalacticsourcesandtakinginto
account the different Aνi for events coming from the North and South hemispheres, then 6.2 events
IC
areexpectedfromtheSouth(theyreduceto5.8eventsforaE−2.5flux). Thisexcessof∼7.5eventsin
theSouthernskycorrespondsto∼40%ofthetotalsignal.
AnevenstrongerexcessfromtheSouthisderivedfromdatain[4],usingeventswith E > 25
dep
TeV. Here, to the 5.7 signal events coming from the North with sinδ > 0.06 (assuming the same
considerationsofabove)shouldcorrespond∼7.2eventsfromtheSouthwithsinδ < −0.06. As24.2
events are observed, more than 50% of the number of signal events in the whole sky seems to be
produced by a non-isotropic cosmic component, likely of Galactic origin. A possible contribution
fromtransientextragalacticobjectslocatedintheSouthernskycanbeconsideredaswell.
TheaboveconclusionsarederivedforΓ = 2.0;howeversimilarresultsareobtainedusingsofter
spectralindexes(i.e. Γ > 2.0)forthecosmicneutrinoflux. Thisisrelevantbecausedifferentmodels
involvingGalactic,extragalacticorexoticoriginoftheICsignalexistintheliterature. Theneutrino
fluxpredictedbyeachmodelhasapreferredvalue,usuallyrangingintheintervalΓ=2.0÷2.7.
In the following, the effects of the hypothesis that a sizeable fraction of the cosmic neutrinos
observed by IC is originated in our Galaxy is considered. Following the methods reported in [5],
the neutrino flux from point-like or extended sources compatible with the above evaluated Galactic
fractionoftheICsignalisderivedfordifferentvaluesofΓ.
A signal originating from the Southern sky region can be observed by the ANTARES neutrino
telescope[6], locatedintheMediterraneanSea. ExistingANTARESupperlimitsderivedunderthe
hypothesisofaΓ=2.0neutrinospectrumareusedtoinferupperlimitsforΓ>2.0. TheANTARES
expectedsensitivitiesforextendedsourcesareusedtodiscusstheconditionsunderwhichanIceCube
hotspotcanbeobserved.
2 The ANTARES and IceCube effective areas
Theneutrinoeffectiveareaatagivenenergy,A (E),isdefinedastheratiobetweentheneutrinoevent
eff
rateinadetector(units: s−1)andtheneutrinoflux(units: cm−2 s−1)atthatenergy. Theeffectivearea
dependsontheflavorandcross-sectionofneutrinos,ontheirabsorptionprobabilityduringthepassage
throughtheEarth,andondetector-dependentefficiencies. Detectorefficienciesarecorrelatedtoeach
particular analysis, referring to the criteria used to trigger and to reconstruct the events, and to the
cutsappliedtoreducethebackground. Afractionoftheirreduciblebackgroundduetoatmospheric
neutrinos contaminates in any case the signal, with a percentage depending on the strength of cuts
usedtodefineA (E).
eff
RICAP-14TheRomaInternationalConferenceonAstroparticlePhysics
Figure1. Fullblackline:ANTARESν effectivearea[7].Theneutrinotrackisdeterminedwithamedianangle
µ
of(cid:46)0.4◦. Coloreddashedlines: IceCubeν ,ν andν effectiveareas[1]fromtheanalysisyieldingtheHESE.
e µ τ
ThebackgroundsduetoatmosphericmuonsandneutrinosislargelysuppressedabovefewtensofTeV.
Figure1showstheANTARESeffectivearea(Aνµ ,fullblackline)fortheν flavorasderivedin
ANT µ
theanalysis[7]forthesearchforcosmicneutrinopointsourcesinthedeclinationbandcontainingthe
GalacticCenter. Thered(Aνe),green(Aνµ)andblue(Aντ)linesrefertotheIceCubeanalysisona4π
IC IC IC
sryieldingtheHESE.
Despite the fact that the ANTARES instrumented volume is much smaller than 1 km3, Fig. 1
shows that the ANTARES ν effective area is larger than Aνe, Aνµ and Aντ below ∼ 60 TeV. At the
µ IC IC IC
highestenergieswhereneutrinosweredetected,2PeV,Aνµ isafactoroftwolargerthanAνµ while
IC ANT
thetotalICeffectiveareaforHESE(Aνe +Aνµ +Aντ)isonly7.3timeslargerthanAνµ .
IC IC IC ANT
These values of the A of the two experiments are largely dominated by the different criteria
eff
hiddenintheanalysis. StrongcutsareusedintheICanalysistoselectahigh-puritysampleofdiffuse
high-energycosmicneutrinoswithinteractionvertexinsidetheinstrumentedvolume. Theestimated
angularresolutionis∼1◦forν and∼10◦−15◦forneutrinointeractionsproducingshowers(mainly
µ
from charged current interactions of ν ,ν ). The criteria used in the ANTARES analysis allow a
e τ
largercontaminationoflower-energyatmosphericneutrinos,butenabletoreconstructν eventswith
µ
superiorangularresolution,∼ 0.4◦. TheconsequenceisthatANTAREShasequivalent(orsuperior,
dependingonthesignalspectralindex)capabilitytoextractasignalifthecosmicsourceislocatedin
theSouthernsky,anditispoint-likeorconfinedinaregionseenwithinasmallsolidangle∆Ωbythe
detector.
3 Normalization factors for different cosmic spectral indexes
ThestandarddiffusiveshockaccelerationmodelyieldsaΓ=2.0spectralindexforprimaryCRs,and
consequentlyforsecondaryγ-raysandneutrinos. However,mostγ-raysourcesobservedintheGeV
andTeVrangeshowspectralindexeslargerthan2.0. Thereasonwhyγ-rayspectrafromsupernovae
remnants are observed with spectral indexes Γ (cid:39) 2.2−2.3 remains unclear. A softer spectral index
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units: (GeVcm−2s−1)
Φp,Γ(fromHESE) ANTARES
0
Γ= n =1 n =2 n =3 n =4 n =5 90%C.L.limit
p p p p p
2.0 6.910−9 1.410−8 2.110−8 2.810−8 3.510−8 4.010−8
2.2 9.010−8 1.810−7 2.710−7 3.610−7 - 3.210−7
2.3 3.310−7 6.610−7 9.910−7 - - 8.410−7
2.4 1.210−6 2.310−6 - - - 2.210−6
Table2.Column2to6:normalizationfactorsΦp,Γyieldingn =1,...,5HESEinIceCubevs.Γ.Thelast
0 p
columnshowsthe90%C.L.upperlimitsforaΓ=2.0point-likesourcederivedfromANTARES[9].The
valuesforΓ>2.0werederivedin[5].Thefirstvalueineachrowexcludedbytheselimitsisunderlined.
(Γ(cid:39)2.4−2.5)isconsistentwiththetheoreticalmodelofCRinjectionbydiffusiveshockacceleration
followedbyescapethroughtheGalacticmagneticfieldwithKolmogorovturbulence[8].
Thus, it is important to consider the normalization factors ΦD,Γ for the IC signal for different
0
models of cosmic fluxes EΓΦD,Γ(E) ≡ ΦD,Γ(E) (in units: GeV cm−2 s−1 sr−1. The D stands for
0
diffuse.) Thesamenumber N ofeventsfordifferentΦD,Γ(E)isobtainedusingtheeffectivearea
IC
A (E)≡[Aνe +Aνµ +Aντ]anddetectorlivetimeT:
IC IC IC IC
(cid:90) (cid:90)
NIC =T · ΦD,Γ(E)·AIC(E)·dE·dΩ=4πT ·Φ0D,Γ· E−Γ·AIC(E)·dE =4πT ·Φ0D,Γ·DΓ. (1)
TheintegralDΓ (thedetectorresponse)extendsovertheenergyrangewhere AIC(E)isnotnull,and
iscomputednumerically.
Letusassumethatn eventsoutofN areproducedbyapoint-likesourcewithgenericspectrum:
p IC
EΓΦp,Γ(E) = Φp,Γ (units: GeVcm−2s−1 .The pstandsfor point−like.) Thenormalizationfactor
0
Φp,Γnecessarytoproducen eventsisobtainedbyrequiringthat:
0 p
(cid:90) (cid:90)
np =T · Φp,Γ(E)·AIC(E)·dE =T ·Φ0p,Γ E−Γ·AIC(E)·dE =T ·Φ0p,Γ·DΓ (2)
whereT, AIC(E)and, consequently, thedetectorresponseDΓ arethesameasinEq. (1). Then, the
normalizationfactorforapoint-likesourcefluxofagivenspectralindexΓisgivenby:
(cid:18) n (cid:19)
Φp,Γ =4π· p ·ΦD,Γ. (3)
0 N 0
IC
If a fraction n∆Ω of the IceCube signal is produced in a region of the Southern sky of angular
extension ∆Ω (cid:28) 4π sr, and flux EΓΦD(cid:48),Γ(E) = ΦD(cid:48),Γ, the signal can be observed as an enhanced
0
diffuseflux. SimilarlytoEq. (3),usingthedetectorresponsederivedin(1),weobtain
ΦD(cid:48),Γ =(cid:18)n∆Ω(cid:19)·(cid:18)4π(cid:19)·ΦD,Γ. (4)
0 N ∆Ω 0
IC
4 ANTARES constraints for the IC signal from the Southern sky
Point-like sources. Table 2 reports the normalization factor Φp,Γ for a point-like source necessary
0
to produce n = 1÷5 HESE, as derived from Eq. (3). Four different values of Γ are considered.
p
RICAP-14TheRomaInternationalConferenceonAstroparticlePhysics
units: (GeVcm−2s−1sr−1)
∆Ω ΦD(cid:48),Γ(fromHESE) ANTARES
0
(sr) Γ= n∆Ω =3 n∆Ω =4 n∆Ω =5 n∆Ω =6 sensitivity
0.06 2.0 3.510−7 4.610−7 5.810−7 7.010−7 3.110−7
2.2 4.510−6 6.010−6 7.510−6 9.010−6 3.610−6
2.3 1.710−5 2.210−5 2.810−5 3.310−5 1.110−5
2.4 5.910−5 7.810−5 9.810−5 1.210−4 3.410−5
Table3.Column3to6:NormalizationfactorsΦ0D(cid:48),Γforanenhanceddiffuseflux,obtainedassumingn∆Ω=3to
6HESEinacircularwindowof8◦(∆Ω=0.06sr).Inthelastcolumn,thevalueforΓ=2.0correspondstothe
ANTARESsensitivitiesfromtheFBregions[11].ThesensitivitiesforΓ>2.0areobtainedin[5].
Point-likesourcesintheGalacticcentralregionweresearchedforbyANTARES[9]andupperlimits
as a function of the source declination were derived assuming a spectral index Γ = 2.0. Following
theproceduredefinedin[5],thethe90%C.L.upperlimitforapoint-likesourcehasbeentranslated
toupperlimitsforsofterspectralindexes. TheANTARESresultsforΓ = 2.0intheGalacticCenter
regionandthederivedvaluedforΓ =2.2,2.3and2.4arereportedinthelastcolumnofTable2. The
ANTARES 90% C.L. upper limit excludes a single point-like source with Γ = 2.0 producing more
than5HESE.Thederivedlimitexcludesasinglepoint-likesourceyieldingaclusterofmorethan2
eventsforΓ=2.3,whilethepresenceofaclustermadeoftwoormoreeventsisexcludedforΓ>2.3.
Enhanceddiffuseflux. Table3(columnsfrom3to6)showsthenormalizationfactorsfromEq. (4),
assumingn∆Ω =3÷6HESEwithinasolidangleregion∆Ω=2π(1−cosθ)correspondingtoacircular
windowsofθ=8◦. TheANTARESstrategyforthestudyofanenhanceddiffusefluxisdifferentwith
respecttothatforthesearchforpoint-likesources. Thislatterreliesmainlyonthepointingaccuracy
of the telescope. The expected background due to atmospheric neutrinos within a circular windows
ofθ (cid:46) 1◦ issmallandthisisnotanymoretrueforlargervaluesofθ. Astheenergyspectrumfroma
cosmicsignal(eitherpoint-likeordiffuse)isexpectedtobeharderthanthatofatmosphericneutrinos,
thesignalshouldexceedthebackgroundaboveacertainthresholdofthereconstructedenergy. Thus,
thediscriminationbetweensignalandbackgroundneedstheuseoftheestimatedenergyoftheevent,
similarlytothecaseofthesearchforadiffusefluxofhighenergyν [10].
µ
ANTARES has used an Artificial Neural Network to estimate the energy of the muons entering
thedetectorforstudyingtheFermibubbles(FB)[11]. ThereportedANTARESsensitivityinterms
of an enhanced diffuse flux from the FB region, assuming a E2Φ(E) spectrum without cutoff up to
thePeVenergiesis3.1×10−7GeVcm−2s−1sr−1. Intheanalysis,using806dayslivetime,16events
werefound,withanexpectedbackgroundof11events. ForΓ = 2.0thebackgroundcorrespondsto
7.5events/(sr·y).Thederived90%C.L.upperlimitisE2ΦFB(E)=5.4×10−7GeVcm−2s−1sr−1.As
thesensitivitydependsonthebackgroundrate,differentoptimizationsmustbededucedfordifferent
spectralindexes;ingeneral,itcanbeassumedthatthebackgroundlevelslightlyincreasesforsofter
spectralindexes[5]. ThesensitivitiesextrapolatedfromtheANTARESFBanalysisforΓ > 2.0are
reportedinthelastcolumnofTable3. Accordingtothesevalues,adedicatedsearchforadirectional
neutrino flux, for instance around the IC hot spot, would produce a positive result for any spectral
indexesΓ≥2.0,if∆Ω≤0.06sr(orcircularwindowofθ <8◦)andn∆Ω >2. Forasignalspreadout
onalargercircularwindow,theminimumsensitivitywouldcorrespondtoahighern∆Ω.
Regionsoflargeangularsize(Fermibubbles,Galacticplane). Recentpredictionsoftheneutrino
flux from the FB regions [12] allow to estimate the expected number of events for the IC detector
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assuming the effective area of the HESE. By folding the predicted ν spectra with the ANTARES
effectivearea,thenumberofν inducedeventsinANTARESwouldcorrespondto∼ 30%,50%and
µ
100%oftheν +ν +ν HESEinthesamelivetime,forΓ=2.0,2.1and2.3,respectively. However,
e µ τ
thiscorrespondstoasmallerANTARESsensitivitywithrespecttoIC,duetothelargerbackground
inducedbyatmosphericneutrinosfromthewideFBregion(∆Ω∼0.8sr).
The excess of HESE events from the Galactic region could finally be produced by interaction
during propagation of freshly injected CRs with spectral index Γ (cid:39) 2.4−2.5 [8]. The preliminary
ANTARESupperlimitsfromtheGalacticplanearereportedin[13].
5 Conclusions
The ANTARES detector has sufficient sensitivity to test many models that explain a fraction of the
HESEsampleinIceCubeintermsofaGalacticcomponent. Modelsinwhichmorethan2HESEare
originatedfromapoint-likeandsteadysourceintheSouthernhemisphereareexcludedforspectral
indexesΓ≥2.3.Thepossibilitythataclusteringofeventsisproducedinaregionofsmallangularsize
(∆Ω(cid:39)0.1−0.2sr)in(ornear)theGalacticPlaneisunderinvestigationinANTARES.Asreportedin
Table3,theestimatedANTARESsensitivityisbelowthesignallevel(allowingapositivedetection)
foranyspectralindexesΓ≥2.0,if∆Ω≤0.06sr(i.e. acircularwindowofθ<8◦)andn∆Ω >2. Fora
signalspreadoutonalargersolodangle,theminimumsensitivitywouldcorrespondtoahighern∆Ω.
Forverylargeregions(theFB,theGalacticplane)thepresentsensitivitiesusingtheν channelalone
µ
areabovethemodelpredictions. Theinclusionofshoweringevents,witharelativelylooserangular
precision,wouldsignificantlyincreasetheANTARESsensitivitiesforthestudyofextendedregions.
Acknowledgments
IwouldliketothankmanymembersoftheANTARESandKM3NeTCollaborationsforcomments
andinparticularJ.BrunnerandA.Kouchner.
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