Sensitivity of the KM3NeT/ARCA neutrino telescope to point-like neutrino sources

University of Groningen

Sensitivity of the KM3NeT/ARCA neutrino telescope to point-like neutrino sources

KM3NeT Collaboration; van den Berg, A.

Published in:

Astroparticle Physics

DOI:

10.1016/j.astropartphys.2019.04.002

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KM3NeT Collaboration, & van den Berg, A. (2019). Sensitivity of the KM3NeT/ARCA neutrino telescope to

point-like neutrino sources. Astroparticle Physics, 111, 100-110.

https://doi.org/10.1016/j.astropartphys.2019.04.002

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ContentslistsavailableatScienceDirect

Astroparticle

Physics

journalhomepage:www.elsevier.com/locate/astropartphys

Sensitivity

of

the

KM3NeT/ARCA

neutrino

telescope

to

point-like

neutrino

sources

S.

Aiello

a

_,

_S.E.

_Akrame

b

_,

_F.

_Ameli

c

_,

_E.

_G.

_Anassontzis

d

_,

_M.

_Andre

e

_,

_G.

_Androulakis

f

_,

M.

Anghinolfi

g

_,

_G.

_Anton

h

_,

_M.

_Ardid

i

_,

_J.

_Aublin

j

_,

_T.

_Avgitas

j

_,

_C.

_Bagatelas

f

_,

_G.

_Barbarino

k,l

_,

B.

Baret

j

_,

_J.

_{Barrios-Martí}

m

_,

_A.

_Belias

f

_,

_E.

_Berbee

n

_,

_A.

_van

_den

_Berg

o

_,

_V.

_Bertin

p

_,

_S.

_Biagi

q

_,

A.

Biagioni

c

_,

_C.

_Biernoth

h

_,

_J.

_Boumaaza

r

_,

_S.

_Bourret

j

_,

_M.

_Bouta

s

_,

_M.

_Bouwhuis

n

_,

_C.

_Bozza

t

_,

H.

Brânza

¸s

u

_,

_M.

_Bruchner

h

_,

_R.

_Bruijn

n,v

_,

_J.

_Brunner

p

_,

_E.

_Buis

w

_,

_R.

_Buompane

k,x

_,

_J.

_Busto

p

_,

D.

Calvo

m

_,

_A.

_Capone

y,c

_,

_S.

_Celli

y,c,V

_,

_M.

_Chabab

b

_,

_N.

_Chau

j

_,

_S.

_Cherubini

q,z

_,

_V.

_Chiarella

A

_,

T.

Chiarusi

B

_,

_M.

_Circella

C

_,

_R.

_Cocimano

q

_,

_J.A.B.

_Coelho

j

_,

_A.

_Coleiro

m

_,

_M.

_Colomer

_Molla

j,m

_,

R.

Coniglione

q

_,

_P.

_Coyle

p

_,

_A.

_Creusot

j

_,

_G.

_Cuttone

q

_,

_A.

_D’Onofrio

k,x

_,

_R.

_Dallier

D

_,

_C.

_De

_Sio

t

_,

I.

Di

Palma

y,c

_,

_A.F.

_Díaz

E

_,

_D.

_{Diego-Tortosa}

i

_,

_C.

_Distefano

q

_,

_A.

_Domi

g,p,F

_,

_R.

_Donà

B,G

_,

C.

Donzaud

j

_,

_D.

_Dornic

p

_,

_M.

_Dörr

H

_,

_M.

_Durocher

q,V

_,

_T.

_Eberl

h

_,

_D.

_van

_Eijk

n

_,

_I.

_El

_Bojaddaini

s

_,

H.

Eljarrari

r

_,

_D.

_Elsaesser

H

_,

_A.

_Enzenhöfer

h,p

_,

_P.

_Fermani

y,c

_,

_G.

_Ferrara

q,z

_,

_M.

_D.

_Filipovi

_´c

I

_,

L.A.

Fusco

j

_,

_T.

_Gal

h

_,

_A.

_Garcia

n

_,

_F.

_Garufi

k,l

_,

_L.

_Gialanella

k,x

_,

_E.

_Giorgio

q

_,

_A.

_Giuliante

J

_,

S.R.

Gozzini

m

_,

_R.

_Gracia

K

_,

_K.

_Graf

h

_,

_D.

_Grasso

L

_,

_T.

_Grégoire

j

_,

_G.

_Grella

t

_,

_S.

_Hallmann

h

_,

H.

Hamdaoui

r

_,

_H.

_van

_Haren

M

_,

_T.

_Heid

h

_,

_A.

_Heijboer

n

_,

_A.

_Hekalo

H

_,

_J.J.

_{Hernández-Rey}

m

_,

J.

Hofestädt

h

_,

_G.

_Illuminati

m

_,

_C.W.

_James

h

_,

_M.

_Jongen

n

_,

_M.

_de

_Jong

n

_,

_P.

_de

_Jong

n,v

_,

M.

Kadler

H

_,

_P.

_Kalaczy

_´nski

N

_,

_O.

_Kalekin

h

_,

_U.F.

_Katz

h

_,

_N.R.

_Khan

_Chowdhury

m

_,

_D.

_Kießling

h

_,

E.N.

Koffeman

n,v

_,

_P.

_Kooijman

v,W

_,

_A.

_Kouchner

j

_,

_M.

_Kreter

H

_,

_V.

_Kulikovskiy

g

_,

M.

Kunhikannan

Kannichankandy

N

_,

_R.

_Lahmann

h

_,

_G.

_Larosa

q

_,

_R.

_Le

_Breton

j

_,

_F.

_Leone

q,z

_,

E.

Leonora

a

_,

_G.

_Levi

B,G

_,

_M.

_Lincetto

p

_,

_A.

_Lonardo

c

_,

_F.

_Longhitano

a

_,

_D.

_Lopez

_Coto

O

_,

M.

Lotze

m

_,

_L.

_Maderer

h

_,

_G.

_Maggi

p

_,

_J.

_Ma

_´nczak

N

_,

_K.

_Mannheim

H

_,

_A.

_Margiotta

B,G

_,

A.

Marinelli

P,L

_,

_C.

_Markou

f

_,

_L.

_Martin

D

_,

_J.A.

_{Martínez-Mora}

i

_,

_A.

_Martini

A

_,

_F.

_Marzaioli

k,x

_,

R.

Mele

k,l

_,

_K.W.

_Melis

n

_,

_P.

_Migliozzi

k

_,

_E.

_Migneco

q

_,

_P.

_Mijakowski

N

_,

_L.S.

_Miranda

Q

_,

C.M.

Mollo

k

_,

_M.

_Morganti

L,X

_,

_M.

_Moser

h

_,

_A.

_Moussa

s

_,

_R.

_Muller

n

_,

_M.

_Musumeci

q

_,

_L.

_Nauta

n

_,

S.

Navas

O

_,

_C.A.

_Nicolau

c

_,

_C.

_Nielsen

j

_,

_B.

_{Ó Fearraigh}

n

_,

_M.

_Organokov

K

_,

_A.

_Orlando

q

_,

S.

Ottonello

g

_,

_V.

_Panagopoulos

f

_,

_G.

_Papalashvili

R

_,

_R.

_Papaleo

q

_,

_G.E.

_P

_˘av

_˘ala

_¸s

u

_,

C.

Pellegrino

G,Y

_,

_M.

_{Perrin-Terrin}

p

_,

_P.

_Piattelli

q

_,

_K.

_Pikounis

f

_,

_O.

_Pisanti

k,l

_,

_C.

_Poiré

i

_,

G.

Polydefki

f

_,

_V.

_Popa

u

_,

_M.

_Post

v

_,

_T.

_Pradier

K

_,

_G.

_Pühlhofer

S

_,

_S.

_Pulvirenti

q

_,

_L.

_Quinn

p

_,

F.

Raffaelli

L

_,

_N.

_Randazzo

a

_,

_S.

_Razzaque

Q

_,

_D.

_Real

m

_,

_L.

_Resvanis

d

_,

_J.

_Reubelt

h

_,

_G.

_Riccobene

q

_,

M.

Richer

K

_,

_L.

_Rigalleau

D

_,

_A.

_Rovelli

q

_,

_M.

_Saffer

h

_,

_I.

_Salvadori

p

_,

_D.F.E.

_Samtleben

n,T

_,

A.

Sánchez

Losa

C

_,

_M.

_Sanguineti

g

_,

_A.

_Santangelo

S

_,

_D.

_Santonocito

q

_,

_P.

_Sapienza

q

_,

J.

Schumann

h

_,

_V.

_Sciacca

q

_,

_J.

_Seneca

n

_,

_I.

_Sgura

C

_,

_R.

_Shanidze

R

_,

_A.

_Sharma

P

_,

_F.

_Simeone

c

_,

A.

Sinopoulou

f

_,

_B.

_Spisso

t,k

_,

_M.

_Spurio

B,G

_,

_D.

_Stavropoulos

f

_,

_J.

_Steijger

n

_,

_S.M.

_Stellacci

t,k

_,

B.

Strandberg

n

_,

_D.

_Stransky

h

_,

_T.

_Stüven

h

_,

_M.

_Taiuti

g,F

_,

_F.

_Tatone

a

_,

_Y.

_Tayalati

r

_,

_E.

_Tenllado

O

_,

T.

Thakore

m

_,

_A.

_Trovato

q,∗

_,

_E.

_Tzamariudaki

f

_,

_D.

_Tzanetatos

f

_,

_V.

_Van

_Elewyck

j

_,

_F.

_Versari

B,G

_,

S.

Viola

q

_,

_D.

_Vivolo

k,l

_,

_J.

_Wilms

U

_,

_E.

_de

_Wolf

n,v

_,

_D.

_Zaborov

p

_,

_J.D.

_Zornoza

m

_,

_J.

_Zúñiga

m

_,

_(The

KM3NeT

collaboration)

∗ _{Corresponding author.}

E-mail addresses: sapienza@lns.infn.it (P. Sapienza), trovato@apc.in2p3.fr (A. Trovato). https://doi.org/10.1016/j.astropartphys.2019.04.002

a INFN, Sezione di Catania, Via Santa Sofia 64, Catania, 95123, Italy

b Cadi Ayyad University, Physics Department, Faculty of Science Semlalia, Av. My Abdellah, P.O.B. 2390, Marrakech, 40 0 0 0, Morocco c INFN, Sezione di Roma, Piazzale Aldo Moro 2, Roma, 00185, Italy

d Physics Department, N. and K. University of Athens, Athens, Greece

e Universitat Politécnica de Catalunya, Laboratori d’Aplicacions Bioacústiques, Centre Tecnológic de Vilanova i la Geltrú, Avda. Rambla Exposició, s/n,

Vilanova i la Geltrú, 08800, Spain

f NCSR Demokritos, Institute of Nuclear and Particle Physics, Ag. Paraskevi Attikis, Athens, 15310, Greece g INFN, Sezione di Genova, Via Dodecaneso 33, Genova, 16146, Italy

h Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Straße 1, Erlangen 91058, Germany i Universitat Politécnica de Valéncia, Instituto de Investigación para la Gestión Integrada de las Zonas Costeras, C/ Paranimf, 1, Gandia, 46730, Spain j APC, Université Paris Diderot, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris, Sorbonne Paris Cité, Paris 75205, France

k INFN, Sezione di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia ed. G, Napoli, 80126, Italy

l Universitá di Napoli “Federico II”, Dip. Scienze Fisiche “E. Pancini”, Complesso Universitario di Monte S. Angelo, Via Cintia ed. G, Napoli, 80126, Italy m IFIC - Instituto de Física Corpuscular (CSIC - Universitat de Valéncia), c/Catedrático José Beltrán, 2, Paterna 46980, Valencia, Spain

n Nikhef, National Institute for Subatomic Physics, PO Box 41882, Amsterdam, 1009 DB, the Netherlands o KVI-CART University of Groningen, Groningen, the Netherlands

p Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France

q INFN, Laboratori Nazionali del Sud, Via S. Sofia 62, Catania, 95123, Italy

r University Mohammed V in Rabat, Faculty of Sciences, 4 av. Ibn Battouta, B.P. 1014, Rabat R.P. 10 0 0 0, Morocco s University Mohammed I, Faculty of Sciences, BV Mohammed VI, B.P. 717, Oujda R.P. 60 0 0 0, Morocco

t Universitá di Salerno e INFN Gruppo Collegato di Salerno, Dipartimento di Fisica, Via Giovanni Paolo II 132, Fisciano, 84084, Italy u ISS, Atomistilor 409, M ˘agurele, RO-077125, Romania

v University of Amsterdam, Institute of Physics/IHEF, PO Box 94216, Amsterdam, 1090 GE, the Netherlands w TNO, Technical Sciences, PO Box 155, Delft, 2600 AD, the Netherlands

x Universitá degli Studi della Campania “Luigi Vanvitelli”, Dipartimento di Matematica e Fisica, viale Lincoln 5, Caserta, 81100, Italy y Universitá La Sapienza, Dipartimento di Fisica, Piazzale Aldo Moro 2, Roma, 00185, Italy

z Universitá di Catania, Dipartimento di Fisica e Astronomia, Via Santa Sofia 64, Catania, 95123, Italy A INFN, LNF, Via Enrico Fermi, 40, Frascati, 0 0 044, Italy

B INFN, Sezione di Bologna, v.le C. Berti-Pichat, 6/2, Bologna, 40127, Italy C INFN, Sezione di Bari, Via Amendola 173, Bari, 70126, Italy

D Subatech, IMT Atlantique, IN2P3-CNRS, 4 rue Alfred Kastler - La Chantrerie, Nantes, BP 20722, 44307, France E University of Granada, Dept. of Computer Architecture and Technology/CITIC, Granada 18071, Spain F Universitá di Genova, Via Dodecaneso 33, Genova, 16146, Italy

G Universitá di Bologna, Dipartimento di Fisica e Astronomia, v.le C. Berti-Pichat, 6/2, Bologna, 40127, Italy H University Würzburg, Emil-Fischer-Straße 31, Würzburg, 97074, Germany

I Western Sydney University, School of Computing, Engineering and Mathematics, Locked Bag 1797, Penrith, NSW 2751, Australia J Universitá di Pisa, DIMNP, Via Diotisalvi 2, Pisa, 56122, Italy

K Université de Strasbourg, CNRS, IPHC, 23 rue du Loess, Strasbourg, 67037, France L INFN, Sezione di Pisa, Largo Bruno Pontecorvo 3, Pisa, 56127, Italy

M NIOZ (Royal Netherlands Institute for Sea Research) and Utrecht University, PO Box 59, Den Burg, Texel, 1790 AB, the Netherlands N National Centre for Nuclear Research, Warsaw, 00-681, Poland

O University of Granada, Dpto. de Física Teórica y del Cosmos & C.A.F.P.E., Granada 18071, Spain P Universitá di Pisa, Dipartimento di Fisica, Largo Bruno Pontecorvo 3, Pisa, 56127, Italy Q University of Johannesburg, Department Physics, PO Box 524, Auckland Park, 2006, South Africa R Tbilisi State University, Department of Physics, 3, Chavchavadze Ave., Tbilisi, 0179, Georgia

S Eberhard Karls Universität Tübingen, Institut für Astronomie und Astrophysik, Sand 1, Tübingen, 72076, Germany T Leiden University, Leiden Institute of Physics, PO Box 9504, Leiden, 2300 RA, the Netherlands

U Friedrich-Alexander-Universität Erlangen-Nürnberg, Remeis Sternwarte, Sternwartstraße 7, Bamberg 96049, Germany V Gran Sasso Science Institute, GSSI, Viale Francesco Crispi 7, L’Aquila, 67100, Italy

W Utrecht University, Department of Physics and Astronomy, PO Box 80 0 0 0, Utrecht, 3508 TA, the Netherlands X Accademia Navale di Livorno, Viale Italia 72, Livorno, 57100, Italy

Y INFN, CNAF, v.le C. Berti-Pichat, 6/2, Bologna, 40127, Italy

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Article history:

Received 19 October 2018 Revised 2 April 2019 Accepted 4 April 2019 Available online 5 April 2019

Keywords:

Astrophysical neutrino sources

Cherenkov underwater neutrino telescope KM3NeT

a

b

s

t

r

a

c

t

KM3NeTwillbeanetworkofdeep-seaneutrinotelescopesintheMediterraneanSea.TheKM3NeT/ARCA detector,tobeinstalledattheCapoPasserosite(Italy),isoptimisedforthedetectionofhigh-energy neu-trinosofcosmicorigin.ThankstoitsgeographicallocationontheNorthernhemisphere,KM3NeT/ARCA canobserveupgoingneutrinosfrommostoftheGalacticPlane,includingtheGalacticCentre.Givenits effectiveareaandexcellentpointingresolution,KM3NeT/ARCAwillmeasureorsignificantlyconstrainthe neutrinofluxfrompotentialastrophysicalneutrinosources.Atthesametime,itwilltestfluxpredictions basedongamma-raymeasurementsandtheassumptionthatthegamma-rayfluxisofhadronicorigin. Assumingthisscenario,discoverypotentialsandsensitivitiesforaselectedlistofGalacticsourcesandto genericpointsourceswithanE−2spectrumarepresented.Thesespectraareassumedtobetime inde-pendent.Theresultsindicatethatanobservationwith3σ significanceispossibleinaboutsixyearsof operationforthemostintensesources,suchasSupernovaeRemnantsRXJ1713.7-3946andVelaJr.Ifno signalwillbefoundduringthistime,thefractionofthegamma-rayfluxcomingfromhadronicprocesses canbeconstrainedtobebelow50%forthesetwoobjects.

© 2019TheAuthors.PublishedbyElsevierB.V. ThisisanopenaccessarticleundertheCCBY-NC-NDlicense. (http://creativecommons.org/licenses/by-nc-nd/4.0/)

1. Introduction

Neutrinos arean optimal probeto observe highenergy astro-physical phenomena, since they interact only weakly with mat-terandarenotdeflectedbymagneticfields.Therefore,theypoint backtotheirorigin,canbridgelargedistanceswithoutabsorption, andmayprovideinformationonprocessesindensesources,which canbe opaque to the electromagnetic radiation.Theyare unique messengers from the most violent and highestenergy processes inourGalaxyandfar beyond. The discovery by theIceCube Col-laborationofahigh-energyneutrinofluxofextra-terrestrialorigin [1–3] hasthus opened a newobservational window on our Uni-verseandinitiateda neweraofneutrinoastronomy.KM3NeT1 _is

a large research infrastructure that will consist of a network of deep-seaneutrino telescopes in the Mediterranean Sea. KM3NeT will include two detectors with the same technology but differ-ent granularity, KM3NeT/ARCA and KM3NeT/ORCA (Astroparticle andOscillationResearchwithCosmics inthe Abyss, respectively) [4].WhileKM3NeT/ORCA,installedattheKM3NeT-Francesite off-shoreToulon (France),willstudyoscillations ofatmospheric neu-trinoswiththeprimary objectivetodeterminetheneutrinomass ordering, KM3NeT/ARCA will be dedicated to high-energy neu-trino astronomy, including the investigation of the cosmic neu-trinofluxdiscoveredby IceCube.KM3NeT/ARCAisbeinginstalled attheKM3NeT-ItalysiteoffshoreCapoPassero(Italy)andwillhave cubic-kilometerscalesize,suitedtomeasureneutrinosintheTeV– PeVenergy range.KM3NeT/ARCAwill have a wider and comple-mentaryfield ofview withrespecttoIceCube.Oneofitsprimary targetsisthedetectionofGalacticsourcesvisiblealsoatrelatively lowenergyaroundtensofTeVforwhichtheIceCubesensitivityto muonneutrinosislow.

InKM3NeT,neutrinosaredetectedbymeasuringtheCherenkov lightinducedbychargedsecondaryparticlesemergingfroma neu-trino interaction in the sea water, which serves as target mate-rialandCherenkovradiatoraswell asashieldfordowngoing at-mosphericmuons.Thelight isdetectedby photo-multipliertubes (PMTs)arrangedinglassspheresthatwithstandthewaterpressure (digitalopticalmodules, DOMs [5,6]).Eachopticalmodulehouses 313-inchPMTsoptimisingthephoto-cathodearea,thedirectional sensitivity,theangularcoverageper DOM,andthephoton count-ing capability. The DOMs of the KM3NeT/ARCA detector are ar-rangedalong flexible strings witha total height ofabout700 m. KM3NeT/ARCAwill consist of two building blocks of 115 strings each, with 18 DOMsper string, vertically spaced by 36 m. Each block will have a roughly circular footprint withan average dis-tancebetweenstringsofabout90m.Thetwoblockstogetherwill coveran instrumented volume ofabout1 km3_{.They willbe} de-ployedandanchoredintheCapoPasserositelocatedat36◦16N 16◦06E,atadepthof3500m,andwillbeconnectedtotheshore stationvia a 100 km electro-optical cable to transfer power and databetweenshoreandthedetector.

Different populations of Galactic astrophysical objects have beenproposedasproductionsitesofneutrinosuptotheTeV–PeV range.SupernovaRemnants (SNRs)are the bestmotivated candi-datesinourGalaxy [7].Theyareoftenaddressedasthemain con-tributors to the flux of Galactic Cosmic Rays (the so-called SNR paradigmon the originof GCR). Evidence forthe acceleration of protonsintheremnantswasprovidedin2013whenFermi-LAT re-portedanindicationofthepion-decaysignaturefromtheSNRsW 44andIC443 [8].However,beingmodeldependent,this measure-mentisnotaconclusiveproof.Beingasmokinggunforhadronic acceleration,neutrinoscouldcontributetothechallengeof unveil-ingcosmic-rayaccelerators.

1_{http://www.km3net.org .}

Inthelastdecades,veryhighenergy(VHE:E_γ>100GeV) emis-sionfromalargenumberofGalacticSNRshasbeenidentified by

γ

-raytelescopes.Theobserved

γ

-rayspectracanextenduptotens ofTeV,provingthattheseobjectsareefficientparticleaccelerators. Theseparticlescouldbe protonsyielding

γ

-raysviainelastic pro-duction ofneutral pions,but could also be electrons which emit VHE

γ

-rays via Inverse Compton scattering on ambient low en-ergyphotons.Theobservationofhigh-energyneutrinosfromthese sourceswouldestablishanunambiguousproofthat hadronic pro-cesses are atwork; dueto strongmodel dependences,this proof cannoteasilybeachievedwiththecurrent

γ

-raysobservations.

AnotherclassofGalactic objectsobserved inTeV

γ

-rays com-prises Pulsar Wind Nebulae (PWNe), in which emission of non-thermal radiation is powered by the relativistic outflows from a pulsar, i.e. a rapidly spinning, strongly magnetised neutron star. The interaction of the pulsar wind with the slower supernova ejectaorwiththeinterstellarmediumcreatesaterminationshock where particles can be accelerated to very high energies. Even thoughtheTeVemissionofPWNisusuallyinterpretedinapurely leptonicscenario [9],someauthors [10]alsoconsiderthepresence of a hadronic contribution, which could be tested withneutrino telescopes.

The scientific potential of KM3NeT/ARCA to detect neutrino point-like sources in our Galaxy andbeyond isdiscussed inthis paper. This subject has already been covered in Ref. [4]. How-ever, since then, the event reconstruction and analysis methods havebeenimprovedsignificantly,leadingtonewresultspresented in thispaper. Moreover,the recent publication ofnew andmore precise

γ

-ray observations [11,12] has also allowed for updated neutrino flux predictions. In addition, an extended set of poten-tialneutrinosources isnowinvestigated,includingseveral candi-datesourcesformeasurableneutrinosignals.Astackinganalysisof SNRswiththemostintenseVHE

γ

-rayfluxisalsopresented.

The recent detection by IceCube of a high-energy neutrino eventcoincidentindirectionandtimewitha

γ

-rayflaringstateof theblazarTXS0506₊056isreportedinRef. [13].Thisobservation suggeststhat blazars [14]arelikelysourcesofextra-galactic high-energyneutrinos.Inaddition,aninvestigationoverthefullIceCube neutrinoarchivehasshownan excesswithmorethan3

σ

signifi-canceofhigh-energyneutrinoeventsatthepositionofthisblazar compatiblewithaneutrinofluxwithE−2energydependence [15]. ANTAREShasalsosearchedforneutrinosfromthissource [16]but noevidencehasbeenfound.Toillustratethedetectioncapabilities ofKM3NeT/ARCAforthistype ofextragalacticsources, the sensi-tivity of KM3NeT/ARCA to a E−2 neutrino flux froma point-like sourceisalsodiscussed.

Theanalysisfocussesoncharged-currentinteractionsof muon-neutrinos, producing a high-energy muon in the final state. Due to its path length of up to several kilometres in water, the di-rection ofthe muon andthus – at sufficiently highneutrino en-ergy– theneutrinodirectioncanbemeasuredwithgoodaccuracy (see Section 4).Suchtrack-likeeventsthereforeprovidethe dom-inantcontributiontothesensitivityforpoint-likesources [4].The main backgrounds are due to atmospheric neutrinos and muons produced bytheinteraction ofcosmicrays withnucleiinthe at-mosphere. To eliminate atmospheric muons, only events recon-structedasupgoingorcomingfromslightlyabovethehorizonare selectedsincetheEarthortheslantwaterlayertraversed absorbs all particles except neutrinos. The cosmic neutrino signal is ob-served asan excess on the backgroundofatmospheric neutrinos andofremaining atmosphericmuonsfalselyreconstructedas up-going.Giventhelatitudeofthedetector,KM3NeTwilldetect upgo-ingneutrinosfromabout3.5

π

srofthesky,includingmostofthe Galactic Plane.The visibility ofa given candidatesource (i.e. the fractionoftimeitisobservable)dependsonitsdeclination,

δ

,and ontheangularacceptanceabovethehorizon, see Fig.1.In

partic-Fig. 1. Source visibility for KM3NeT/ARCA as a function of declination for a zenith cut of 10 ◦_{above the horizon (black line). The markers represent the visibility of}

the specific sources discussed in this paper according to their declination and the zenith cuts used in the analyses (see Table 2 for the individual zenith cuts).

ular, note that the region of full visibilityextends to

δ

−45◦ _if eventsupto10◦ abovethehorizonareincluded,asinthepresent analysis(see Section5).

In thispaper, neutrino fluxesexpectedfrom a selected listof Galactic

γ

-raysourcesareestimatedassumingahadronicscenario for the

γ

-ray production and transparent sources. This topic is discussed in Section 2. The details of the simulation codes are described in Section 3 and the reconstruction performances in Section 4. The analysis procedure and the results are presented in Section5. Section6isdevotedtotheKM3NeT/ARCAsensitivity anddiscoverypotentialforagenericE−2flux.Theeffectof system-aticuncertaintiesisdiscussedin Section7,andtheconclusionsare summarisedin Section8.

2. SelectedGalacticsourcesandestimatedneutrinofluxes GalacticcandidatesourceshavebeenselectedfromtheTeVCat catalogue [17] on the basis oftheir visibility,the

γ

-ray intensity andtheenergyspectrum.Inparticular,itwasrequiredthatthe

γ

-rayfluxismeasureduptoafewtensofTeV.Theselectedsources are: RXJ1713.7-3946,VelaX,VelaJr,HESSJ1614-518,theGalactic Centre andMGROJ1908+06 (see Table 1fortheindividual refer-ences).Thevisibilityofthesesourcesisindicatedin Fig.1.Except forMGROJ1908+06,allthesourceshaveavisibilityabove70%.

For all the sources (with the only exception of MGRO J1908₊06), the neutrino flux is derived from the mea-sured

γ

-ray flux usingthe methoddescribedin Refs. [18,19] and references therein. Another method has been tested [20], using as a test case the source RX J1713.7-3946, obtaining compatible results. All neutrino fluxes are estimated for the

ν

_μ2 _channel,

assuming that,duetooscillation,forcosmicneutrinostheflavour ratio at Earth will be

ν

e:

ν

μ:

ν

τ=1:1:1 [21]. For all cases a

100%hadronicemissionandatransparentsourceareassumed,but the resultscan also be interpreted interms ofthe percentageof hadronic emission, provided that the hadronic andnon-hadronic contributions have the sameenergy spectrum. If

ξ

_had is the per-centage of the

γ

-ray flux that has hadronic origin, the neutrino fluxesarecalculatedunderthehypothesis that

ξ

had=1,butfrom theseresultsthediscoverypotentialsandsensitivities for

ξ

_had<1 canbederived.

Allneutrinofluxesinthispublicationareparameterisedby

ν

(

E

)

= k0



_E 1 TeV



− exp



−



_EE cut



β



, (1)

wherek0isthenormalisationconstant,

isthespectralindex,Ecut istheenergycutoff,

β

isthecutoff exponent [22]. Table1liststhe

2 In this paper the notation _{νis used to refer to both neutrinos and antineutrinos.}

Fig. 2. Muon neutrino fluxes ( νμ+ ¯νμ) used in the analysis. The corresponding pa-

rameters are given in Table 1 .

sourcesconsidered, theirdeclination

δ

andangularextension (in-dicatedasradius),asmeasured by

γ

-ray detectors,aswellasthe parametersofthe Eq.(1).Forseveralsources,different parameter-isationsareconsistentwiththe

γ

-raydataandthecorresponding neutrino fluxesare included in the analysis. The fluxeslisted in Table1areshownin Fig.2.

In the followingsubsections shortdescriptions ofthe sources aregivenwithdetails onthederivationoftheneutrinofluxfrom themeasured

γ

-rayflux.

2.1. RXJ1713.7-3946

Theyoungshell-type SNRRXJ1713.7-3946isatpresentoneof the best studied SNRs in the VHE regime. Its high-energy

γ

-ray emission has been observed by H.E.S.S. in several campaigns, in theyears 2003–05 [22,26,27] andin 2011and2012 [11].The re-portedspectrumextendsuptoabout100TeV,suggestingthatthe hadronicparticlepopulation mayhaveenergies uptoseveralPeV ifthe

γ

-rayproductionishadronic.

TheoriginoftheTeV

γ

-rayemissionfromRXJ1713.7-3946has been a matter of active debate. A detailed discussion of the in-terpretation ofthe H.E.S.S. data inhadronic orleptonic scenarios canbefounde.g.inRefs. [11,27]andreferencestherein.Fermi-LAT reportedan observation ofGeV

γ

-ray emission fromRX J1713.7-3946 [28]. Whilethe hard spectrumatGeVenergies reportedby the Fermi Collaboration is generally interpreted as an argument in favour of a leptonic scenario, some authors argue that both hadronicandleptonicscenarioscanreproducethedataunder cer-tain assumptions(e.g. Ref. [29]).On theother hand,the observa-tionofmolecularcloudsinthevicinityofthesource [30,31]could provide an additional hint infavour of the hadronic scenario. In Ref. [32],adetailednumericaltreatmentoftheSNRshock interac-tioninanonhomogenousmediumhasbeenreported,consistently describing the broadband GeV–TeVspectrum of RX J1713.7-3946

Table 1

Parameters of the candidate sources investigated, references for the corresponding γ-ray measure- ments and source type. The neutrino flux is expressed according to Eq. (1) , with the normalisation constant k 0 in units of 10 −11 TeV −1 s −1 cm −2 and E cut in units of TeV. See the text for further details

(note that ξhad = 1 is assumed).

Source δ radius k0 Ecut β γ-ray data type

RX J1713.7-3946 −39 . 77 ◦ _0.6 ◦ _{0.89 2.06 8.04 1 [11] SNR}

Vela X −45.6 ◦ _0.8 ◦ _{0.72 1.36 7 1 [23] PWN}

Vela Jr −46.36 ◦ ₁ ◦ _{1.30 1.87 4.5 1 [12] SNR}

HESS J1614-518 (1) −51.82 ◦ _0.42 ◦ _{0.26 2.42 – – [24] SNR}

HESS J1614-518 (2) −51.82 ◦ _0.42 ◦ _{0.51 2 3.71 0.5 [24] SNR}

Galactic Centre −28.87 ◦ _0.45 ◦ _{0.25 2.3 85.53 0.5 [25] UNID}

MGRO J1908 + 06 (1) 6.27 ◦ _0.34 ◦ _{0.18 2 17.7 0.5 see text UNID}

MGRO J1908 + 06 (2) 6.27 ◦ _0.34 ◦ _{0.16 2 177 0.5 see text UNID}

MGRO J1908 + 06 (3) 6.27 ◦ _0.34 ◦ _{0.16 2 472 0.5 see text UNID}

Table 2

Zenith cut ( θcut ) and expected number of signal events for the candidate sources

in five years of data taking. The number of events is specified at three stages: after reconstruction; after zenith cut; and after the αcut (see text). The sum of νμand ντ events is shown, where the ντcontribution is between 8% and 10%.

Sources θcut Reconstructed Events with Events with

[ ◦_{] events θ θcut θ θcut}

AND α≤ 10 ◦ RX J1713.7-3946 78 22.0 20.0 16.4 Vela X 81 41.5 40.7 34.9 Vela Jr 80 26.0 25.6 21.1 HESS J1614-518 (1) 86 10.7 10.5 9.1 HESS J1614-518 (2) 86 9.3 9.1 7.7 Galactic center 78 9.1 7.0 5.7 MGRO J1908 + 06 (1) 80 6.7 4.1 3.5 MGRO J1908 + 06 (2) 80 11.9 7.1 6.1 MGRO J1908 + 06 (3) 80 14.0 8.3 7.1

interms of a hadronic model.In Ref. [11], the X-ray, the Fermi-LATandthe updatedH.E.S.S. dataarecombinedtoderiveinboth scenariostheparticlespectrafromtheSNR spectralenergy distri-bution.Thedatacanbefitbothwithhadronicandleptonicmodels soneitherofthetwoscenarioscancurrentlybeexcluded.

PreviousKM3NeTresults [4]werederived fromH.E.S.S.datain Ref. [22].Thesearesuperseded bya mostrecentH.E.S.S. publica-tion [11] which showsa softer spectrum at the highestenergies comparedto the previous paper. This newspectrum is basedon anewanalysiswhichmakes useofmoredata,refinedcalibration anddataanalysismethods. Theresultspresented inthis publica-tionarebasedonthedatareportedinRef. [11].In Table1onlythe fluxderivedwiththemethodinRefs. [18,33,34]isreported.

2.2.VelaX

Vela Xisone ofthe nearestpulsar wind nebulaeandis asso-ciatedwiththeenergeticVelapulsarPSRB0833-45.EvenifPWNe aregenerallyconsidered asleptonic sources,interpretationofTeV

γ

-rayemission fromVelaX interms ofhadronicinteractions has beendiscussed [10,35].

TheVHE

γ

-rayemission fromVelaXwasfirstreportedby the H.E.S.S.Collaboration [36]andwasfoundto becoincidentwitha regionofX-ray emissiondiscovered withROSAT asafilamentary structure extending south-west from the pulsar to the centre of VelaX.ThefirstresultofH.E.S.S. hasbeenupdated [23] withdata fromthe2005–07and2008–09observationcampaignsandusing animprovedmethodforthebackgroundsubtraction.Thenewdata arecharacterisedby a25%higherintegralfluxabove 1TeVanda harderenergyspectrumandareusedheretoderive theneutrino spectrum.

2.3. VelaJr

RX J0852.0-4622,commonlyreferred asVelaJunior (VelaJrin Table1and Fig.2)isayoungshell-typeSNR withproperties sim-ilar to RX J1713.7-3946. Vela Junior emits

γ

-rays up to energies of few tens of TeV [12]. Also forthis source the

γ

-ray emission hasbeen interpreted bothin thehadronicand leptonic scenarios (see Ref. [12] for an overview on the arguments). In particular, a recent analysis [37] reports a good spatial correspondence be-tweentheTeV

γ

-raysandinterstellar hydrogenclouds,suggesting ahadronicinterpretationoftheoriginoftheobserved

γ

-raysfrom thissource.

2.4. HESSJ1614-518

The

γ

-ray highenergyemission ofthesource HESS J1614-518 has been observed by H.E.S.S. up to about 10 TeV [24] and was studiedin termsofmorphological,spectralandmulti-wavelength properties and classified as candidate shell-type SNR. The

γ

-ray fluxfromthesourceHESSJ1614-518hasbeenfittedinRef. [24]as a pure powerlaw and the neutrinoflux derived from it is indi-cated in the following as HESS J1614-518 (1). To test the effect ofa possiblecutoff in thespectrum, inthisstudythe H.E.S.S.

γ

-ray datawerefittedalsowithapower lawwithexponential cut-off.The neutrino flux derived from this

γ

-ray flux isreferred as HESSJ1614-518(2).

2.5. GalacticCentre

Recently,theH.E.S.S.Collaborationhasreported

γ

-ray observa-tionsoftheregionsurroundingtheGalacticCentre [25].The

γ

-ray fluxreportedisderivedfortworegions:apointsourcewithradius 0.1◦ (PS) HESS J1745-290 centred on Sgr A∗ and a diffuse emis-sion(DF)fromanannulusbetween0.15◦and0.45◦.Thestrong cor-relationbetweenthebrightnessdistributionof diffuseVHE

γ

-ray emissioninthewidervicinityoftheGalacticCentreandthe loca-tions ofmolecularcloudspointstowards ahadronicoriginofthe

γ

-ray emission [25]. Sincethe DF

γ

-ray dataare consistentwith ahard power-law,the spectrumoftheparent protonsshould ex-tendtoPeVenergies.Theneutrinospectraexpectedfromthetwo regions PSandDF havebeenevaluated inRef. [19],wherea few possibleneutrinospectraare proposed startingfromplausible

γ

-rayfluxesandexploringdifferentenergycutoffs.The flux consid-eredhereisthesumofthePSandDFregions,choosingasfluxfor thePSareatheonederivedfromthe

γ

-rayfluxwithEcut,γ =10.7 TeVandfortheDFregiontheonefromEcut,γ =0.6PeV.For sim-plicity,the source shapeis approximatedas ahomogeneous disk ofradius0.45◦.

2.6. MGROJ1908+06

The source MGRO J1908+06 has been detected both by air Cherenkovtelescopes (H.E.S.S. [38] andVERITAS [39]) and exten-sive air-shower detectors (Milagro [40–42], ARGO-YJB [43] and HAWC [44]).Thenatureofthissourceiscurrentlyunclear.Itcould be a PWN associated with the pulsar PSR J1907₊0602 [45]. Its largesizeandthelackofsofteningoftheTeVspectrumwith dis-tance fromthepulsar, however,are uncommon forTeV PWNeof similar age,suggestingthat it could alsobe a SNR [39,46].Using themeasured

γ

-rayspectra,theprospectsfordetectingneutrinos fromthissourcewithIceCubearediscussedinRef. [46].Three pos-sibleassumptions on the

γ

-ray flux areconsidered, witha spec-tral index

_γ =2 and cutoff energies Ecut,γ =30,300,800 TeV. The corresponding neutrino fluxes derived in Ref. [46], listed in Table 1as(1), (2)and(3),respectively, are usedin thisanalysis. The sourceposition andextensionare takenfromtheH.E.S.S. re-sults [38].

3. Simulations

For this analysis the Monte Carlo (MC) chain discussed in Ref. [4] is used. Neutrinos and anti-neutrinos of all flavours are considered, and both charged and neutral current reactions are simulated. Forthe generationequal fluxesof neutrinos and anti-neutrinos are assumed. Since neutrino andanti-neutrino interac-tionscannotbedistinguishedinKM3NeTonanevent-by-event ba-sis,the leptonsymbols(

ν

,

μ

,e,and

τ

)denotebothparticles and anti-particles in the following. Neutrinos are generated over the fullsolidangletosimulatethebackgroundofatmospheric neutri-nos.Neutrinosfromthespecificsourcesdescribedin Section2are simulated asoriginating fromhomogeneous disks centred atthe declination shown in Table 1 and with a radius given in the sametable.Eventsaregeneratedintheenergyrangebetween102 and 108 _{GeV according to an E}−1.4 _{spectrum and subsequently} reweighted todifferentfluxmodels.The neutrinointeractions are simulatedusingLEPTO [47]withthepartondistributionfunctions CTEQ6 [48](fordeepinelastic scattering).Themuon produced at theinteraction vertexispropagated throughrockandwaterwith MUSIC [49].

The Cherenkov photonsinduced by charged particles travers-ingthewaterarepropagated totheDOMs.TosaveCPU time,this isdoneusingtabulatedphotonpropagationprobabilitiesbasedon fullGEANT3.21 [50]simulationsandtakingintoaccounttheDOM properties (effective area, quantum efficiency and collection effi-ciency of the photomultipliers; transmission probability through glass andgel), theDOM orientation withrespect to the incident directionofthephotonandtheopticalwaterpropertiesmeasured at the KM3NeT-Italy site [51]. Foreach event, the PMTs measur-ingasignalaredetermined,eachsignal(“hit”)beingcharacterised by the photon arrival time and the signal amplitude (deposited charge). The hit data are converted to digitised arrival time and time-over-threshold(ToT),i.e.thetimetheanalogsignalexceedsa predefinedthreshold.

Opticalbackgroundduetothepresenceof40_{Kinsaltwateris} simulated by adding an uncorrelatedhit rate of 5 kHzper PMT. Moreover, theprobability oftwo-,three-andfour-fold hit coinci-dences on a DOM from a single 40_{K decay have been estimated} byGEANTsimulationsandare includedwithratesof500,50and 5HzperDOM,respectively.Boththesingleandcoincidencerates are in agreement with the results from the prototype detection unit of the KM3NeT detectordeployedat Capo Passero [52].The effect of bioluminescence light is negligible at the KM3NeT-Italy site [53].

At theend ofthe simulationchain, triggeralgorithms are ap-pliedin orderto selectpotentiallyinteresting events thatwill be

reconstructedandanalysedwiththestatisticalmethodsdescribed below.The triggeris basedonthe L1hits, i.e.hitsonmore than onePMTofthesameDOMinatimewindowof10ns.Eventspass thetriggerconditionifthereare atleast5causally connectedL1 hits.Details onthetrigger andtrigger efficiencyaregivenin Ref. [4].

3.1. Atmosphericneutrinosandmuons

Only a very small fraction of the high energy neutrino flux arriving at the detector is of astrophysical origin. The dominant contribution is due to atmospheric neutrinos from extended air showerscausedby cosmic ray interactions withnucleiin the at-mosphere.However, at sufficientlylarge energies, the astrophysi-calfluxwilldominatethatofatmosphericorigin.Theatmospheric neutrinoflux has two components: the conventional one due to thedecayofchargedpionsandkaonsandthepromptonedueto thedecay ofcharmed hadrons, produced in theprimary interac-tion.Theatmosphericneutrinofluxissimulatedassumingthe con-ventionalatmosphericmodelasinRef. [54]andtheprompt com-ponent asdescribedinRef. [55].Corrections dueto the breakin the cosmic ray spectrum (knee) are applied as describedin Ref. [56]. Also other models of prompt neutrino fluxes [57–59] have beentested,buttheyleavethefinalresultsessentiallyunaltered.

Inadditiontoatmosphericneutrinos,cosmicrayinteractionsin theatmospherealsoproduce atmosphericmuons. Eachinitial in-teractioncreatesanumberofmuonsthatarecollimatedand coin-cident intime (muonbundle).Atmospheric muonsare simulated usingtheMUPAGE eventgenerator [60].Intheanalysispresented heretwosimulatedmuoneventsamplesareused,onewithmuon bundleenergies E_b>10TeV, correspondingto alivetimeofabout 3months,theother withEb>50TeV,equivalenttoabout3years oflivetime.

4. Eventreconstructionperformances

The neutrino induced events are observed in two topologies, track-like and cascade-like events, each class requiring specific eventreconstructionalgorithms.

Track-like events are due to charged-current

ν

_μ interactions that, forE_ν࣡1TeV, produce in the final state muons withtrack lengthsoftheorderofkilometresandtrajectoriesalmostcolinear withtheparentneutrinodirection.Also

ν

_τ charged-current inter-actionscanproduceahigh-energymuoninthefinalstatethrough amuonicdecayofthefinal-state

τ

withabranchingratioofabout 17%.Thereconstruction algorithmusedfortrack-likeeventsis de-scribedinRef. [61].Themuondirectionisreconstructedfromthe sequenceofCherenkovphotonhitsonthePMTs,takingadvantage ofthefactthat photonsareemittedalong theparticletrackatan angle of about42◦. The reconstruction algorithm starts by a pr-efit scanning the full solid angle.Then, startingfrom the twelve best fitted directionsin the prefit, a maximum likelihood search is performed. The likelihood is derived from a probability den-sity function depending on the position and orientation of the PMTs with respect to the muon trajectory and on the hit times. Among these intermediate tracks, the one with the best likeli-hood ischosen. Areconstruction quality parameter is definedas



=− logL_{− 0.}1Ncomp, wherelogListhelog-likelihoodof thefit andNcomp isthenumberofintermediatetracksduringthe recon-structionwithin1◦fromthechosenone.

Theangularresolution,calculatedasthemediananglebetween thereconstructedtrackandtheneutrinodirection,issmallerthan 0.2◦ at E_ν>10 TeV. The energyis reconstructed fromthe spatial distributionofhitandnon-hitPMTs.Theresolutionisbetterthan 0.3unitsin log₁₀

(

Ereco/Eμ

)

,where Ereco is thereconstructed and E_μ thetruemuonenergyatthedetectorlevel.

All neutral-current reactions, aswell ascharged-current reac-tions of

ν

e and most

ν

τ, produce cascade-like event topologies.

Particle cascades evolve from the hadronic final state and the final-statecharged lepton (e) or its decay products (

τ

, except in muonicdecays). Thesecascades are typically severalmetres long andthereforesmallcomparedtointer-DOMdistances.The recon-struction for such cascade-like events has an angular resolution worsethanthetrack-likecaseandisdescribedin [61].

Track-like eventsare ofparticular relevance forthe search for point-like, i.e. very localised sources of neutrino emission, since theyallowforfullyexploitingthelargeeffectiveareaandthegood angularresolutionofKM3NeT/ARCA.Theanalysisdiscussedinthis paperthereforefocussesontrack-likeevents,andconsequentlythe eventreconstruction specificfortrack-likeeventsisapplied toall events,includingcascadeevents.Onlyeventswithsufficient recon-structionqualityareretained.

5. Galacticsources:searchmethodandresults

Sincetheneutrinosignalfromanypointsourcemustbe identi-fiedontopofalargebackgroundofatmosphericmuonsand neu-trinos,statisticaltechniquesarerequiredtoquantifyapossible ex-cessofeventsaroundthesourceposition.Thetwoquantitiesused to describe the detector performance are the discovery potential andthe sensitivity.Thediscovery potential refers tothe fluxthat couldproduceasignificant(e.g.3

σ

or5

σ

)observationwith proba-bility50%.Thesensitivityreferstothefluxthatcanbeexcludedat agivenconfidencelevel(90%inthispaper),ifnosignificantsignal isobserved(see Section5.3).

ThesearchforGalacticpoint-likeneutrinosourcesisperformed inthefollowingsteps:

• Selection cuts are applied to reduce the background events (Section5.1).

• AmultivariateanalysisemployingaRandomDecisionForest al-gorithm [62] isperformed on the remaining eventsto distin-guishsignalfrombackgroundevents(Section5.2).

• An unbinnedlikelihood method isused todetermine the dis-coverypotentialandthesensitivity(Section5.3).

5.1.Selectioncuts

For signal events,onlycharged-current interactions of

ν

_μ and of

ν

_τ (withsubsequent

τ

→

μνν

decay)are consideredsincethe remaining event classes (other decays of

ν

_τ producing cascades, charged-current

ν

eandallneutral-currentinteractions)arealmost

completelyrejectedbyapplyingtrackreconstructionquality crite-ria.Foratmosphericneutrinos, bothcharged-andneutral-current

ν

μ and

ν

e eventsaretakenintoaccount.

Thelooseselectioncutsappliedare:

1.A zenith cut at about 10◦ above the horizon to reduce the backgroundofatmosphericmuons(see Section 1),slightly op-timisedforeachcandidatesourcetakingintoaccountits maxi-mumelevation(see Table2).

2. A cut on the angle

α

between the reconstructed track direc-tion andthe nominalsource position.A cut

α

<10◦ has been selectedasacompromisetoreducethebackgroundwithout re-ducingsignificantlytheefficiencyforselectingsignalevents. The numbers of signal events after these selection cuts, ex-pected fromthe differentsources for the flux assumptions from Table1,arereportedin Table2.

5.2.RandomDecisionForesttraining

A multivariateanalysisemployingtheRandomDecision Forest algorithmisperformedtodistinguishthreeclassesofevents:

neu-trinos comingfromthe source,atmosphericneutrinos,and atmo-sphericmuons.Morespecificallyweusetheextremelyrandomised treesclassifierfromRef. [63].

Thefeaturesusedinthetrainingtocharacterisetheeventsare: the angle

α

between the reconstructed track direction and the nominalsourceposition;thereconstructedzenithangle

θ

;the re-constructedmuonenergyatthedetectorlevel;thenumbersofhits used atdifferent stagesof the reconstruction;the errorestimate on this fit

β

; andthe track reconstruction quality parameter,



, defined in Section 4. The distributions of the mostimportant of thesefeaturesareshownin Fig.3forthethreeeventclasses.Note that

α

istheconvolutionofthesourceextension andtheangular resolution.

In a first stepthe algorithm is trainedon a sample ofevents to optimise its performance in distinguishing the differentevent classes.Thetrainedclassifier hasthenbeenapplied toaseparate eventsampletotestitsperformance.Foreacheventtheclassifier returnstheprobabilityto belongtoeachone ofthethreeclasses. The distributions of the probability to belong to the signal class (Fig. 4) areused forallevents asprobability densityfunctionsin thesubsequentanalysisstep.

An example of thedistribution of the simulated neutrino en-ergyatthedifferentstagesoftheanalysisisshownin Fig.5forthe sourceSNRRXJ1713.7-3946 (see Table1).Theenergydistribution for all the reconstructed events, those passing the selection cuts (see Table 2) and those passing the cuts of the “cut-and-count” analysis (see [64] fora description of thismethod) is shown. In lattercase, theoutput oftheRandomDecision Forestclassifier is usedaasvariabletocuton.Thisisshownheretoillustratethe en-ergyofinterestofthesekindofanalyses,typicallypeakingaround 10TeV.However,thecut-and-countmethodisnotusedforthe re-sultsdescribedinthenextsections.

5.3. Unbinnedmethod

Inordertotestthecompatibilityofthedatawithtwodifferent hypotheses H0 and H1, a test statisticis defined. The test statis-tic can in principle be any function of the data butis optimally selected such that its distributions under thetwo competing hy-pothesesaremaximallyseparated.Inthesearchforneutrinopoint sourcesthehypothesisH0=Hbreferstothecaseinwhichthedata setconsistsofbackgroundeventsonly.HypothesisH1=Hs+brefers tothecasewhereeventsfromacosmic sourcearepresentin ad-ditiontothebackground.Tocalculatetheteststatistic,alikelihood ratio [65]hasbeendefinedastheratiooftheprobabilitiesto ob-tainthedataassumingthehypothesesH_s+b orH_b:

LR = log



P

(

data

|

Hs+b

)

P

(

data

|

Hb

)



. (2)

Thelikelihoodratiocanbe writtenintermsofthte probability densityfunctions(PDFs) describingthe distribution ofsignal and background eventsas a function of a given variablex, f(x|s) and f(x|b): LR = n  i=1 log

⎡

⎣

ns n · f

(

xi

|

s

)

+



1 −ns n



· f

(

xi

|

b

)

f

(

xi

|

b

)

⎤

⎦

, (3)

wherenisthetotalnumberofrecordedeventsinagivenperiodof timeandnsistheexpectednumberofsignaleventsinthesample ofnevents;nsisafreeparameterconstrainedtobenon-negative. Notethat thesourcepositionisassumedtobe knownandisnot determinedfromthedata.Foreachsample, LRismaximised.The maximumvalueofLR isusedasthetest statisticandwillbe de-notedwiththesymbol

λ

.Thevariablexin Eq.(3)isthe probabil-itythattheeventbelongstothe“signal” classascalculatedbythe

Fig. 3. Distributions of the most significant features used in the training ( α, θ, β, ), for the three different event classes: atmospheric muons (blue lines), atmospheric neutrinos (green lines) and neutrinos from the source (red lines). Shown are the distributions for SNR RX J1713.7-3946 (see Table 1 ) with the zenith and αcuts applied. In the top left plot the blue and green lines are superimposed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Distribution of the probability to be signal according to the Random De- cision Forest output for signal and backgrounds events for the source RX J1713.7- 3946.

RandomDecisionForestclassifier.As anexamplethePDFsforthe sourceRXJ1713.7-3946areshownin Fig.4.

Theoutputofthealgorithmisthe

λ

valueandthe correspond-ingfittednsvalue.Thedistributionof2

λ

forthebackground-only caseisexpectedtofollowahalf-

χ

2_{-distributionasdefinedinRef.} [66]andcanbeusedtoestimatethepre-trialp-value.

Inordertoestimatethedistributionoftheteststatisticforthe background-only assumption, the algorithm is applied to several thousandsamples of backgroundevents sampledfromthe simu-latedatmosphericneutrinoandmuonevents.Foreachsample,the maximumvalue ofLR,

λ

,isrecorded.Thenormaliseddistribution of

λ

,g(

λ

|b)isthendetermined.Selectingtherequiredsignificance andthecorrespondingtwo-sidedGaussianprobability,e.g.3

σ

and

Fig. 5. Distribution of the generated neutrino energy for the source RX J1713.7- 3946 at reconstruction level (red line), after the selection cuts θ≥ 78 ◦_{and α< 10} ◦

(blue line) and after the cuts of the cut-and-count analysis (green line), that cor- responds to an additional cut on the Random Decision Forest output greater than 0.92. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2.7× 10−3_{,acriticalvalue}

_λ

_{3σ iscalculatedfrom ∞}

λ3σ

g

(

λ|

b

)

d

λ

= 2 .7 × 10 −3_{. (4)} Subsequently,theprocedure isrepeatedaddingthePoissonian expectation, Ns, ofone simulated signal eventtothe background sample,thenthePoissonianexpectationfortwosignalevents,and so on.For each Ns,

λ

isagain calculated andits normalised dis-tributionwillbe indicatedwithg(

λ|

Ns+b).The“power” P

(

Ns

)

is calculatedas:

 _∞

λ3σ

Fig. 6. Ratio of the discovery potential 3σ(left) and sensitivity 90 (right) to the expectation flux νas a function of the observation time for the three fluxes assumed for

the source MGRO J1908 + 06 (see Table 1 ). The fluxes (1), (2) and (3) correspond to a γ-ray spectral index γ= 2 and cutoff energies of E cut,γ= 30 , 300 , 800 TeV, respectively.

Fig. 7. Ratio of the discovery potential 3σ(left) and sensitivity 90 (right) to the expectation flux νas a function of the observation time for the first seven source fluxes

listed in Table 1 .

Let n3σ be thevalue ofNs forwhichP

(

Ns

)

=0.5. Thenn3σ is thenumberofexpectedsignaleventsthatwouldleadtoa detec-tionwithasignificanceofatleast3

σ

in50%ofthecases.

Iftheanalysishasbeenperformedwithamodelforthesource thatpredicts aflux

sandameannumberofsignal events

ns

,

thediscoverypotentialwillbegivenby

3σ =

s ·

n3σ

ns

.

(6) Thesensitivityiscalculatedasthe90%confidencelevelmedian upperlimit by using theNeyman method [65]. The procedure is similar to that described previously but in this casea reference value

λ

90 is calculated as the median of the g(

λ

|b) distribution. Thepowerisevaluated asin Eq.(5)butwith

λ

90 insteadof

λ

3σ. Thenumberofeventsneededtoreachtherequiredsensitivity,n90, isthenumberofeventssuchthatP

(

Ns

)

=0.9.

Asin Eq.(6),thesensitivityflux

90 iscalculatedas:

90 =

s · n90

ns

.

(7)

5.4.Results

The results are shown in Figs. 6 and 7. Fig. 6 refers to the source MGRO J1908+06 with the three neutrino flux assump-tionslisted in Table1,whiletheresultsfortheother sourcesare shownin Fig.7.Thefluxescorrespondingtothediscovery poten-tialat3

σ

,

3σ, areshown inthe left plots of Figs. 6and 7and the sensitivity at 90% confidence level,

90, is shown the right plots.In Figs. 6 and 7both

3σ and

90 are reportedasa ratio overthefluxexpectation

_ν ofeachsource,givenby Eq.(1)and shownin Table1.Therefore

3σ/

ν =1indicatesthetimeneeded fora3 sigmadetectionofthe sourcefor

ξ

had=1.If

3σ/

ν<1,

3σ/

ν=y gives the time needed to observe the source at 3

σ

forthecase

ξ

had=y.Thesamenotationappliestothesensitivity.

Note that, by definition,

3σ and

ν have the samespectral shape(see Eq.(6))sotheratiobetweenthetwofluxescorresponds to the ratio betweentheir normalisation constants anddoes not dependontheenergy.

The sensitivity to exclude the predicted fluxes at 90% confi-dencelevel is reachedforall the sources after about5 (7)years for

ξ

had=1 (

ξ

had=0.8). ForMGRO J1908+06, a 3

σ

discovery is possible after about 5.5 years (7.5 years) if the cutoff in the

γ

-rayspectrum isEcut,γ =800TeV(300TeV)andif

ξ

_had=1.Inthe case of a cutoff at much lower energies, however, longer obser-vation time would be necessary, e.g. 27 years for a cutoff at 30 TeV. For RX J1713.7-3946, 5 years of observation time are suffi-cienttoconstrainthehadronicfractionto

ξ

had<0.5.Eventhough hadronic scenarios forVela X are disfavoured, it is worth noting that KM3NeT/ARCA could constrain the hadronic contribution to

ξ

had<0.6(

ξ

had<0.2)inabout1year(5.5years).

Itshouldbenotedthattheresultsforagivenneutrinoflux de-gradewithincreasingextensionofthesource.Thiseffectdepends mainly on the source radius, but also on the source spectrum. StudiesconcerningthiseffecthavebeenreportedbytheANTARES Collaboration [67,68]and other authors [69]. Toquantify the im-pactofthesourceextension, thediscoverypotential andthe sen-sitivityhavebeendeterminedfortwosources,assuming thatthey are point-like instead of having finite extension. For RX J1713.7-3946, with a radius of 0.6◦,

3σ is reduced by about 25% and

90byabout20%.ForMGROJ1908+06(0.34◦radiusandaharder spectrum),therelativereductionisabouthalfaslarge.Systematic effectsfromtheuncertaintiesofthesourceextensionsorpossible inhomogeneities oftheneutrinoemission fromthesource region areexpectedtobenegligible.

A stacking analysis has beenperformed forthe two most in-tense SNRs, RX J1713.7-3946 and Vela Jr. The analysis is simi-lar tothe one describedabove for singlesources, withthe PDFs in Eq. (3) obtained as weighted sums of the PDFs of the single

Fig. 8. Ratio of the discovery potentials 3σ and 5σ to the expectation flux νas a function of the observation time for the stacking analysis including RX J1713.7-3946

and Vela Jr. The neutrino fluxes assumed for the individual sources are listed in Table 1 . In this case, νis taken as the sum of the fluxes of the two sources.

Fig. 9. Sensitivity, defined as the median upper limit at 90% confidence level (left), and discovery flux at 5 σ (right) for sources with a generic, unbroken neutrino flux proportional to E −2 _{, as a function of the source declination. An observation time of 6 years is assumed. For comparison, the corresponding IceCube [70] and ANTARES}

[67] results are also shown. Note that the IceCube discovery potential [70] follows the one-sided gaussian probability convention, while in this paper the two-sided one is used. For the KM3NeT results the difference deriving from using one or the other convention has been evaluated to be less than 4%, within the line thickness of the figure.

sources, both forthe signal andthebackground, usingasweight the numberof eventsexpectedin each case. Since thesesources are quite distant in the sky (about 80◦), there is no overlap be-tween theselected eventsaroundthe twosources. Itistherefore possible touse theoriginal classifier designedforeach source as PDFforthestackedsearch.

In Fig. 8, the resulting values of

3σ/

ν and

5σ/

ν are shownasafunctionoftheobservationtime.Notethatinthiscase

ν indicatesthesumofthefluxesofthetwostackedsources.An observationat3

σ

ispossibleafter3yearsandat5

σ

after9years.

6. GenericpointsourceswithE−2spectrum

Thesensitivitytoastrophysical neutrinosources lackinga spe-cificneutrinofluxpredictionbasedon

γ

-raymeasurementsis per-formedassumingageneric,unbrokenpowerlawenergyspectrum proportionaltoE−2.Thisassumptionisinagreementwiththe re-centIceCubefindings [15]andprovidesabenchmarkscenariothat canbe comparedwithother detectors(seee.g. thecorresponding resultsfromANTARES [67]andIceCube [70]).

Inthiscasenospecificsourcegenerationisperformed.Instead ofa specific trainingforeachpossible sourcelocation inthesky, onlyonetrainingisperformedassumingas“signal” anevent sam-plegeneratedwithaE−2spectrumandimposingtheexperimental pointspreadfunction.Onlytracksreconstructedbelowthehorizon andupto10◦abovethehorizonareconsidered.Thefeaturesused forthetrainingarethesameasforGalacticsources,exceptthatin thiscasethedistancefromthesourcepositionisnotusedatthis stage of the analysis.The output of the Random Decision Forest classifierisusedasacutvariableintheanalysis.

The likelihood ratio in Eq. (3) is built in this case from the PDFsthatdescribethereconstructeddirectionsandenergiesofthe events,followingaprocedurewidelyusedbytheANTARES Collab-oration(seee.g. [67]).Moreprecisely,

f

(

xi

|

s

)

= f

(

ψ

i

|

s

)

f

(

Ei,rec

|

s

)

(8) wheref(

ψ

i|s) isa parameterisation ofthe point spread function,

i.e.theprobabilitydensityfunctionofreconstructingeventiatan angular distance

ψ

i from the true source location, and f(Ei,rec|s) is theprobability densityfunction for signal events to be recon-structedwithan energyErec.Forthebackground,thespatialpart of the PDF depends only on the event declination

δ

i while the

probabilityinrightascensionisuniformlydistributed,so

f

(

xi

|

b

)

= f

(

δ

i

|

b

)

/

(

2

π

)

f

(

Ei,rec

|

b

)

(9) Here, f(

δ

i|b)/(2

π

) is the probability density for background

eventsasa function ofthe declinationandf(E_i,rec|b)is the prob-abilitydensityfunctionforbackgroundeventstobereconstructed withanenergyErec.

The resulting sensitivity and 5

σ

discovery flux are shown in Fig.9asafunctionofthesourcedeclination.Anobservationtime of6 yearshas beenused, whichis similar toIceCube results re-portedinRef. [70].

Previously [4], the 5

σ

discovery flux was reportedfor an ob-servationtimeofthreeyears.The presentanalysisleadstoa25% improvementwithrespecttoRef. [4]inthe5

σ

discoveryflux. 7. Systematicuncertainties

A detailed investigation of the systematic effects for point sourcesearcheshasbeenreportedinRef. [4].The main