On the Detectability of High-Energy Galactic Neutrino Sources

0.21 MB·English

by  Francesco Vissani

On the Detectability of High-Energy
Galactic Neutrino Sources
FrancescoVissani
LaboratoriNazionalidelGranSasso,Assergi(AQ),Italy
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1 FelixAharonian
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2 DublinInstituteforAdvancedStudies,31FitzwilliamPlace,Dublin2,Ireland

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NarekSahakyan
a
J Universita`diRoma“LaSapienza”andICRANet,Pescara,Italy

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2

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H
Abstract
.
h With the arrival of km3 volume scale neutrino detectors the chances to detect the first astronomical sources of TeV neutrinos will be
p
dramaticallyincreased.Whilethetheoreticalestimatesoftheneutrinofluxescontainlargeuncertainties,wecanformulatetheconditions
-
o forthedetectabilityofcertainneutrinosourcesphenomenologically.Infact,sincemostgalacticneutrinosourcesaretransparentforTeV
r γ-rays, theirdetectabilityimpliesaminimumfluxoftheaccompanyingγ-rays. Foratypicalenergy-dependenceofdetectionareasof
st km3volumeneutrinodetectors,weobtainthequantitativeconditionIγ(20TeV)>2×10−15ph/cm2s,thatthankstothenormalizationof
a theγ-rayspectrumat20TeVappearstobequiterobust,i.e.almostindependentoftheshapeofenergyspectrumofneutrinos.Weremark
[
thatthisconditionissatisfiedbytheyoungsupernovaremnantsRXJ1713.7-3946andRXJ0852.0-4622(VelaJr)-twoofthestrongest

1 galacticγ-raysources. Thepreliminaryconditionforthedetectabilityofhighenergyneutrinosisthatthebulkofγ-rayshasahadronic
v origin:AnewwaytotestthishypothesisforRXJ1713.7-3946isproposed.Finally,weassesstherelevanceofaneutrinodetectorlocated
2 intheNorthernHemisphereforthesearchforgalacticneutrinosources.Inparticular,wearguethatiftheTeVneutrinosourcescorrelate
4 withthegalacticmassdistribution,theprobabilitythatsomeofthemwillbeobservedbyadetectorintheMediterraneanSeaislargerby
8 afactorof1.4-2.9comparedtotheoneofIceCube.
4
. Keywords: highenergyneutrinosources;highenergyγ-rayobservations;galacticastronomy
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PreprintsubmittedtoAstroparticlePhysics January26,2011
1. Introduction Nγ Ec =
100 100.5 101 101.5 102 102.5 103
The search for neutrinos with E > TeV (Lipari 2006
ν 1.8 70 16 4.9 2.1 1.1 0.7 0.5
[18]) with telescopes of volumes in the km3 scale is con-
1.9 86 20 6.7 3.0 1.7 1.1 0.8
sidered important. IceCube is collecting exposures of the
α= 2.0 110 25 9.0 4.2 2.5 1.7 1.3
order of km2×year and this will be continued and extended
2.1 130 32 12 5.9 3.5 2.5 2.0
by KM3NeT.1 As has happened in the past (e.g., for X-ray
2.2 160 41 16 8.0 5.0 3.6 3.0
searches) the new instruments could eventually lead to sur-
prising outcomes. The hope for surprises is certainly one
Table1:Normalizationoftheγ-rayfluxesNγ,inunitsof10−12/(cm2sTeV)
strong motivation of the search for the sources of high en- thatcorrespondstoaninducedfluxIµ+µ¯(>1TeV)=1/(km2yr). Firstcol-
ergyneutrinosthatplausiblyarealsosourcesofcosmicrays. umn: slopeoftheγ-rayflux,α. Firstrow: cutoffenergyoftheγ-rayspec-
At the same time, there are many reasons why we would trum,Ec,measuredinTeV.SeeEq.1.
like to have defined expectations on high energy neutrinos:
tointerprettheresultsoftheobservations,toplanthefuture
2. Using high energy γ-rays as a guide for high energy
research, to better focus our goals, to optimize the new in-
neutrinosearch
struments. The trouble is that the hypotheses on which the
present expectations are based are still rather uncertain and Hereweconsiderapreciseassumptionontheastrophys-
difficulttotest. Thuswedonothavereliablepredictionsyet, ical neutrino sources: We suppose that they are transparent
andthislimitsourcapabilitytoplanthenextsteps. to the very high energy gamma radiation. In this way we
In this paper, we focus on this aspect of the search for can derive upper limits on neutrinos, by postulating that all
highenergyneutrinosourcesthatwearenowbeginning. We γ-rays originate from proton-proton collisions. In fact, the
discussseveralaspects:Weemphasizetherelevanceofγ-ray yield of neutral mesons and of charged mesons are strictly
observationsinthe10-100TeVenergyrangeforhighenergy connectedandthereisalinearrelationbetweenthefluxesof
neutrinos; weanalyzetheprospecttounderstandbettercer- highenergyγ-raysandneutrinos(Vissani2006[29],Villante
tain γ-ray sources that have a special theoretical interest in &Vissani2008[28]).
connectionwithhighenergyneutrinos; weclarifytheargu-
mentinfavorofaneutrinotelescopeintheNorthernhemi- We can then quantify the concept of “promising” γ-ray
sphere. We focus on the subclass of galactic sources that sources. Supposethattheγ-rayfluxhastheform:
areofparticularinterestforfutureinstrumentslocatedinthe
(cid:20) (cid:113) (cid:21)
Northernhemisphere. I (E )= N ×(E /1TeV)−α×exp − E /E , (1)
γ γ γ γ γ c
Theoutlineofthispaperisasfollows. First,considering
thehighenergyγ-raysourcesthataretransparenttothera- whereweconsidertherangesofparameters: α = 1.8−2.2
diation, we characterize those of them that could be, at the (=slope)andE =1TeV−1PeV(=energycutoff).Thisform
c
same time, bright enough neutrino sources (Sec. 2). Next, corresponds to an exponential cutoff in the spectrum of the
after recalling the relevant theoretical context, we discuss
cosmicraysthatgeneratetheγ-rays,seeKappesetal.2007
theprospectsofobtainingmoredefinedexpectationsforone
[17],andhasbeentestedforadequacyontheavailableγ-ray
of the most interesting of these γ-ray sources, namely, the
data. FollowingVillante&Vissani2008[28],andrequiring
youngsupernovaremnantnamedRXJ1713.7-3946(Sec.3).
an induced flux of 1 muon or antimuon per km2 per year
Finally,wequantifyinSec.4theimportanceofmonitoring
above 1 TeV (i.e. 1 signal event in a conventional neutrino
the Southern high energy neutrino sky on the basis of the
telescope with an exposure of 1 km2×yr), we determine N
γ
Galacticmatterdistribution.
foreachvalueofαandE ;theresultsaregiveninTab.1and
c
arefurtherillustratedinFig.1.
1Allconsiderationsbelowapplytoanylarge(i.e.,withvolumesoftheor- This table and this figure identify the transparent γ-ray
derofonekm3)neutrinotelescopeoftheNorthernhemisphere;KM3NeTis sources that could be interesting neutrino sources. There is
takenasanexample,beingthemostadvancedprojectofthistypeatpresent. a wide variety of possibilities, ranging from intense γ-ray
2
Γ(cid:45)raysourceswith1Ν SNRasamajorexampleoftransparentγ-raysource. Shell
perkm2(cid:180)yearabove1TeV typesupernovaremnants(SNR)areanimportantexampleof
astronomical sources of γ-rays that is expected to be trans-
IΓin1(cid:144)(cid:72)cm2sTeV(cid:76) parent to their γ-ray emission. A few young SNR, recently
observedintheTeVrange,2 areknowntoexceedthebound
in Eq. 2, thus being of particular interest. We discuss them
10(cid:45)12
heretoillustratetheissuefurther:
10(cid:45)14 • ThefirstexampleisthesupernovaremnantcalledRX
J1713.7-3946 and measured by HESS, Aharonian et
10(cid:45)16
al.2007[3]. Ithasaγ-rayspectrumthatisreasonably
well described assuming α = 1.79 ± 0.06 and E =
10(cid:45)18 c
3.7±1 TeV in Eq. 1 (Villante & Vissani 2007 [27])
10(cid:45)201 5 10 50 100 500 1000 EΓinTeV 1an0d−14th/at(chmas2asnTeinVte).nsCitoyrraetsp2o0nTdienVglyo,ft(h1e.7m±ax0im.3)um×
Figure1: Fluxesofγ-rayscorrespondingto1eventabove1TeVinaneu- value ofthe neutrinosignal fromthis sourceis larger
trinotelescopewithanexposureof1km2×yr,asgiveninEq.1andTab.1. than1eventperkm2 peryear,andmorepreciselywe
Weselectedthevaluesα=1.8and2.2(thefirstbeingsmalleratlowener- have:
giesandlargerathighenergies)andEc=1,101.5and103TeV(continuous,
dashedanddottedlines,respectively).
Iµ+µ¯(>1TeV)=(2.4±0.3±0.5)/(km2yr) (3)
sourcestoweakones;thisisduetothefactthatthemaincon- seeTab.2andSect.IVofVillante&Vissani2008[28].
tributiontotheneutrinosignalisfromenergieslargerthan1
• AsecondexampleistheSNRcalledVelaJr(RXJ0852.
TeV,wherewestillhavelimitedinformationfromγ-rayob-
0-4622) as observed in Aharonian et al. 2007b [4].
servations.
Theavailableγ-rayobservationsofthisobjectareless
Byasystematicexplorationoftheγ-rayskytillE ∼100
γ complete. Its measured spectrum has been described
TeVandofthesourceswithanintensityabove10−12/(cm2
simplybyapowerlawanditsemissionat20TeVisin
s TeV) at 1 TeV, we could have a guide for the search of
therange(1−3)×10−14 /(cm2sTeV); however, 20
very high energy neutrinos, at least for the sources that are
TeVisthehighestmeasuredenergy.
transparent to their γ-ray radiation. It is interesting to note
thatat20TeV,allfluxesofTab.1areinthenarrowrange NotethatboththeseSNR’sareintheSouthγ-raysky,andare
thuspotentiallyinterestingforneutrinotelescopeslocatedin
I (20TeV)=(2−6)×10−15ph/(cm2sTeV) (2)
γ theNorthernhemisphere(seeSec.4).Inthenextsection,we
recallthereasonswhysuchSNR’sareconsideredinteresting
that characterizes the region of energies and of intensities
anddiscussinmoredetailsthepresentunderstandingofRX
wheretheγ-rayobservationsaremorerelevantforthehigh
J1713.7-3946andtheprospectsofimprovement.
energyneutrinodetectors: seeagainFig.1.
We remark that Eq. 2 is a new result. Its relevance can
Remarks and caveats. We did not include latitude depen-
beunderstoodbetterbyrecallingthattheexistingγ-rayde- dent effects, such as the limited time to observe a source
tectors have explored mostly the region of energy around
(discussedinthelastsection)ortheabsorptionintheEarth,
the TeV. Thus, the future γ-ray measurements in the region inordertosimplifythediscussion. Thelattereffectismore
10-100 TeV–e.g. those by the Cherenkov Telescope Array
severe for the fluxes that extend up to the highest energies,
(CTA)instrument–willhaveanimportantimpactontheex-
pectations of high energy neutrinos. In summary, it will be
possibletoclarifytheexpectationsofneutrinoastronomy,by 2AusefulresourceistheHESSSourceCatalogthatcanbeconsultedat:
themeasurementsoffutureγ-rayobservatories. http://www.mpi-hd.mpg.de/hfm/HESS/pages/home/sources/.
3
namely those with a smaller value of α and/or a higher en- SNR? To address this question, we have to consolidate the
ergy cutoff. We roughly take into account this, by limiting physicalpictureofRXJ1713.7-3946,thatcanbedoneonly
thespectrumtoE <1PeV. employinginthebestwaytheavailable(theoreticalandob-
ν
Letusrepeatthattheγ-raydatapermitustoobtainanup- servational)information.
perboundonneutrinos, postulatingthatthesourceistrans-
parenttoitsγ-rays.Butinsomecases,highenergyneutrinos Amodelforthespectrum. Arelatedquestionthatweshould
andγ-raysdonotcorrelate.Thisisthoughttohappenforcer- tackleiswhichisthenatureoftheelectromagneticspectrum
taininterestingastrophysicalobjectssuchasgalacticbinary ofRXJ1713.7-3946. Therearevariousmodelsinthelitera-
systemscontainingaluminousopticalstarandacompactob- ture, e.g., Malkovetal.2005[19], Berezhko&Vo¨lk2006-
ject(microquasars)wheretheabsorptionofγ-raysisconsid- 2010[7],Morlinoetal.2009[23],Zirakashvili&Aharonian
erable. Caseslikethisincreasetheapriorichanceofhaving 2010 [31], Ellison et al. 2010 [11], Fan et al. 2010 [12]. In
surprisingly large neutrino fluxes. At the same time, such a typical model, the γ-ray emission is dominated by a sin-
a situation causes further difficulties to obtain reliable ex- glemechanismatallenergies,whichreducesthequestionof
pectations,duetotheincreaseddependenceonanuncertain neutrinostoadichotomy.
theoreticalmodelling. Foramorecompletediscussionofal- We focus on one proposal of Zirakashvili & Aharonian
ternativegalacticneutrinosources,seeAharonian2007[2]. 2010 [31], where the spectrum is instead composite: it has
significantcontributionsbothfromtheInverseCompton(IC,
i.e. leptonic mechanism) and from neutral pion decays (π0,
3. TowardreliablepredictionsfortheSNRRXJ1713.7-
i.e.hadronicmechanism). Eveniftheirmodelwillturnout
3946
tobeincorrectinsomequantitativeaspect,suchahypothesis
allows us to make one step ahead in the right direction: to
In this section we analyze why supernova remnants are
understandneutrinos,weneedtoknowwhichpartoftheγ-
expectedtoactasemittersofhighenergyneutrinosanddis-
rayemissionishadronic.
cussindetailsthepromisingcaseofthesupernovaremnant
Thisproposalhasgoodphysicalmotivations:1)Thesim-
calledRXJ1713.7-3946.
ilarityofthefeaturesobservedinX-raysisexplained,since
Thekineticenergyofsupernovaremnants(SNR)isone theICdominatestheintegratedspectrummeasuredbyHESS.
order of magnitude larger than the cosmic ray losses of the 2)Attributingthehighenergytailofthespectrumtoπ0 de-
Galaxy(Ginzburg&Syrovatskii1964[15])anddiffusiveac- caysinsteadovercomesthedifficultiesinaccountingforitby
celerationontheshockwave(Fermi1949[13])canprovide IC,whosespectrumshouldbecut-offabruptly. Thequestion
themechanismforcosmic-rayacceleration(seee.g.,Malkov wewanttoaddressbecomes:Howdowetestthepredictions
&O’Drury2001[20]): Thus,weexpectthatSNR’scontain ofthismodel?
highdensitiesofcosmicrays. TheSNRcanalsobesources
Prospectsofobservationaltests. ThemeasurementsofAg-
of high energy γ-rays and neutrinos, especially when they
ileandFermiintheenergyrange1-10GeVwillbeakeytest
areassociatedwithmolecularclouds,thatactasatargetfor
(seee.g.,Morlino,Blasi,Amato2009[23]),fortheshapeof
the cosmic ray collisions (Aharonian, O’Drury, Vo¨lk 1994
the γ-ray spectrum at GeVenergies depends on the mecha-
[5],O’Drury,Aharonian,Vo¨lk1994[24]).
nismofemission: Assumingthatprotonsandelectronshave
ThespectrumoftheyoungSNRRXJ1713.7-3946,mea-
power-law spectra with the same slope, ∝ E−α, π0 decays
suredbyHESS(Aharonianetal.2007[3])till100TeVand p,e
(as already discussed) exceeding the bound of Eq. 2, has a give ∝ Eγ−(α−0.1) whereas IC gives ∝ Eγ−(α+1)/2; thus, extrap-
special interest in the discussion of high energy neutrinos. olatingtheγ-rayspectrafromthelowestpointmeasuredby
This is even more true when one realizes that this SNR in- HESS,Eγ =300GeV,wefindthatthehadronicmechanism
teractswithasystemofmolecularcloudsdetectedbyNAN- leads to an emission 3 times more intense than the one due
TEN(Fukuietal.2003[14],Sanoetal.2010[25]). Itisin to the leptonic mechanism already at 10 GeV. But it is un-
theSouthernneutrinosky,relativelyclosetous,D∼1kpc. clear whether Agile or Fermi will attain sufficient angular
These are the reasons why it is urgent to ask: How far resolutiontorevealthattheγ-rayscomepreferentiallyfrom
wearefromunderstandingthehighenergyneutrinosofthis
4
Energy Dominant Observational Relevant mentofphotonstatistics,foranyreasonableobservationtime,
ofγ-rays emission test data onlybyafactoroftwoorso. Arealbreakthroughinthisre-
∼1−10GeV π0 intensity&shape Fermi,Agile gardisexpectedonlywiththenextgenerationγ-rayinstru-
∼1−10TeV IC SNRshell HESS
mentslikeCTA.
>10TeV π0 molecularclouds HESS
Implicationsforhighenergyneutrinoastronomy. Ifthemodel
Table2:TestsoftheZirakashvili&Aharonianmodelfortheγ-rayspectrum
oftheSNRnamedRXJ1713.7-3946. Firstcolumn,theenergyrangeofthe ofZirakashvili&Aharonianwillbevalidatedbyfuturedata
measurement;second,thedominantmechanismofemissionexpectedinthe analyses,theinducedmuonfluxfromRXJ1713.7-3946will
model;third,thepossibletest;lastcolumn,therelevantexperiment.Seethe be lower than the upper limit that we derive in the extreme
textfordetails.
hypothesisofhadronicemissionfromEq.3,namely: Iµ+µ¯(>
1TeV) < 3.5 / (km2yr) at 90 % CL. This will make the
thesiteswherecosmicraycollisionsandπ0decaysaremore searchforasignalmoredifficultbutwillbeaccompaniedby
frequent,i.e.themolecularclouds. adecreaseofthebackground,forthesourcesofhighenergy
This qualifying hypothesis could be verifiable at much neutrinos are the relatively small molecular clouds and not
largerenergies. Infact,themodelofZirakashvili&Aharo- themuchlargerSNR.
nian2010[31]predicts,atseveraltensofTeV,aγ-raysignal Bycomparingthesizeoftheoverdensecloudswiththe
enhancedinthedirectionoftheoverdensemolecularclouds oneoftheSNR,onewouldexpectinidealconditionsade-
ofNANTEN,ofsize(2−8)µsr(Sanoetal.2010[25]), on crease of the background by a factor of ten; however, the
top of the known background distribution due to misiden- decrease will be limited by instrumental features, if the an-
tified cosmic rays and of a minor component of the signal gularresolutionoftheneutrinotelescopeswillbelargerthan
distributedastheSNRshell. thecloudsize. Justforillustration, anangularresolutionof
Dowehaveenoughdatatotestthispicture?HESS(Aha- δθ = 0.2◦ at the relevant energies corresponds to a search
ronianetal.2007[3])has1021(resp.,474)eventsONagainst windowofπδθ2 =40µsr. Multiplyingbythenumberofthe
751(resp.,338)OFFabove20(resp.,30)TeV,namelyabout main overdense clouds, i.e. four (see Sano et al. 2010 [25])
250 (resp., 130) signal events3. To illustrate the meaning andcomparingtothesizeoftheSNRimpliesadecreaseof
of these numbers, suppose that 750 background events are thebackgroundbyafactoroftwo.
uniformly distributed in 25 patches of equal area; thus, 30 WewouldliketoemphasizethatthemodelofZirakashvili
signaleventsinasinglepatchareenoughtodoubletheaver- &Aharonian2010[31]doesnotnecessarilyimplyastrong
age density of events. The low statistics conditions suggest reductionoftheneutrinodetectionratecomparedtothepure
an unbinned likelihood analysis of the γ-ray data, as those hadronicmodel,becauseinthecompositespectrumthemost
proposed for similar applications in neutrino astronomy by relevant γ-rays–those with energy above 10 TeV–are con-
Braunetal.2008[8]andbyIannietal.2009[16]. tributed mainly by cosmic ray interactions. On the other
Are´sume´ ofpossibletestsisprovidedinTab2. hand, the composite model implies that the γ-rays and of
The above estimates show that the existing HESS data coursethehighenergyneutrinosareproducedinmorecom-
can only marginally provide a decisive study of energy de- pactregions,whichleadstoasignificantreductionofback-
pendent γ-ray morphology of RX J1713.7-3946. Thus, it groundevents.
is highly desirable to increase significantly the TeV photon Wewillbeinabetterpositiontoquantifytheexpected,
statisticsbynewobservationsofthesource. Presentlysuch neutrino-induced muon flux when we will know the results
observations can be performed only by the HESS array of of the analyses of Agile and Fermi. In order to predict the
telescopes. However, because of the limited potential of very high-energy neutrino flux, it would be even more im-
HESS at energies above 10 TeV, we can hope for enhance- portanttoknowtheamountofveryhigh-energyγ-rayscor-
related with the molecular clouds. HESS could provide us
withsomeevidenceforsuchahadronicemission,butfuture
3TheterminologyON/OFFreferstothetwocaseswhenthegammaray highstatisticsobservationswillbecrucialtoobtainreliable
telescopepointstothesourceandwheninsteaditpointstoaregionwhere measurements(orstronglimits)ofthiscomponentoftheγ-
nosignalisexpected.
rayemission.
5
the galactic center is at δ ∼ −π/6, thus the region δ < 0 is
morepopulated;finally,thefeaturesarelessprominentwhen
weinclude1/r2 sincethisemphasizesthelocalpatchofthe
Galaxyratherthanthedistantregions.
Let us consider the traditional signal of induced muons
(see Markov 1963 [21] for the original references). High
energy neutrino detectors observe only downward to safely
avoid atmospheric muons; thus, a source at declination δ is
seenonlyforafractionoftime:
Re[arccos(−tanδtanφ)]
f[δ,φ]=1− (4)
π
as a function of the latitude φ of the detector, as discussed
e.g., in Costantini & Vissani 2005 [9]. For instance, the
Figure2: Continuousline: normalizedmassdistributionoftheGalaxy,as galactic center, that is in the Southern sky at δ = −29◦, is
afunctionofthedeclination. Dashedline,thesamebutweightedwiththe
invisible in IceCube (f = 0) and it is seen for a fraction of
inversesquareddistancefromthemass.
time f =67%,63%,64%or75%inAntares,NEMO,Nestor
orBaikalrespectively.
4. AnappraisalofatelescopeintheNorthernHemisphere By convoluting f with the distribution of the matter in
the Milky Way we estimate the relevance of a high energy
Inthelastsection, wediscusstheimportanceofoperat- neutrino detector. The result is shown in the curves Fig. 3.
inganewtelescopeforhighenergyneutrinosintheNorth- They are symmetric around 1/2 when φ → −φ, just as f:
ern Hemisphere. The discussion elaborates quantitatively f[δ,φ] + f[δ,−φ] = 1, for two antipodal detectors see the
the oft-heard observation: most of the Galaxy, being in the entiresky. Fromthisfigure,weverifythattheSouthPoleis
SouthernHemisphere,liesintheNorthernneutrinosky. a less promising place to search for neutrinos from galactic
sources. A detector in the Mediterranean, say with latitude
Webeginbyconsideringaneducatedguessonthegalac- φ = 36◦30(cid:48), has 2.9 times better chances; when we weight
tic sources of neutrinos. It is plausible that the distribution themassdistributionwith1/r2,theimprovementisafactor
of neutrino sources follows the mass distribution of super- of 1.4 instead. The first number applies if the hypothetical
novae,youngmatterand/orstar-formingregions. Weusethe sourcesaresointense,thatallofthemcanbeseen;thesec-
distributionofneutronstarsofYusifov&Ku¨c¸u¨k2004[30], ond one is plausibly a better estimation of the factor of im-
adoptedforthestudyofsupernovaneutrinosofMirizzietal. provementifthereisasortof“standardsource”withafixed
2006 [22] (see their Eqs. (1), (2) and (4) and compare with intensity, andtheneutrinodetectorsareabletoseeonlythe
Costantini,Ianni,Vissani2005[9]). Wesetthe x-axisfrom closestones.
thegalacticcentertotheEarth,thez-axisinthedirectionof Similar arguments are frequently invoked in favor of a
the galactic North, obtaining for components of the Earth’s detectorintheNorthernhemisphere; however,thequantita-
rotationaxistheunitvector(ux,uy,uz)=(.484,.747,.456). tiveevaluationofthefactorofimprovementthatweobtained
Now we derive the normalized mass distribution as a is,tothebestofourknowledge,anewresult.
function of the declination of the sources δ, that we regard
astheprobabilityoffindingneutrinosources. Similarly,we Remarksandcaveats. Thereareotheraspectstobekeptin
derivethemassdistributionweightedwiththeinverseofthe mind; e.g., it seems possible to cover safely a few degrees
squared distance of the source, accounting for the fact that abovethehorizonalreadywithIceCube(Abbasietal.2009
the number of events scales as 1/r2 for a standard source. [1]). Also,whenKM3NeTwilloperate,aportionofthesky
The results are given in Fig. 2 and are easy to understand: willbealreadyexploredbyIceCube;however,someredun-
TheanglebetweenthegalacticplaneandtheCelestialequa- dancy in the observations could be precious to cross check
toris∼π/3,thusmostofthematterisat|δ|<1;furthermore, theproperfunctioningofthedetectors.
6
A new high-energy neutrino telescope in the Northern
Hemisphere is considered highly desirable. We discussed
in Sec. 2 which γ-ray sources may yield a minimum sig-
nal in neutrino telescopes, assuming that the γ-rays are not
absorbed. We found that, for high energy neutrino search,
itwouldbeparticularlyimportanttoknowtheγ-raysources
withasufficientlyintenseemissionaround20TeV:seeEq.2.
WearguedinSec.3thattherearereasonablechancesofget-
tingareliablepredictionforRXJ1713.7-3946,thatcouldbe-
comeareferencetargetforatelescopelocatedintheNorth-
ern hemisphere. The chances are linked to future analyses
of existing γ-ray data: those by Agile and Fermi, which
shouldrevealanemissionmoreintensethantheoneexpected
assuming the leptonic mechanism; those by HESS above
20 − 30 TeV, that should reveal the correlation of the γ-
Figure 3: Fraction of the Galaxy seen by a neutrino detector at a given ray events with the molecular clouds that interact with RX
latitude.Forthedashedlines,themassdistributionwasweightedwith1/r2 J1713.7-3946. Finally,wediscussedtheimportancetohave
totakeintoaccountthatnearbysourcesareeasiertodetect.
anewhigh-energyneutrinotelescopeintheNorthernHemi-
sphere. We developed in Sec. 4 the oft-heard argument in
favorofsuchaninstrument,concludingthatitwillbesupe-
One can repeat the calculations including the halo, or a
riorbyafactorof1.4-2.9toIceCubeasamonitorofgalactic
“bar” as part of the Milky Way, possibilities that above we
neutrinosourcesdistributedasthematteroftheMilkyWay.
disregarded. Alternatively,onecouldconsidersuitablegen-
eralizations,forinstancethecaseofdarkmatterdecayoran-
Acknowledgement
nihilation;thelatterrequirestoweightthefraction f withthe
WearegratefultoR.RuffiniforhospitalityinICRANet, Pescara, where
squareofthedensityofthedarkmatterdistribution.Manyof
thisworkwasbegun. WethankP.L.Ghia,P.Lipari,A.Masiero,N.Paver,
thesecasesresemblecloselyasourceinthegalacticcenter,
G.RiccobeneandO.Ryashzkayaforusefuldiscussions,hintsandhelp.F.V.
discussedabove.
thanksM.L.CostantiniandF.L.Villanteforcollaborationonrelatedtopics.
5. Summaryanddiscussion
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