Issue |
A&A
Volume 508, Number 2, December III 2009
|
|
---|---|---|
Page(s) | 561 - 564 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361/200913323 | |
Published online | 21 October 2009 |
A&A 508, 561-564 (2009)
Probing the ATIC peak in the cosmic-ray electron spectrum with H.E.S.S.
F. Aharonian1,13
- A. G. Akhperjanian2 -
G. Anton16 - U. Barres de
Almeida8,
- A. R. Bazer-Bachi3 -
Y. Becherini12 - B. Behera14
- K. Bernlöhr1,5 - A. Bochow1
- C. Boisson6 - J. Bolmont19
- V. Borrel3 - J. Brucker16
- F. Brun19 - P. Brun7
- R. Bühler1 - T. Bulik24
- I. Büsching9 - 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-Ataï12 -
W. Domainko1 -
L. O'C. Drury13 -
F. Dubois11 - G. Dubus17
- J. Dyks24 - M. Dyrda28
- K. Egberts1
- D. Emmanoulopoulos14 -
P. Espigat12 - C. Farnier15
- F. Feinstein15 - A. Fiasson11
- A. Förster1 - G. Fontaine10
- M. Füßling5 - S. Gabici13
- Y. A. Gallant15 -
L. Gérard12 - D. Gerbig21
- B. Giebels10 -
J. F. Glicenstein7 -
B. Glück16 - P. Goret7
- D. Göring16 - D. Hauser14
- M. Hauser14 - S. Heinz16
- G. Heinzelmann4 - G. Henri17
- G. Hermann1 -
J. A. Hinton25 -
A. Hoffmann18 - W. Hofmann1 - M. Holleran9
- S. Hoppe1 - D. Horns4
- A. Jacholkowska19 -
O. C. de Jager9 - C.
Jahn16 - I. Jung16
- K. Katarzynski27 - U. Katz16
- S. Kaufmann14 - E. Kendziorra18
- M. Kerschhaggl5 -
D. Khangulyan1 - B. Khélifi10
- D. Keogh8 - W. Kluzniak24
- 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 -
R. Moderski24 - E. Moulin7
- M. Naumann-Godo10 -
M. de Naurois19 -
D. Nedbal20 - D. Nekrassov1
- B. Nicholas26 - J. Niemiec28
- S. J. Nolan8 -
S. Ohm1 - J.-F. Olive3
- E. de Oña Wilhelmi1,12,29 -
K. J. Orford8 -
M. Ostrowski23 - M. Panter1
- M. Paz Arribas5 -
G. Pedaletti14 - G. Pelletier17
- P.-O. Petrucci17 - S. Pita12
- G. Pühlhofer14 - M. Punch12
- A. Quirrenbach14 -
B. C. Raubenheimer9 -
M. Raue1,29 -
S. M. Rayner8 -
O. Reimer30 - M. Renaud1
- F. Rieger1,29 - J. Ripken4
- L. Rob20 - S. Rosier-Lees11
- G. Rowell26 - B. Rudak24
- C. B. Rulten8 -
J. Ruppel21 - V. Sahakian2
- A. Santangelo18 -
R. Schlickeiser21 -
F. M. Schöck16 -
R. Schröder21 - U. Schwanke5
- S. Schwarzburg 18 -
S. Schwemmer14 - A. Shalchi21
- M. Sikora24 - J. L. Skilton25
- H. Sol6 - D. Spangler8
-
.
Stawarz23 - R. Steenkamp22
- C. Stegmann16 - F. Stinzing16
- G. Superina10 - A. Szostek23,17
- P. H. Tam14 -
J.-P. Tavernet19 - R. Terrier12
- O. Tibolla1 - M. Tluczykont4
- C. van Eldik1 -
G. Vasileiadis15 - C. Venter9
- L. Venter6 -
J. P. Vialle11 -
P. Vincent19 - M. Vivier7
- H. J. Völk1 -
F. Volpe1 -
S. J. Wagner14 -
M. Ward8 -
A. A. Zdziarski24 -
A. Zech6
1 - Max-Planck-Institut für Kernphysik, PO Box 103980, 69029
Heidelberg, Germany
2 - Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036
Yerevan,
Armenia
3 - Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du
Colonel Roche, BP
4346, 31029 Toulouse Cedex 4, France
4 - Universität Hamburg, Institut für Experimentalphysik, Luruper
Chaussee
149, 22761 Hamburg, Germany
5 - Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15,
12489 Berlin, Germany
6 - LUTH, Observatoire de Paris, CNRS, Université Paris Diderot,
5 place Jules Janssen, 92190 Meudon, France
7 - IRFU/DSM/CEA, CE Saclay, 91191
Gif-sur-Yvette, Cedex, France
8 - University of Durham, Department of Physics, South Road, Durham DH1
3LE,
UK
9 - Unit for Space Physics, North-West University, Potchefstroom 2520,
South Africa
10 - Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3,
91128 Palaiseau, France
11 - Laboratoire d'Annecy-le-Vieux de Physique des Particules,
Université de Savoie, CNRS/IN2P3,
9 chemin de Bellevue, BP 110, 74941 Annecy-le-Vieux Cedex,
France
12 - Astroparticule et Cosmologie (APC), CNRS, Université Paris 7 Denis
Diderot,
10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France, UMR
7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris)
13 - Dublin Institute for Advanced Studies, 5 Merrion Square,
Dublin 2,
Ireland
14 - Landessternwarte, Universität Heidelberg, Königstuhl, 69117
Heidelberg, Germany
15 - Laboratoire de Physique Théorique et Astroparticules, Université
Montpellier 2, CNRS/IN2P3, CC 70, Place Eugène Bataillon, 34095
Montpellier Cedex 5, France
16 - Universität Erlangen-Nürnberg, Physikalisches Institut,
Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
17 - Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université
Joseph Fourier, BP
53, 38041 Grenoble Cedex 9, France
18 - Institut für Astronomie und Astrophysik, Universität Tübingen,
Sand 1, 72076 Tübingen, Germany
19 - LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis
Diderot
Paris 7, CNRS/IN2P3, 4 place Jussieu, 75252 Paris Cedex 5,
France
20 - Charles University, Faculty of Mathematics and Physics, Institute
of Particle and Nuclear Physics, V Holesovickách 2, 180 00
Prague 8, Czech Republic
21 - Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und
Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
22 - University of Namibia, Private Bag 13301, Windhoek, Namibia
23 - Obserwatorium Astronomiczne, Uniwersytet Jagiellonski, Kraków,
Poland
24 - Nicolaus Copernicus Astronomical Center, ul. Bartycka 18,
00-716 Warsaw,
Poland
25 - School of Physics & Astronomy, University of Leeds, Leeds
LS2 9JT, UK
26 - School of Chemistry & Physics, University of Adelaide,
Adelaide 5005, Australia
27 - Torun Centre for Astronomy, Nicolaus Copernicus University,
ul. Gagarina 11, 87-100 Torun, Poland
28 - Instytut Fizyki Jadrowej PAN, ul. Radzikowskiego 152, 31-342
Kraków,
Poland
29 - European Associated Laboratory for Gamma-Ray Astronomy, jointly
supported by CNRS and MPG
30 - Stanford University, HEPL & KIPAC, Stanford, CA
94305-4085, USA
Received 19 September 2009 / Accepted 18 October 2009
Abstract
The measurement of an excess in the cosmic-ray electron spectrum
between 300 and 800 GeV by the
ATIC experiment has - together with the PAMELA detection of a rise
in the positron fraction up to 100 GeV - motivated
many interpretations
in terms of dark matter scenarios; alternative explanations assume a
nearby electron source like a pulsar or supernova remnant.
Here we present a measurement of the cosmic-ray electron spectrum with
H.E.S.S. starting at 340 GeV.
While the overall electron flux measured by
H.E.S.S. is consistent with the ATIC data within statistical and
systematic errors, the H.E.S.S. data exclude a pronounced peak in the
electron spectrum as suggested for interpretation by ATIC. The H.E.S.S.
data follow a power-law spectrum with spectral index of
,
which steepens at about 1 TeV.
Key words: cosmic rays - methods: data analysis
1 Introduction
Very-high-energy ( GeV)
cosmic-ray electrons
lose their energy rapidly via inverse Compton scattering and
synchrotron radiation resulting in short cooling time and hence range.
Therefore, they must come
from a few nearby sources (Kobayashi et al. 2004; Aharonian
et al. 1995; Shen 1970).
Recently, the ATIC collaboration reported the measurement of
an excess in the electron spectrum (Chang
et al. 2008). The excess appears as a peak in E3
where
is the differential electron flux; it can be approximated as a
component with a power law index around 2 and a sharp cutoff around
620 GeV. Combined with the excess in the positron
fraction measured by PAMELA (Adriani
et al. 2009), the peak feature of the ATIC
measurement has been interpreted in terms of a dark matter signal or a
contribution of a nearby pulsar (e.g. Malyshev
et al. 2009, and references
given there). In the case of dark matter, the structure in the electron
spectrum can be explained as
caused by dark matter annihilation into low multiplicity
final states, while in the case of a pulsar scenario the structure
arises from a competition between energy loss processes of pulsar
electrons (which impose an energy cutoff depending on pulsar age) and
energy-dependent diffusion (which favors high-energy particles in case
of more distant pulsars).
The possibility to distinguish between a nearby electron source and a dark matter explanation with imaging atmospheric Cherenkov telescopes has been discussed by Hall & Hooper (2009). Imaging atmospheric Cherenkov telescopes have five orders of magnitude larger collection areas than balloon and satellite experiments and can therefore measure TeV electrons with excellent statistics. Hall and Hooper assume that a structure in the electron spectrum should be visible even in the presence of a strong background of misidentified nucleonic cosmic rays. However, the assumption of a smooth background is oversimplified; in typical analyses the background rejection varies strongly with energy and without reliable control or better subtraction of the background, decisive results are difficult to achieve. In a recent publication, the High Energy Stereoscopic System (H.E.S.S.) collaboration has shown that such a subtraction is indeed possible, reporting a measurement of the electron spectrum in the range of 700 GeV to 5 TeV (Aharonian et al. 2008).
2 The low-energy extension of the H.E.S.S. electron measurement
Here an extension of the H.E.S.S. measurement towards lower energies is
presented, partially covering the range of the reported ATIC excess.
H.E.S.S. (Hinton 2004)
is a system
of four imaging atmospheric Cherenkov telescopes in Namibia.
While designed for the measurement of -ray initiated air-showers,
it can be used to measure cosmic-ray electrons as well.
The basic properties of
the analysis of cosmic-ray electrons with H.E.S.S. have been presented
in Aharonian et al. (2008).
For the analysis, data from extragalactic fields (with a minimum of 7
above or below the Galactic plane) are used excluding any known
or potential
-ray
source in order to avoid an almost
indistinguishable
-ray
contribution to the electron signal.
As the diffuse extragalactic
-ray background is strongly
suppressed by pair creation on cosmic radiation fields (Coppi & Aharonian 1997),
its contribution to the measured flux can be estimated following Coppi & Aharonian (1997) to
be less than
,
assuming a blazar spectrum of an unbroken powerlaw
up to 3 TeV with a Gaussian spectral index distribution
centered at
with
.
For an improved rejection of the hadronic background a Random Forest
algorithm (Breiman &
Cutler 2004) is used. The algorithm uses image information
to estimate the electron likeness
of
each event. Since
some of the image parameters used to derive the
parameter are energy
dependent, also
depends
on energy.
To derive an electron spectrum, a cut on
of
is applied and the number of electrons is
determined in independent energy bands by a fit of the distribution in
with
contributions of simulated electrons and protons.
The contribution of heavier nuclei is sufficiently suppressed for
as not to
play a role.
The result does not depend on the particular choice of
.
For an extension of the spectrum towards lower
energies, the analysis has been modified to improve the sensitivity at
low energies.
In the event selection cuts, the minimum image amplitude
has been reduced from 200 to 80 photo
electrons to allow for lower energy events.
In order to guarantee good shower reconstruction, only events with a
reconstructed distance from the projected core position on the ground
to the array center of less
than 100 m are included.
Additionally, only data taken between 2004 and 2005 are used. The
reason is that the H.E.S.S. mirror reflectivity
degrades over time and a reduced light yield corresponds to an
increased energy threshold.
The new data and event selection reduces the event statistics but
enables to lower
the analysis threshold to 340 GeV. The effective collection
area at 340 GeV is
m2.
With a live-time of 77 h of good quality data, a
total effective exposure of
m2 sr s
is achieved at 340 GeV. Owing to the steepness of the electron
spectrum, the measurement at lower
energies is facilitated by the comparatively higher fluxes.
The
distribution
in the energy range of 340 to 700 GeV is shown
in Fig. 1.
![]() |
Figure 1:
The measured distribution of the parameter |
Open with DEXTER |
![]() |
Figure 2:
The energy spectrum E3 dN/dE
of cosmic-ray electrons as measured by ATIC (Chang et al. 2008),
PPB-BETS (Torii
et al. 2008), emulsion chamber experiments (Kobayashi et al. 2004),
FERMI (Abdo et al. 2009)
(the gray band shows the FERMI systematic uncertainty, the double arrow
labeled with |
Open with DEXTER |
The low-energy electron spectrum resulting from this analysis is shown
in Fig. 2
together with previous data of H.E.S.S. and direct measurements.
The spectrum is well described by a broken power law
(
,
p=0.23) with a normalization
TeV-1 m-2 sr-1 s-1,
and a break energy
TeV,
where the transition between the two spectral indices
and
occurs. The parameter
denotes the sharpness of the transition, the fit prefers a sharp
transition,
.
The shaded band indicates the uncertainties in the flux normalization
that arise from uncertainties in the modeling of hadronic interactions
and
in the atmospheric model. The uncertainties amount to about 30% and are
derived in the same fashion as in the initial paper (Aharonian et al. 2008),
i.e. by comparison of the spectra derived from two independent data
sets taken in summer and autumn 2004 for the effect of atmospheric
variations and by comparison of the spectra derived using the SIBYLL
and QGSJET-II hadronic interaction model for the effect of the
uncertainties in the proton simulations.
The band is centered around the broken power law fit.
The systematic error on the spectral indices
,
is
.
The H.E.S.S. energy scale uncertainty of
is visualized by the
double arrow.
3 Interpretation
The H.E.S.S. measurement yields a smooth spectrum with a steepening towards higher energies, confirming the earlier findings above 600 GeV (Aharonian et al. 2008).
When compared to ATIC, the H.E.S.S. data show no indication of
an excess and sharp cutoff in the electron spectrum as reported by the
ATIC collaboration. Since H.E.S.S. measures the electron spectrum only
above 340 GeV, one cannot test the rising section of the
ATIC-reported excess.
Although different in shape, an overall consistency of the ATIC
spectrum with the H.E.S.S. result can be obtained within the
uncertainty of the H.E.S.S. energy scale of about .
The deviation between the ATIC and the H.E.S.S. data is minimal at the
confidence level (assuming Gaussian errors for the systematic
uncertainty dominating the H.E.S.S. measurement) when applying an
upward shift of
in energy to the H.E.S.S. data. The shift is well within the
uncertainty of the H.E.S.S. energy scale. In this case the H.E.S.S.
data overshoot the measurement of balloon experiments above
800 GeV, but are consistent given the large statistical errors
from balloon experiments at these energies. However, the nominal
H.E.S.S. data are in very good agreement with the high precision FERMI
measurement up to 1 TeV. The combined
H.E.S.S. and FERMI measurements make a feature in the electron spectrum
in the region of overlap of both experiments rather unlikely.
![]() |
Figure 3: The energy spectrum E3 dN/dE of cosmic-ray electrons measured by H.E.S.S. and balloon experiments. Also shown are calculations for a Kaluza-Klein signature in the H.E.S.S. data with a mass of 620 GeV and a flux as determined from the ATIC data (dashed-dotted line), the background model fitted to low-energy ATIC and high-energy H.E.S.S. data (dashed line) and the sum of the two contributions (solid line). The shaded regions represent the approximate systematic error as in Fig. 2. |
Open with DEXTER |

Despite superior statistics, the H.E.S.S. data do not rule out the existence of the ATIC-reported excess owing to a possible energy scale shift inherent to the presented measurement. Whereas compatibility with FERMI and ATIC data is confirmed, the KK scenario of Chang et al. (2008) cannot be easily reconciled with the H.E.S.S. measurement. The spectrum rather exhibits a steepening towards higher energies and is therefore compatible with conventional electron populations of astrophysical origin within the uncertainties related to the injection spectra and propagation effects.
AcknowledgementsThe support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Science and Technology Facilities Council (STFC), the IPNP of the Charles University, the Polish Ministry of Science and Higher Education, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment.
References
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Footnotes
- ...
- Supported by CAPES Foundation, Ministry of Education of Brazil.
- ... electrons
- The term electrons is used generically in the following to refer to both electrons and positrons since most experiments do not discriminate between particle and antiparticle.
All Figures
![]() |
Figure 1:
The measured distribution of the parameter |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
The energy spectrum E3 dN/dE
of cosmic-ray electrons as measured by ATIC (Chang et al. 2008),
PPB-BETS (Torii
et al. 2008), emulsion chamber experiments (Kobayashi et al. 2004),
FERMI (Abdo et al. 2009)
(the gray band shows the FERMI systematic uncertainty, the double arrow
labeled with |
Open with DEXTER | |
In the text |
![]() |
Figure 3: The energy spectrum E3 dN/dE of cosmic-ray electrons measured by H.E.S.S. and balloon experiments. Also shown are calculations for a Kaluza-Klein signature in the H.E.S.S. data with a mass of 620 GeV and a flux as determined from the ATIC data (dashed-dotted line), the background model fitted to low-energy ATIC and high-energy H.E.S.S. data (dashed line) and the sum of the two contributions (solid line). The shaded regions represent the approximate systematic error as in Fig. 2. |
Open with DEXTER | |
In the text |
Copyright ESO 2009
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