Probing the ATIC peak in the cosmic-ray electron spectrum with H.E.S.S

0.09 MB·English

by  H. E. S. S. Collaboration

Astronomy&Astrophysicsmanuscriptno.13323 (cid:13)c ESO2010
January20,2010
Probing the ATIC peak in the cosmic-ray electron spectrum with
H.E.S.S.
F.Aharonian1,13,A.G.Akhperjanian2,G.Anton16,U.BarresdeAlmeida8 ⋆,A.R.Bazer-Bachi3,Y.Becherini12,
B.Behera14,K.Bernlo¨hr1,5,A.Bochow1,C.Boisson6,J.Bolmont19,V.Borrel3,J.Brucker16,F.Brun19,P.Brun7,
R.Bu¨hler1,T.Bulik24,I.Bu¨sching9,T.Boutelier17,P.M.Chadwick8,A.Charbonnier19,R.C.G.Chaves1,
A.Cheesebrough8,L.-M.Chounet10,A.C.Clapson1,G.Coignet11,M.Dalton5,M.K.Daniel8,I.D.Davids22,9,
B.Degrange10,C.Deil1,H.J.Dickinson8,A.Djannati-Ata¨ı12,W.Domainko1,L.O’C.Drury13,F.Dubois11,
G.Dubus17,J.Dyks24,M.Dyrda28,K.Egberts1 ⋆⋆,D.Emmanoulopoulos14,P.Espigat12,C.Farnier15,F.Feinstein15,
0 A.Fiasson11,A.Fo¨rster1,G.Fontaine10,M.Fu¨ßling5,S.Gabici13,Y.A.Gallant15,L.Ge´rard12,D.Gerbig21,
1 B.Giebels10,J.F.Glicenstein7,B.Glu¨ck16,P.Goret7,D.Go¨ring16,D.Hauser14,M.Hauser14,S.Heinz16,
0
G.Heinzelmann4,G.Henri17,G.Hermann1,J.A.Hinton25,A.Hoffmann18,W.Hofmann1 ⋆⋆⋆,M.Holleran9,
2
S.Hoppe1,D.Horns4,A.Jacholkowska19,O.C.deJager9,C.Jahn16,I.Jung16,K.Katarzyn´ski27,U.Katz16,

n S.Kaufmann14,E.Kendziorra18,M.Kerschhaggl5,D.Khangulyan1,B.Khe´lifi10,D.Keogh8,W.Kluz´niak24,
a
J T.Kneiske4,Nu.Komin15,K.Kosack1,R.Kossakowski11,G.Lamanna11,J.-P.Lenain6,T.Lohse5,V.Marandon12,
J.M.Martin6,O.Martineau-Huynh19,A.Marcowith15,J.Masbou11,D.Maurin19,T.J.L.McComb8,M.C.Medina6,
0
2 R.Moderski24,E.Moulin7,M.Naumann-Godo10,M.deNaurois19,D.Nedbal20,D.Nekrassov1,B.Nicholas26,
J.Niemiec28,S.J.Nolan8,S.Ohm1,J-F.Olive3,E.deOn˜aWilhelmi1,12,29,K.J.Orford8,M.Ostrowski23,M.Panter1,

] M.PazArribas5,G.Pedaletti14,G.Pelletier17,P.-O.Petrucci17,S.Pita12,G.Pu¨hlhofer14,M.Punch12,
E
A.Quirrenbach14,B.C.Raubenheimer9,M.Raue1,29,S.M.Rayner8,O.Reimer30,M.Renaud1,F.Rieger1,29,
H
J.Ripken4,L.Rob20,S.Rosier-Lees11,G.Rowell26,B.Rudak24,C.B.Rulten8,J.Ruppel21,V.Sahakian2,
h. A.Santangelo18,R.Schlickeiser21,F.M.Scho¨ck16,R.Schro¨der21,U.Schwanke5,S.Schwarzburg18,S.Schwemmer14,
p A.Shalchi21,M.Sikora24,J.L.Skilton25,H.Sol6,D.Spangler8,Ł.Stawarz23,R.Steenkamp22,C.Stegmann16,F.
-
o Stinzing16,G.Superina10,A.Szostek23,17,P.H.Tam14,J.-P.Tavernet19,R.Terrier12,O.Tibolla1,M.Tluczykont4,
r C.vanEldik1,G.Vasileiadis15,C.Venter9,L.Venter6,J.P.Vialle11,P.Vincent19,M.Vivier7,H.J.Vo¨lk1,F.Volpe1,
t
s S.J.Wagner14,M.Ward8,A.A.Zdziarski24,andA.Zech6
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[ (Affiliationscanbefoundafterthereferences)

2
Received/Accepted
v
5
0
ABSTRACT
1
0
The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has - together with
.
5 the PAMELA detection of a risein the positron fraction up to ≈100 GeV - motivated many interpretations in termsof dark matter scenarios;
0 alternativeexplanationsassumeanearbyelectronsourcelikeapulsarorsupernovaremnant.Herewepresentameasurementofthecosmic-ray
9 electronspectrumwithH.E.S.S.startingat340GeV.WhiletheoverallelectronfluxmeasuredbyH.E.S.S.isconsistentwiththeATICdatawithin
0 statisticalandsystematicerrors,theH.E.S.S.dataexcludeapronouncedpeakintheelectronspectrumassuggestedforinterpretationbyATIC.
: TheH.E.S.S.datafollowapower-lawspectrumwithspectralindexof3.0±0.1(stat.)±0.3(syst.),whichsteepensatabout1TeV.
v
i Keywords.(ISM:)cosmic-rays-Methods:dataanalysis
X
r
a
1. Introduction Kobayashietal.2004). Recently, the ATIC collaboration re-
ported the measurement of an excess in the electron spectrum
Very-high-energy (E & 100 GeV) cosmic-ray electrons1 (Changetal.2008). The excess appears as a peak in E3 Φ(E)
lose their energy rapidly via inverse Compton scatter- whereΦisthedifferentialelectronflux;itcanbeapproximated
ing and synchrotron radiation resulting in short cooling
asacomponentwithapowerlawindexaround2andasharpcut-
time and hence range. Therefore, they must come from off around620 GeV. Combinedwith the excess in the positron
a few nearby sources (Shen1970, Aharonianetal.1995,
fraction measured by PAMELA (Adrianietal.2009), the peak
featureoftheATICmeasurementhasbeeninterpretedinterms
⋆ supportedbyCAPESFoundation,MinistryofEducationofBrazil ofadarkmattersignaloracontributionofanearbypulsar(e.g.
⋆⋆ [email protected] Malyshevetal.2009andreferencesgiventhere).Inthecaseof
⋆⋆⋆ [email protected] dark matter, the structure in the electron spectrum can be ex-
1 Thetermelectronsisusedgenericallyinthefollowingtoreferto plainedascausedbydarkmatterannihilationintolowmultiplic-
bothelectronsandpositronssincemostexperimentsdonotdiscriminate ity final states, while in the case of a pulsar scenario the struc-
betweenparticleandantiparticle.
2 Aharonianetal.:ProbingtheATICpeakintheCRe±spectrumwithH.E.S.S.
turearisesfromacompetitionbetweenenergylossprocessesof mirrorreflectivitydegradesovertime anda reducedlightyield
pulsar electrons (which impose an energy cutoff depending on correspondstoanincreasedenergythreshold.Thenewdataand
pulsarage)andenergy-dependentdiffusion(whichfavorshigh- eventselection reducesthe eventstatistics butenablesto lower
energyparticlesincaseofmoredistantpulsars). theanalysisthresholdto340GeV.Theeffectivecollectionarea
The possibility to distinguish between a nearby electron at340GeVis≈4×104m2.Withalive-timeof77hoursofgood
source and a dark matter explanation with imaging at- quality data, a total effective exposure of ≈ 2.2 × 107 m2srs
mospheric Cherenkov telescopes has been discussed by is achievedat 340GeV. Owing to the steepnessof the electron
Hall&Hooper2008. Imaging atmospheric Cherenkov tele- spectrum, the measurement at lower energies is facilitated by
scopes have five orders of magnitude larger collection areas thecomparativelyhigherfluxes.Theζ distributionintheenergy
than balloon and satellite experiments and can therefore mea- rangeof340to700GeVisshowninFig.1.
sure TeV electrons with excellent statistics. Hall and Hooper The low-energyelectronspectrum resultingfrom this analysis
assume that a structure in the electron spectrum should be vis-
ible even in the presence of a strong background of misiden-
tified nucleonic cosmic rays. However, the assumption of a s1400
smooth background is oversimplified; in typical analyses the nt
e
background rejection varies strongly with energy and without v
E
reliable control or better subtraction of the background, deci- 1200 H.E.S.S. 0.34-0.7 TeV
sive results are difficult to achieve. In a recent publication, the
High EnergyStereoscopic System (H.E.S.S.) collaborationhas Electrons
1000
shown that such a subtraction is indeed possible, reporting a
Protons
measurementoftheelectronspectrumintherangeof700GeV
to5TeV(Aharonianetal.2008). 800 Best Fit Model
2. Thelow-energyextensionoftheH.E.S.S.electron
600
measurement
Here an extension of the H.E.S.S. measurementtowards lower
400
energiesispresented,partiallycoveringtherangeofthereported
ATICexcess.H.E.S.S.(Hinton2004)isasystemoffourimag-
ing atmospheric Cherenkov telescopes in Namibia. While de- 200
signedforthemeasurementofγ-rayinitiatedair-showers,itcan
beusedtomeasurecosmic-rayelectronsaswell.Thebasicprop-
ertiesoftheanalysisofcosmic-rayelectronswithH.E.S.S.have 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1
ζ
been presented in Aharonianetal.2008. For the analysis, data
from extragalactic fields (with a minimum of 7◦ above or be-
Fig.1.Themeasureddistributionoftheparameterζ,compared
lowtheGalacticplane)areusedexcludinganyknownorpoten-
withdistributionsforsimulatedprotonsandelectrons,forshow-
tial γ-ray source in order to avoid an almost indistinguishable
ers with reconstructed energy between 0.34 and 0.7 TeV (the
γ-ray contribution to the electron signal. As the diffuse extra-
energyrangeoftheextensiontowardslowerenergiescompared
galactic γ-ray background is strongly suppressed by pair cre-
to the analysis presented in Aharonianetal.2008). The best
ation on cosmic radiation fields (Coppi&Aharonian1997), its
fit model combination of electrons and protons is shown as a
contribution to the measured flux can be estimated following
shadedband.TheprotonsimulationsusetheSIBYLLhadronic
Coppi&Aharonian1997tobelessthan6%,assumingablazar
interaction model. Distributions differ from the ones presented
spectrumofanunbrokenpowerlawupto3TeVwithaGausian
inFig.1ofAharonianetal.2008becauseoftheenergydepen-
spectralindexdistributioncenteredatΓ=−2.1withσ =0.35.
Γ denceoftheζ parameter.
ForanimprovedrejectionofthehadronicbackgroundaRandom
Forest algorithm (Breiman&Cutler2004) is used. The algo-
rithm uses image information to estimate the electron likeness is shown in Fig. 2 together with previousdata of H.E.S.S. and
ζ ofeachevent.Sincesomeoftheimageparametersusedtode- directmeasurements.The spectrumis welldescribedby a bro-
rive the ζ parameter are energy dependent, also ζ depends on kenpowerlawdN/dE =k·(E/Eb)−Γ1 ·(1+(E/Eb)1/α)−(Γ2−Γ1)α
energy.Toderiveanelectronspectrum,acutonζ ofζ > 0.6is (χ2/d.o.f. = 5.6/4, p = 0.23) with a normalization k =
applied and the number of electrons is determined in indepen- (1.5±0.1)×10−4 TeV−1m−2sr−1s−1,andabreakenergyE =
b
dent energy bands by a fit of the distribution in ζ with contri- 0.9±0.1TeV,wherethetransitionbetweenthetwospectralin-
butionsofsimulatedelectronsandprotons.Thecontributionof dices Γ = 3.0 ± 0.1 and Γ = 4.1± 0.3 occurs. The param-
1 2
heavier nuclei is sufficiently suppressed for ζ > 0.6 as not to eter α denotes the sharpness of the transition, the fit prefers a
playarole.Theresultdoesnotdependontheparticularchoice sharptransition,α < 0.3.Theshadedbandindicatestheuncer-
of ζ . For an extension of the spectrum towards lower ener- tainties in the flux normalization that arise from uncertainties
min
gies, the analysis has been modified to improve the sensitivity inthemodelingofhadronicinteractionsandintheatmospheric
atlowenergies.Intheeventselectioncuts,theminimumimage model.Theuncertaintiesamounttoabout30%andarederivedin
amplitudehasbeenreducedfrom200to80photoelectronstoal- thesame fashionasin theinitialpaper(Aharonianetal.2008),
lowforlowerenergyevents.Inordertoguaranteegoodshower i.e.bycomparisonofthespectraderivedfromtwoindependent
reconstruction, only events with a reconstructed distance from datasetstakeninsummerandautumn2004fortheeffectofat-
theprojectedcorepositiononthe groundto thearraycenterof mosphericvariationsand by comparisonof the spectra derived
less than100m areincluded.Additionally,onlydata takenbe- using the SIBYLL and QGSJET-II hadronic interaction model
tween 2004and 2005are used. The reason is that the H.E.S.S. fortheeffectoftheuncertaintiesintheprotonsimulations.The
Aharonianetal.:ProbingtheATICpeakintheCRe±spectrumwithH.E.S.S. 3
energies.However,the nominalH.E.S.S. data are in verygood
1)
-r agreement with the high precision FERMI measurement up to
s
1 1TeV.ThecombinedH.E.S.S.andFERMImeasurementsmake
-s ∆ E+ 5%
-2 m -10% ∆ E ± 15% a feature in the electron spectrum in the region of overlap of
2 bothexperimentsratherunlikely.
V Beside comparing the H.E.S.S. measurement with ATIC and
e
G
E (
d
3 dN/102 -1)sr
E 1
-s
-2 m ∆ E ± 15%
ATIC 2
V
PPB-BETS e
Kobayashi G
Fermi E (
H.E.S.S. d
H.E.S.S. - low-energy analysis N/102
Systematic error d
Systematic error - low-energy analysis 3 E
Broken power-law fit
102 103 ATIC
Energy (GeV) PPB-BETS
Kobayashi
H.E.S.S.
Fig.2. The energy spectrum E3 dN/dE of cosmic-ray H.E.S.S. - low-energy analysis
Background model
electrons as measured by ATIC (Changetal.2008),
KK signature, smeared with
PPB-BETS (Toriietal.2008), emulsion chamber experi- H.E.S.S. energy resolution
Sum of background model
ments (Kobayashietal.2004), FERMI (Abdoetal.2009)
and KK signature
(the gray band shows the FERMI systematic uncertainty,
the double arrow labeled with −+150%% the uncertainty of 102 103
the FERMI energy scale), and H.E.S.S. Previous H.E.S.S. Energy (GeV)
data(Aharonianetal.2008)areshownasbluepoints,theresult
Fig.3. The energy spectrum E3 dN/dE of cosmic-ray electrons
of the low-energy analysis presented here as red points. The
measuredbyH.E.S.S.andballoonexperiments.Alsoshownare
shadedbandsindicate the approximatesystematic errorarising
calculations for a Kaluza-Klein signature in the H.E.S.S. data
from uncertainties in the modeling of hadronic interactions
withamassof620GeVandafluxasdeterminedfromtheATIC
and in the atmospheric model in the two analyses. The double
data (dashed-dotted line), the background model fitted to low-
arrow indicates the effect of an energy scale shift of 15%, the
energy ATIC and high-energy H.E.S.S. data (dashed line) and
approximate systematic uncertainty on the H.E.S.S. energy
thesumofthetwocontributions(solidline).Theshadedregions
scale.Thefitfunctionisdescribedinthetext.
representtheapproximatesystematicerrorasinFig.2.
band is centered aroundthe brokenpower law fit. The system-
FERMI data,we also putthe Kaluza-Klein(KK)interpretation
atic erroron thespectralindicesΓ , Γ is ∆Γ(syst.) . 0.3.The
1 2 suggested by Changetal.2008 to test: A model calculation of
H.E.S.S. energy scale uncertainty of 15% is visualized by the
howthe thereinproposedKK particlewith a massof620GeV
doublearrow.
andafluxapproximatedtofittheATICdatawouldappearinthe
H.E.S.S. data is shownin Fig. 3. Here electronair showersare
simulatedwithanenergydistributionfollowingtheenergyspec-
3. Interpretation
trumofthe KK signaturepresentedbythe ATICcollaboration.
The H.E.S.S. measurement yields a smooth spectrum with The simulated eventsand their energyare reconstructedby the
a steepening towards higher energies, confirming the earlier H.E.S.S. data analysis. With the use of the effective collection
findingsabove600GeV(Aharonianetal.2008). areaandthe “observationtime”thatthe numberofsimulations
WhencomparedtoATIC,theH.E.S.S.datashownoindication corresponds to, the KK spectrum is obtained as it would be
of an excess and sharp cutoff in the electron spectrum as resolved by H.E.S.S. Due to the limited energy resolution of
reported by the ATIC collaboration. Since H.E.S.S. measures about15%, a sharp cutoffat the energyofthe KK mass would
the electronspectrumonly above340 GeV, one cannottest the havebeensmearedout.Theresidualbackgroundspectrumtoa
rising section of the ATIC-reported excess. Although different KK signal is modeled by a power law with exponentialcutoff,
inshape,anoverallconsistencyoftheATICspectrumwith the whichisfittedtothelow-energyATICdata(E <300GeV)and
H.E.S.S. result can be obtained within the uncertainty of the thehigh-energyH.E.S.S.data(E >700GeV).Accordingly,our
H.E.S.S.energyscaleofabout15%.Thedeviationbetweenthe background spectrum deviates from the GALPROP prediction
ATIC and the H.E.S.S. data is minimal at the 20% confidence asusedinChangetal.2008.Fixingthebackgroundspectrumto
level (assuming Gaussian errors for the systematic uncertainty most recent observational data is preferable since the Galactic
dominating the H.E.S.S. measurement) when applying an electronspectrumat highestenergiesmightcarry the signature
upward shift of 10% in energy to the H.E.S.S. data. The shift of nearby electron sources (Pohl&Esposito1998) and can
is well within the uncertainty of the H.E.S.S. energy scale. therefore differ substantially from the model calculation. The
In this case the H.E.S.S. data overshoot the measurement of sumoftheKKsignalandelectronbackgroundspectrumabove
balloon experiments above 800 GeV, but are consistent given 340 GeV is shown as solid curve in Fig. 3. The shape of the
the large statistical errors from balloon experiments at these predictedspectrumforthecaseofaKKsignalisnotcompatible
4 Aharonianetal.:ProbingtheATICpeakintheCRe±spectrumwithH.E.S.S.
withtheH.E.S.S.dataatthe99%confidencelevel. 11 Laboratoire d’Annecy-le-Vieux de Physique des Particules,
Despitesuperiorstatistics,theH.E.S.S.datadonotruleoutthe Universite´ de Savoie, CNRS/IN2P3, 9 Chemin de Bellevue - BP
existence of the ATIC-reported excess owing to a possible en- 110F-74941Annecy-le-VieuxCedex,France
ergyscaleshiftinherenttothepresentedmeasurement.Whereas 12 Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7
Denis Diderot, 10, rue Alice Domon et Leonie Duquet, F-75205
compatibility with FERMI and ATIC data is confirmed, the
Paris Cedex 13, France UMR 7164 (CNRS, Universite´ Paris VII,
KK scenario of Changetal.2008 cannot be easily reconciled
CEA,ObservatoiredeParis)
withthe H.E.S.S.measurement.Thespectrumratherexhibitsa
13 DublinInstituteforAdvancedStudies,5MerrionSquare,Dublin2,
steepeningtowardshigherenergiesandisthereforecompatible
Ireland
with conventional electron populations of astrophysical origin 14 Landessternwarte, Universita¨t Heidelberg, Ko¨nigstuhl, D 69117
within the uncertainties related to the injection spectra and Heidelberg,Germany
propagationeffects. 15 Laboratoire de Physique The´orique et Astroparticules, Universite´
Montpellier 2, CNRS/IN2P3, CC 70, Place Euge`ne Bataillon, F-
The support of the Namibian authorities and of the 34095MontpellierCedex5,France
University of Namibia in facilitating the construction and op- 16 Universita¨t Erlangen-Nu¨rnberg, Physikalisches Institut, Erwin-
Rommel-Str.1,D91058Erlangen,Germany
erationofH.E.S.S.isgratefullyacknowledged,asisthesupport
17 Laboratoired’AstrophysiquedeGrenoble,INSU/CNRS,Universite´
by the German Ministry for Education and Research (BMBF),
JosephFourier,BP53,F-38041GrenobleCedex9,France
the Max Planck Society, the French Ministry for Research,
18 Institut fu¨r Astronomie und Astrophysik, Universita¨t Tu¨bingen,
the CNRS-IN2P3 and the Astroparticle Interdisciplinary
Sand1,D72076Tu¨bingen,Germany
Programme of the CNRS, the U.K. Science and Technology 19 LPNHE,Universite´ PierreetMarieCurieParis6,Universite´ Denis
Facilities Council (STFC), the IPNP of the Charles University, Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris
thePolishMinistryofScienceandHigherEducation,theSouth Cedex5,France
African Department of Science and Technology and National 20 CharlesUniversity,FacultyofMathematicsandPhysics,Instituteof
Research Foundation, and by the University of Namibia. We ParticleandNuclearPhysics,VHolesˇovicˇka´ch2,18000Prague8,
appreciate the excellent work of the technical support staff in CzechRepublic
21 Institut fu¨r Theoretische Physik, Lehrstuhl IV: Weltraum und
Berlin,Durham,Hamburg,Heidelberg,Palaiseau,Paris,Saclay,
Astrophysik, Ruhr-Universita¨t Bochum, D 44780 Bochum,
and in Namibia in the constructionand operationof the equip-
Germany
ment.
22 UniversityofNamibia,PrivateBag13301,Windhoek,Namibia
23 ObserwatoriumAstronomiczne,UniwersytetJagiellon´ski,Krako´w,
Poland
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1 Max-Planck-Institut fu¨r Kernphysik, P.O. Box 103980, D 69029
Heidelberg,Germany
2 Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036
Yerevan,Armenia
3 Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, 9 av. du
ColonelRoche,BP4346,F-31029ToulouseCedex4,France
4 Universita¨t Hamburg, Institut fu¨r Experimentalphysik, Luruper
Chaussee149,D22761Hamburg,Germany
5 Institutfu¨rPhysik,Humboldt-Universita¨tzuBerlin,Newtonstr.15,
D12489Berlin,Germany
6 LUTH, Observatoire de Paris, CNRS, Universite´ Paris Diderot, 5
PlaceJulesJanssen,92190Meudon,France
7 IRFU/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette, Cedex,
France
8 UniversityofDurham,DepartmentofPhysics,SouthRoad,Durham
DH13LE,U.K.
9 Unit for Space Physics, North-West University, Potchefstroom
2520,SouthAfrica
10 LaboratoireLeprince-Ringuet,EcolePolytechnique,CNRS/IN2P3,
F-91128Palaiseau,France