arXiv:1301.1173v1 [astro-ph.HE] 7 Jan 2013
Search for photon line-like signatures from Dark Matter annihilations with H.E.S.S.
A. Abramowski,
1
F. Acero,
2
F. Aharonian,
3, 4, 5
A.G. Akhperjanian,
6, 5
G. Anton,
7
S. Balenderan,
8
A. Balzer,
9, 10
A. Barnacka,
11, 12
Y. Becherini,
13, 14
J. Becker Tjus,
15
K. Bernl¨ohr,
3, 16
E. Birsin,
16
J. Biteau,
14
A. Bochow,
3
C. Boisson,
17
J. Bolmont,
18
P. Bordas,
19
J. Brucker,
7
F. Brun,
14
P. Brun,
12
T. Bulik,
20
S. Carrigan,
3
S. Casanova,
21, 3
M. Cerruti,
17
P.M. Chadwick,
8
R.C.G. Chaves,
12, 3
A. Cheesebrough,
8
S. Colafrancesco,
22
G. Cologna,
23
J. Conrad,
24
C. Couturier,
18
M. Dalton,
16, 25, 26
M.K. Daniel,
8
I.D. Davids,
27
B. Degrange,
14
C. Deil,
3
P. deWilt,
28
H.J. Dickinson,
24
A. Djannati-Ata¨ı,
13
W. Domainko,
3
L.O’C. Drury,
4
G. Dubus,
29
K. Dutson,
30
J. Dyks,
11
M. Dyrda,
31
K. Egberts,
32
P. Eger,
7
P. Espigat,
13
L. Fallon,
4
C. Farnier,
24
S. Fegan,
14
F. Feinstein,
2
M.V. Fernandes,
1
D. Fernandez,
2
A. Fiasson,
33
G. Fontaine,
14
A. F¨orster,
3
M. F¨ußling,
16
M. Gajdus,
16
Y.A. Gallant,
2
T. Garrigoux,
18
H. Gast,
3
B. Giebels,
14
J.F. Glicenstein,
12
B. Gl¨uck,
7
D. G¨oring,
7
M.-H. Grondin,
3, 23
S. H¨affner,
7
J.D. Hague,
3
J. Hahn,
3
D. Hampf,
1
J. Harris,
8
S. Heinz,
7
G. Heinzelmann,
1
G. Henri,
29
G. Hermann,
3
A. Hillert,
3
J.A. Hinton,
30
W. Hofmann,
3
P. Hofverberg,
3
M. Holler,
10
D. Horns,
1
A. Jacholkowska,
18
C. Jahn,
7
M. Jamrozy,
34
I. Jung,
7
M.A. Kastendieck,
1
K. Katarzy´nski,
35
U. Katz,
7
S. Kaufmann,
23
B. Kh´elifi,
14
S. Klepser,
9
D. Klochkov,
19
W. Klu´zniak,
11
T. Kneiske,
1
Nu. Komin,
33
K. Kosack,
12
R. Kossakowski,
33
F. Krayzel,
33
P.P. Kr¨uger,
21, 3
H. Laffon,
14
G. Lamanna,
33
J. Lefaucheur,
13
M. Lemoine-Goumard,
25
J.-P. Lenain,
13
D. Lennarz,
3
T. Lohse,
16
A. Lopatin,
7
C.-C. Lu,
3
V. Marandon,
3
A. Marcowith,
2
J. Masbou,
33
G. Maurin,
33
N. Maxted,
28
M. Mayer,
10
T.J.L. McComb,
8
M.C. Medina,
12
J. M´ehault,
2, 25, 26
U. Menzler,
15
R. Moderski,
11
M. Mohamed,
23
E. Moulin,
12
C.L. Naumann,
18
M. Naumann-Godo,
12
M. de Naurois,
14
D. Nedbal,
36
D. Nekrassov,
3, ∗
N. Nguyen,
1
J. Niemiec,
31
S.J. Nolan,
8
S. Ohm,
30, 3
E. de O˜na Wilhelmi,
3
B. Opitz,
1
M. Ostrowski,
34
I. Oya,
16
M. Panter,
3
R.D. Parsons,
3
M. Paz Arribas,
16
N.W. Pekeur,
21
G. Pelletier,
29
J. Perez,
32
P.-O. Petrucci,
29
B. Peyaud,
12
S. Pita,
13
G. P¨uhlhofer,
19
M. Punch,
13
A. Quirrenbach,
23
M. Raue,
1
A. Reimer,
32
O. Reimer,
32
M. Renaud,
2
R. de los Reyes,
3
F. Rieger,
3
J. Ripken,
24
L. Rob,
36
S. Rosier-Lees,
33
G. Rowell,
28
B. Rudak,
11
C.B. Rulten,
8
V. Sahakian,
6, 5
D.A. Sanchez,
3
A. Santangelo,
19
R. Schlickeiser,
15
A. Schulz,
9
U. Schwanke,
16
S. Schwarzburg,
19
S. Schwemmer,
23
F. Sheidaei,
13, 21
J.L. Skilton,
3
H. Sol,
17
G. Spengler,
16
L. Stawarz,
34
R. Steenkamp,
27
C. Stegmann,
10, 9
F. Stinzing,
7
K. Stycz,
9
I. Sushch,
16
A. Szostek,
34
J.-P. Tavernet,
18
R. Terrier,
13
M. Tluczykont,
1
C. Trichard,
33
K. Valerius,
7
C. van Eldik,
7, 3, †
G. Vasileiadis,
2
C. Venter,
21
A. Viana,
12
P. Vincent,
18
H.J. V¨olk,
3
F. Volpe,
3
S. Vorobiov,
2
M. Vorster,
21
S.J. Wagner,
23
M. Ward,
8
R. White,
30
A. Wierzcholska,
34
D. Wouters,
12
M. Zacharias,
15
A. Zajczyk,
11, 2
A.A. Zdziarski,
11
A. Zech,
17
and H.-S. Zechlin
1
(H.E.S.S. Collaboration)
1
Universit¨at Hamburg, Institut f¨ur Experimentalphysik,
Luruper Chaussee 149, D 22761 Hamburg, Germany
2
Laboratoire Univers et Particules de Montpellier,
Universit´e Montpellier 2, CNR S/IN2P3, CC 72,
Place Eug`ene Bataillon, F-34095 Montpellier Cedex 5, France
3
Max-Planck-Institut f¨ur Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany
4
Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dubli n 2, Ireland
5
National Academy of Sciences of the Republic of Armenia, Yerevan
6
Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
7
Universit¨at Erlangen-N¨urnberg, Physikalisches Institut,
Erwin-Rom mel-Str. 1, D 91058 Erlangen, Germany
8
University of Durham, Department of Physics, South Road, Durham DH1 3LE, U .K.
9
DESY, D-15735 Zeuthen, Germany
10
Institut f¨ur Physik und Astronomie, Universit¨at Potsdam,
Karl-Liebknecht-Strasse 24/25, D 14476 Potsdam, Germany
11
Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
12
CEA Saclay, DSM/Irfu, F-91191 Gi f-Sur-Yvette C edex, France
13
APC, AstroParticule et Cosmologie, Universit´e Paris Diderot,
CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cit´e,
10, rue Alice Domon et L´eonie Duquet, 75205 Paris Cedex 13, France,
14
Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
15
Institut f¨ur Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik,
Ruhr-Universit¨at Bochum, D 44780 Bochum, Germany
16
Institut f¨ur Physik, Humboldt-Universit¨at zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany
17
LUTH, Observatoire de Paris, CNRS, Universit´e Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
2
18
LPNHE, Universit´e Pierre et Marie Curie Paris 6,
Universit´e D enis Diderot Paris 7, CNRS/IN2P3,
4 Place Jussieu, F-75252, Paris Cedex 5, France
19
Institut f¨ur Astronomie und Astrophysik, Universit¨at T¨ubingen, Sand 1, D 72076 T¨ubingen, Germany
20
Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
21
Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
22
School of Physics, University of the Witwatersrand,
1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2050 South Africa
23
Landessternwarte, Universit¨at Heidelberg, K¨onigstuhl, D 69117 Heidelberg, Germany
24
Oskar Klein Centre, Department of Physics, Stockholm University,
Albanova University Center, SE-10691 Stockholm, Sweden
25
Universit´e Bordeaux 1, CNRS/IN2P3, Centre d’
´
Etudes Nucl´eaires de Bordeaux Gradignan, 33175 Gradignan, France
26
Funded by contract ERC-StG-259391 from the European Community,
27
University of Namibia, Department of Physics, Private Bag 13301, Wi ndhoek, Namibia
28
School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
29
UJF-Grenoble 1 / CNRS-INSU, Institut de Plan´etologie et
d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France
30
Department of Physics and Astronomy, The University of Leicester,
University Road, Leicester, LE1 7RH, United Kingdom
31
Instytut Fizyki J¸adrowej PAN, ul. Radzikowskiego 152, 31-342 Krak´ow, Poland
32
Institut f¨ur A stro- und Teilchenphysik, Leopold-Franzens-Universit¨at Innsbruck, A-6020 Innsbruck, Austria
33
Laboratoire d’Annecy-le-Vieux de Physique des Particules,
Universit´e de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France
34
Obserwatorium Astronomiczne, Uniwersytet Jagiello´nski, ul. Orla 171, 30-244 Krak´ow, Poland
35
Toru´n Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toru´n, Poland
36
Charles University, Faculty of Mathematics and Physics,
Institute of Particle and Nuclear Physics, V Holeˇsoviˇck´ach 2, 180 00 Prague 8, Czech Republic
(Dated: January 8, 2013)
Gamma-ray line signatures can be expected in the very-high-energy (VHE; E
γ
> 100 GeV) do-
main due to self-annihilation or decay of dark matter (DM) particles in space. Such a signal would
be readily distinguishable from astrophysical γ-ray sources that in most cases produce continu-
ous spectra which span over several orders of magnitude in energy. Using data collected with the
H.E.S.S. γ-ray instrument, upper limits on line-like emission are obtained in the energy range be-
tween ∼ 500 GeV and ∼ 25 TeV for the central part of th e Milky Way halo and for extragalactic
observations, complementing recent limits obtained with th e Fermi-LAT instrument at lower ener-
gies. No statistically significant signal could be found. For monochromatic γ-ray line emission, flux
limits of (2 × 10
−7
– 2 × 10
−5
) m
−2
s
−1
sr
−1
and (1 × 10
−8
– 2 × 10
−6
) m
−2
s
−1
sr
−1
are obtained for
the central part of the Milky Way halo and extragalactic observations, respectively. For a DM par-
ticle mass of 1 TeV, limits on the velocity-averaged DM annihilation cross section hσvi
χχ→γγ
reach
∼ 10
−27
cm
3
s
−1
, based on the Einasto parametrization of the Galactic DM halo density profile.
INTRODUCTION
In the last few years, imaging atmospheric Cherenkov
telescopes (IACTs) have been used to search for dark
matter (DM) signals in very-high-energy (VHE; E
γ
>
100 GeV) γ rays [1–10]. Objects with large predicted DM
density, like the Galactic centre (GC), the central Galac-
tic halo region (CGH), dwarf galaxies or centres of nearby
galaxies were studied. All such searches concentrated on
the detection of γ rays produced in decays of secondary
particles – mostly neutral mesons – in the process of DM
self-annihilation or decay (see, e. g., [11, 12]). The broad
energy distribution of such γ rays is continuous and there-
fore more difficult to distinguish from γ-ray emission from
astrophysical (particle accelerating) sources, as opposed
to spectral features, which would pose a much more strik-
ing evidence for a DM-induced γ-ray signal. The most
prominent spectral feature is a γ-ray line
1
, which, for
DM self-annihilation into γγ/γZ (and m
χ
≫ m
Z
), is ex-
pected at an energy at or close to the DM particle mass,
E
γ
≈ m
χ
. For a decay χ → γX of a DM particle χ with
m
χ
≫ m
X
, E
γ
≈ m
χ
/2. Such annihilations or decays
are, however, loop-suppressed, since electrically neutral
DM particles do not couple to photons directly. Nonethe-
less, recent theoretical developments show the possibil-
ity of a rather pronounced spectral feature for some im-
plementations of particle physics beyond the Standard
Model (see, e. g., [14]). In these models, spectral signa-
tures may arise due to the radiation of a hard photon
from real or virtual charged particles created in the an-
nihilation process and be a dominant component in the
1
Note, however, that VHE γ-ray line features may also arise due
to unshocked e
+
/e
−
-winds created by pulsars [13].
3
overall γ-ray annihilation spectrum. Here a search for γ-
ray line-like signatures conducted with the H.E.S.S. ex-
periment in the energy range E
γ
∼ 500 GeV − 25 TeV
is reported, complementing a recent search at energies
between 7 GeV and 200 GeV with the Fermi-LAT instru-
ment [15] and studies discussing an indication for a line
feature at an energy of about 130 GeV [16–18].
The search for a DM-induced spectral signature in the
H.E.S.S. data is performed separately for two sky regions
of interest. The first is the CGH, a promising region due
to its proximity and predicted large DM concentration.
Following [8], the search region is defined as a circle of
1
◦
radius centred on the GC, where the Galactic plane is
excluded, by requiring |b| > 0.3
◦
. The second region is
the extragalactic sky covered by H.E.S.S. observations,
with regions containing known VHE γ-ray sources being
excluded from the analysis. For both data sets, the un-
certainty on the strength of a putative DM annihilation
signal is much reduced in comparison to the observations
of centres of galaxies: for the CGH, the very centre is not
considered, thus avoiding a region where the DM profile
is only poorly constrained [8]. For the extragalactic data
set, differences in DM density between individual sub-
structures are averaged out by observing many different
fields of view [19]. One should note, however, that a
potentially large (but highly uncertain) γ-ray flux from
Galactic DM annihilations may contribute to the extra-
galactic analysis [20].
METHODOLOGY AND RESULTS
The CGH data set is composed of 112 h (live time) of
GC observations recorded with the H.E.S.S. VHE γ-ray
instrument (see [21] and references therein) during the
years 2004–2008
2
. The mean distance between the tele-
scope pointing positions and the GC is 0.7
◦
, with a max-
imum of 1.5
◦
[8]. The extragalactic data set comprises
1153 h of H.E.S.S. observations taken during 2004–2007,
targeted at various extragalactic objects. Regions in the
field-of-view (FoV) containing known VHE γ-ray sources
are excluded by masking out a circular region (of radius
0.2
◦
for point sources) around the source position.
Observations with zenith angles larger than 30
◦
are
excluded from the analysis to lower the energy thresh-
old, resulting in a mean zenith angle of 14
◦
(19
◦
) for the
CGH (extragalactic) observations. Only γ-ray-like events
are accepted for which the distance between the recon-
structed γ-ray direction and the observation direction of
the H.E.S.S. array is smaller than 2
◦
, avoiding showers
2
Data from later periods were excluded, since the gradual degra-
dation in time of the optical efficiency of the instrument would
result in an increased energy threshold.
(TeV)
γ
E
1 10
)
-1
sr
-1
s
-2
m
1.7
(TeV
γ
dN/dE
2.7
γ
E
-5
10
-4
10
-3
10
Bckg. P(x)
Bckg. G(x)
Bckg. sum
= 2 TeV)
γ
simulated line (E
Bckg. P(x)
Bckg. G(x)
Bckg. sum
FIG. 1. Reconstructed flux spectrum of the CGH region, us-
ing 25 eq uidistant bins per unit of log
10
(E
γ
). Flux points have
been multiplied by E
2.7
γ
. The d ata consist mostly of hadronic
cosmic ray background events, reconstructed using a γ-ray hy-
pothesis. The spectrum is well described by the parametriza-
tion introduced in Eq. 1, depicted by the black solid line. The
correspondin g χ
2
-test probability is p = 0.34. The two contri-
butions P (x) and G(x) are shown by the dashed-dotted and
the dashed curve, respectively. Note t hat the shape of the
Gaussian function G(x) is much broader than th e expected
monochromatic line feature from DM annihilations. As an
example, the red curve shows the expected signal of a line at
E
γ
= 2 TeV that would be detected with a statistical signifi-
cance of 5 standard deviations above the background.
being reconstructed too close to the edges of the ∼ 5
◦
di-
ameter FoV of the H.E.S.S. cameras [21]. Furthermore,
events are considered only if they pass H.E.S.S. standard
γ-ray selection criteria defined in [21] and triggered all
four telescopes. Only 15 % of the total event sample is
kept by the latter selection. However, compared to the
H.E.S.S. standard analysis, such selection leads to a bet-
ter signal to background ratio and an improved energy
resolution of Gaussian width σ
E
(17 % at 500 GeV and
11 % at 10 TeV), and therefore increases the sensitivity
of the analysis to spectral features by up to 50%. The
energy threshold is 310 GeV (500 GeV) for the CGH (the
extragalactic) data set.
Differential flux spectra are calculated from the re-
constructed event energies separately for the CGH and
extragalactic data sets using zenith angle-, energy- and
offset-dependent effective collection areas from γ-ray sim-
ulations. Since sky regions containing known VHE γ-ray
sources were excluded from the analysis, the spectra con-
sist mostly of γ-ray-like cosmic-ray background events
(and a fraction of ∼ 10% of electrons). These spectra are
well described by the empirical parametrization
dN
dE
γ
= a
0
E
γ
1 TeV
−2.7
[P (x) + βG(x)] , (1)
4
where E
γ
is the reconstructed energy of the event under
γ-ray hypothesis and P (x) = exp (a
1
x + a
2
x
2
+ a
3
x
3
).
G(x) is a Gaussian function with mean µ
x
and rms σ
x
,
and x = log
10
(E
γ
/1 TeV). The free parameters a
0...3
, β,
µ
x
, and σ
x
are optimized simultaneously by a maximum
likelihood approach based on the binned event count
spectrum. Since the number of reconstructed counts n
i
in
energy bin i of the count spectrum is Poisson-distributed,
the log-likelihood function takes the form
ln L =
N
X
i=1
n
i
ln λ
i
− λ
i
,
where λ
i
is the number of counts in bin i that is expected
according to the flux spectrum parametrization given in
Eq. 1, and N is the total number of bins of the count
spectrum. As an example, Fig. 1 shows the differential
flux spectrum and the best-fit background parametriza-
tion obtained for the CGH data set.
On top of the smooth cosmic ray flux spectrum, a
monochromatic γ-ray line
3
may be identified as a Gaus-
sian peak of width σ
E
centred at the line energy E
γ
. To
search for such lines, a Gaussian term with fixed energy
E
γ
and fixed corresponding width σ
E
was added to the
spectrum parametrization given in Eq. 1. The spectrum
was refit, and from the normalization of the Gaussian the
flux of the putative line was reconstructed. By repeat-
ing this procedure, using ten logarithmically equidistant
energies E
γ
per decade of energy, the flux spectrum was
scanned for monochromatic γ-ray signatures. Line scans
were performed in the energy range 0.5 TeV–20 TeV and
0.8 TeV–25 TeV for the CGH and the extragalactic data
sets, respectively.
No γ-ray line flux was found to exceed the a-priori
chosen detection threshold of ∆ ln L = 12.5, correspond-
ing to a significance of 5 standard deviations above the
background level for Gaussian parameters. Thus flux up-
per limits were calculated by constraining the flux nor-
malization of the Gaussian to be non-negative in the fit
and using the MINOS package from the Minuit[22] fit-
ting tool to calculate asymmetric errors with error level
∆ ln L = 1.35, corresponding to a 95% CL one-sided limit
on the flux of the line [15, 23]. These limits are shown
in Fig. 2. To test whether the limits are compatible with
random fluctuations of the background, a large number
of statistically randomized fake background spectra was
simulated using the best-fit background parametrization
as an input, and limits were obtained for each of these
spectra. The resulting mean limits, together with the
68% CL region calculated from the limit distribution at
each test energy, are shown in Fig. 2 for comparison. Also
3
In this context, the term ’monochromatic line’ refers to spec-
tral features wi th energy width much smaller than the energy
resolution σ
E
of the H.E.S.S. instrument.
shown are mean reconstructed fluxes from simulated lines
that are detected with a significance of 5 standard devi-
ations using the above prescription.
Additionally, flux upper limits were determined for
broader spectral features like those arising due to inter-
nal bremsstrahlung (IB). As an example, calculations by
[14] in the framework of supersymmetric models predict
the contribution of IB photons to the γ-ray spectrum
to dominate over secondary γ-ray production for photon
energies close to the DM (neutralino) mass m
χ
. Flux
upper limits for the benchmark models BM2 and BM4
of [14] were calculated following the technique described
above. Firstly, the signal shapes predicted by the models
were convolved with the energy response of the instru-
ment. Together with the background parametrization,
the resulting templates were then fitted (with the nor-
malization of the template and the background parame-
ters being free variables in the fit) to the flux spectrum.
Note that only the IB part of the full annihilation spec-
tra of these models is considered since the contribution
from production of secondary photons steeply decreases
towards m
χ
(see [14]), and is therefore hard to discrim-
inate against the cosmic-ray background. In any case,
since these models were calculated for a very specific set
of MSSM parameters (and hence neutralino mass), they
can only serve as a template to demonstrate the sensi-
tivity of H.E.S.S. for features of similar shape (and are
therefore referred to as BM2-like and BM4-like limits).
Fig. 3 shows that – because of the intrinsic widths of the
expected features – these limits are typically weaker by
a factor two (BM2-like) to ten (BM4-like) compared to
the monochromatic line limits. Note that all flux lim-
its do also constrain putative features in the spectrum of
cosmic ray electrons and positrons, since the H.E.S.S. ex-
periment exhibits a similar sensitivity for detecting these
particles as for γ rays.
Possible systematic uncertainties due to the unknown
shape of the background spectrum have been extensively
studied, e. g. by changing the background parametriza-
tion described in Eq. 1 to one based on Legendre poly-
nomials. The background parametrization does not show
any significant correlation with shape parameters of spec-
tral signatures, in particular with regard to the G(x)
term. The stability of the γ-ray flux reconstruction was
investigated by adding artificial peaks to the background
spectrum and reconstructing them with the fitting pro-
cedure described above. The systematic uncertainty on
the reconstructed peak flux was of the order of a few
percent, and the fit of the background was found to be
very stable and independent of the location and normal-
ization of the artificial peak. On the other hand, despite
detailed Monte-Carlo simulations of the instrument, the
true energy resolution σ
E
of the instrument might be un-
derestimated. When σ
E
is artificially enlarged by e. g.
20 % – i. e. σ
E
= 20 % (13 %) at E
γ
= 500 GeV (10 TeV)
–, upper limits get shifted to larger values by about 15–
5
(TeV)
γ
E
1 10
)
-1
sr
-1
s
-2
(95% CL) (mΦ
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
CGH MC detection
CGH limits
extragalactic limits
FIG. 2. Upper limits on γ-ray flux from monochromatic line
signatures, derived from the CGH region (red arrows with
full data points) and from extragalactic observations (black
arrows with open data points). For both data sets, the solid
black lines show the mean expected limits derived from a large
number of statistically randomized simulations of fake back-
ground spectra, and the gray bands denote the correspond in g
68% CL regions for these limits. Black crosses denote the flux
levels needed for a statistically significant line detection in the
CGH dataset.
(TeV)
χ
m
1 10
)
-1
sr
-1
s
-2
(95% CL) (mΦ
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
CGH BM4-like (IB only)
CGH BM2-like (IB only)
CGH monochromatic
extragalactic BM4-like (IB only)
extragalactic BM2-like (IB only)
extragalactic monochromatic
FIG. 3. Flux upper limits on spectral features arising from
the emission of a hard photon in the DM annihilation pro-
cess. Limits are exemplary shown for features of comparable
shape to those arising in the models BM2 and BM4 given in
[14]. The monochromatic line limits, assuming m
χ
= E
γ
, are
shown for comparison.
20 %, depending on the energy and the statistics in the
individual spectrum bins. The maximum shift is ob-
served in the extragalactic limit curve and amounts to
40 %. In total, the systematic error on the flux upper
limits is estimated to be about 50 %. All flux upper
limits were cross-checked using an alternative analysis
framework [24], with an independent calibration of cam-
era pixel amplitudes, and a different event reconstruction
(TeV)
χ
m
-2
10
-1
10
1 10
/s)
3
(95% CL) (cm
γγ→χχ
v>σ<
-29
10
-28
10
-27
10
-26
10
-25
10
HESS Einasto
Fermi-LAT Einasto
FIG. 4. Limits on the velocity-weighted cross section for DM
annihilation into two photons calculated from the CGH flux
limits (red arrows with full data points). The Einasto density
profile with parameters described in [20] was used. Limits ob-
tained by Fermi-LAT, assuming the Einasto profile as well, are
shown for comparison (black arrows with open data points)
[15].
and event selection method, leading to results well con-
sistent within the quoted systematic error.
For the Einasto parametrization of the DM density
distribution in the Galactic halo [20], limits on the
velocity-weighted DM annihilation cross section into γ
rays, hσvi
χχ→γγ
, are calculated from the CGH flux limits
using the astrophysical factors given in [8]. The result is
shown in Fig. 4 and compared to recent results obtained
at GeV energies with the Fermi-LAT instrument.
SUMMARY AND CONCLUSIONS
For the first time, a search for spectral γ-ray signatures
at very-high energies was performed based on H.E.S.S.
observations of the central Milky Way halo region and ex-
tragalactic sky. Both regions of interest exhibit a reduced
dependency of the putative DM annihilation flux on the
actual DM density profile. Upper limits on monochro-
matic γ-ray line signatures were determined for the first
time for energies between ∼ 500 GeV and ∼ 25 TeV, cov-
ering an important region of the mass range of particle
DM. Additionally, limits were obtained on spectral sig-
natures arising from internal bremsstrahlung processes,
as predicted by the models BM2 and BM4 of [14]. It
should be stressed that the latter results are valid for
all spectral signatures of comparable shape. Besides, all
limits also apply for potential signatures in the spectrum
of cosmic-ray electrons and positrons.
Flux limits on monochromatic line emission from the
central Milky Way halo were used to calculate upper lim-
its on hσvi
χχ→γγ
. Limits are obtained in a neutralino