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Aharonian, F. and Akhperjanian, A. G. and Bazer-Bachi, A. R. and Beilicke, M. and Benbow, W. and Berge,
D. and Bernlohr, K. and Boisson, C. and Bolz, O. and Borrel, V. and Braun, I. and Breitling, F. and Brown,
A. M. and Chadwick, P. M. and Chounet, L. M. and Cornils, R. and Costamante, L. and Degrange, B. and
Dickinson, H. J. and Djannati-Ata, A. and Drury, L. O. and Dubus, G. and Emmanoulopoulos, D. and
Espigat, P. and Feinstein, F. and Fontaine, G. and Fuchs, Y. and Funk, S. and Gallant, Y. A. and Giebels, B.
and Gillessen, S. and Glicenstein, J. F. and Goret, P. and Hadjichristidis, C. and Hauser, D. and Hauser, M.
and Heinzelmann, G. and Henri, G. and Hermann, G. and Hinton, J. A. and Hofmann, W. and Holleran, M.
and Horns, D. and Jacholkowska, A. and de Jager, O. C. and Kheli, B. and Klages, S. and Komin, N. and
Konopelko, A. and Latham, I. J. and Le Gallou, R. and Lemiere, A. and Lemoine-Goumard, M. and Leroy, N.
and Lohse, T. and Marcowith, A. and Martin, J. M. and Martineau-Huynh, O. and Masterson, C. and
McComb, T. J. L. and de Naurois, M. and Nolan, S. J. and Noutsos, A. and Orford, K. J. and Osborne, J. L.
and Ouchrif, M. and Panter, M. and Pelletier, G. and Pita, S. and Puhlhofer, G. and Punch, M. and
Raubenheimer, B. C. and Raue, M. and Raux, J. and Rayner, S. M. and Reimer, A. and Reimer, O. and
Ripken, J and Rob, L. and Rolland, L. and Rowell, G. and Sahakian, V. and Sauge, L. and Schlenker, S. and
Schlickeiser, R. and Schuster, C. and Schwanke, U. and Siewert, M. and Sol, H. and Spangler, D. and
Steenkamp, R. and Stegmann, C. and Tavernet, J. P. and Terrier, R. and Theoret, C. G. and Tluczykont, M.
and van Eldik, C. and Vasileiadis, G. and Venter, C. and Vincent, P. and Volk, H. J. and Wagner, S. J. (2006)
'Discovery of very-high-energy gamma-rays from the Galactic Centre ridge.', Nature., 439 (7077). pp. 695-698.
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http://dx.doi.org/10.1038/nature04467
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arXiv:astro-ph/0603021v1 1 Mar 2006
Discovery of Very-High-Energy γ-Rays from the
Galactic Centre Ridge
F. Aharonian
1
, A.G. Akhperjanian
2
, A.R. Bazer-Bachi
3
, M. Beilicke
4
, W. Benbow
1
, D. Berge
1
,
K. Be rnl¨ohr
1,5
, C. Boisson
6
, O. Bolz
1
, V. Borrel
3
, I. Braun
1
, F. Breitling
5
, A.M. Brown
7
,
P.M. Chadwick
7
, L.-M. Chounet
8
, R. Co rnils
4
, L. Costamante
1,20
, B. Degrange
8
,
H.J. Dickinson
7
, A. Djannati-Ata
¨
i
9
, L.O’C. Drury
10
, G. Dubus
8
, D. Emmanoulopoulo s
11
,
P. Espigat
9,3
, F. Feinstein
12
, G. Fontaine
8
, Y. Fuchs
13
, S. Funk
1
, Y.A. Gallant
12
, B. Giebels
8
,
S. Gillessen
1
, J.F. Glicenstein
14
, P. Goret
14
, C. Hadjichristidis
7
, D. Hauser
1
, M. Hauser
11
,
G. Heinzelmann
4
, G. Henri
13
, G. Hermann
1
, J.A. Hinton
1
, W. Hofmann
1
, M. Holleran
15
,
D. Horns
1
, A. Jacholkowska
12
, O.C. de Jager
15
, B. Kh´elifi
1
, S. Klages
1
, Nu. Komin
5
,
A. Konopelko
5
, I.J. L atham
7
, R. Le Gallou
7
, A. Lemi`ere
9
, M. Lemo ine-Goumard
8
, N. Leroy
8
,
T. Lohse
5
, A. Marcowith
3
, J.M. Martin
6
, O. Martineau-Huynh
16
, C. Ma sterson
1,20
,
T.J.L. McComb
7
, M. de Na urois
16
, S.J. Nolan
7
, A. Noutsos
7
, K.J. Orford
7
, J.L. Osborne
7
,
M. Ouchrif
16,20
, M. Panter
1
, G. Pelletier
13
, S. Pita
9
, G. P¨uhlhofer
11
, M. Punch
9
,
B.C. Raubenheimer
15
, M. Raue
4
, J. Raux
16
, S.M. Rayner
7
, A. Reimer
17
, O. Reimer
17
,
J. Ripken
4
, L. Rob
18
, L. Rolland
16
, G. Rowell
1
, V. Sahakian
2
, L. Saug´e
13
, S. Schlenker
5
,
R. Schlickeiser
17
, C. Schuster
17
, U. Schwanke
5
, M. Siewert
17
, H. Sol
6
, D. Spangler
7
,
R. Steenkamp
19
, C. Stegmann
5
, J.-P. Tavernet
16
, R. Terrier
9
, C.G. Th´eoret
9
, M. Tluczykont
8,20
,
C. van Eldik
1
, G. Vasileiadis
12
, C. Venter
15
, P. Vincent
16
, H.J. V¨olk
1
, S.J. Wagner
11
1
Max-Planck-Institut f¨ur Kernphysik, Heidelberg, Germany
2
Yerevan Physics Institute, Armenia
3
Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France
4
Universit¨at Hamburg, Institut f¨ur Experimentalphysik, Germany
5
Institut f¨u r Physik, Humboldt-Universit¨at zu Berlin, Germany
6
LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, France
7
University of Durham, Department of Physics, U.K.
8
Laboratoire Leprince-Ringuet, IN2P3/CNRS, Ecole Polytechnique, Palaiseau, France
9
APC (UMR 7164, CNRS, Universit´e Paris VII, CEA, Observatoire de Paris), Paris
10
Dublin Institute for Advanced Studies, Ireland
11
Landessternwarte, K¨onigstuhl, D 69117 Heidelberg, Germany
12
Laboratoire de Physique Th´eorique et Astroparticules, IN2P3/CNRS, Universit´e Montpellier II
13
Laboratoire d’Astrophysique de Grenoble, INSU/CNRS, Universit´e Joseph Fourier, France
14
DAPNIA/DSM/CEA, CE Saclay, Gif-sur-Yvette, France
15
Unit for Space Physics, North-West University, Potchefstroom, South Africa
16
Laboratoire de Physique Nucl´eaire et de Hautes Energies, IN2P3/CNRS, Universit´es Paris VI & VII
17
Institut f¨u r Theoretische Physik, Weltraum und Astrophysik, Ruhr-Universit¨at Bochum, Germany
18
Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Repu blic
19
University of Namibia, Windhoek, Namibia
20
European Associated Laboratory for Gamma-Ray A stronomy
The origin of Galactic cosmic rays (with energies up to 10
15
eV) remains unclear,
though it is widely believed that they originate in the shock waves of expanding
supernova remnants [1][2]. Currently the best way to investigate their acceleration
and propagation is by observing the γ-rays produced when cosmic rays interact with
interstellar gas [3]. Here we report observations of an extended region of very high
energy (VHE, >100 GeV) γ-ray emission correlated spatially with a complex of giant
molecular clouds in the central 200 pc of the Milky Way. The hardness of the γ-ray
spectrum and the conditions in those molecular clouds indicate that the cosmic rays
giving rise to the γ-rays are likely to be protons and nuclei rather than electrons.
The energy associated with the cosmic rays could have come from a single supernova
explosion around 10
4
years ago.
1
The observations described here were carried out with the High Energy Stereoscopic System
(H.E.S.S.), a system of four imaging atmospheric-Cherenkov telescopes [4]. The instrument oper-
ates in the Teraelectronvolt energy range (TeV), beyond the regime accessible to satellite-based
detectors (MeV up to ∼10 GeV). At satellite energies, the technique of pro bing the distribution
of cosmic rays (CRs) using γ-ray emission has been demonstrated in the large-scale mapping of
the Galactic plane by EGRET [5]. The γ-r ay flux was found to approximately tra c e the density
of interstellar gas, illustrating that the flux of CRs is roughly constant thr oughout the Galaxy.
However, given its modest angular reso lution (∼ 1
◦
), EGRET could only resolve the few neare st
molecular clouds. The order of magnitude better angular resolution of H.E.S.S. opens up this pos -
sibility of res olving individual clouds out to the distance of the Galactic Centre (GC). Moreover,
in the energy rang e acce ssible to EGRET, the picture is complicated by the contribution of cosmic
electrons [1] to the diffuse γ-ray flux via inverse Compton (IC) scattering and B remsstrahlung.
In the e nergy range of H.E.S.S. the dominant component of the truely diffuse γ-ray emission is
very likely the decay of neutral pions produced in the interactions of CRs with ambient material.
Taken together, the wide field of view (∼5
◦
) and the improved angular resolution (better than
0.1
◦
) of H.E.S.S. have made possible the mapping of extended γ-ray emission.
Early H.E.S.S. observa tions of the GC region led to the detection of a point-like so urce of
VHE γ-r ays at the gravitational centre of the Galaxy (HESS J1745 −290) [6], compatible with the
positions of the supermassive black hole Sagittarius A
⋆
, the supernova remnant (SNR) Sgr A East,
and a GC source reported by other groups [7, 8]. A more sensitive exposure of the region in 2004
revealed a second source: the supernova remnant/pulsar wind nebula G 0.9+0.1 [9]. These two
sources are clearly visible in the upper pa nel of Fig. 1. For pre vious VHE instruments such so urces
were close to the limit of detectability. With the greater sensitivity of the H.E.S.S. instrument it is
possible to subtract these two sources and search for much fainter emission. Subtracting the b e st
fit model for point-like emission at the position of these excesses yields the map shown in Fig. 1
(bottom). Two significant features are appare nt after subtraction: extended emission spatially
coincident with the unidentified EGRET source 3EG J1744-3011 (discussed elsewhere [10]) and
emission e xtending along the Galactic plane for roughly 2
◦
. The latter emission is not only very
clearly extended in longitude l, but also significantly extended in latitude b (beyond the angular
resolution of H.E.S.S.) with a characteristic root mean square (rms) width of 0.2
◦
, as can be seen
in the Galactic latitude slices shown in Fig. 2. The reconstructed γ-ray spectrum for the regio n
−0.8
◦
< l < 0.8
◦
, |b| < 0.3
◦
(with point-source emission subtracted) is well described by a power
law with photon index Γ = 2.29 ± 0.07
stat
± 0.20
sys
(Fig. 3).
Given the pla usible assumption that the γ-ray emission takes place near the centre of the
Galaxy, at a distance of about 8.5 kpc, the observed rms extension in latitude of 0.2
◦
corresponds
to a scale of ≈ 30 pc. This value is similar to that of interstellar material in giant molecular clouds
in this region, as traced by their CO emission and in particular by their CS emission [11]. CS line
emission does not suffer from the problem of ‘standard’ C O lines [12], that clouds are optically
thick for these lines and hence the total mass of clouds may be underestimated. The CS data
suggest that the central region of the Galaxy, |l| < 1.5
◦
and |b| < 0.25
◦
, contains about 3 − 8 × 10
7
solar masses of interstellar gas, structured in a number of overlapping clouds, which provide an
efficient target for the nucleonic CRs permeating these clouds. The r e gion over which the γ-ray
sp e c trum is integrated contains 55% of the CS emission, corresponding to a mass of 1.7−4.4×10
7
solar ma sses. At leas t for |l| < 1
◦
, we find a close ma tch between the distribution of the VHE
γ-ray emission and the density of dense interstellar gas as traced by CS emission (Fig. 1 (bottom)
and Fig. 2).
The clos e correlation between γ-ray emission and availa ble target materia l in the central 200
pc of our galaxy is a strong indication for an origin of this emission in the interactions of CRs.
Following this interpretation, the similarity in the distributions of CS line and VHE γ-ray emission
implies a rather uniform C R density in the region. Since in the case of a power-law energy
distribution the spectral index of the γ-rays closely traces the spectral index of the CRs themselves
(corrections due to scaling violations in the CR interactions are small, ∆Γ < 0.1), the measured
γ-ray spe c trum implies a CR spectrum near the GC with a spectral index close to 2.3, significantly
harder than in the solar neighbourhood (where an index of 2.75 is measured). Given the probable
2
proximity of particle accelerators, pro pagation effects are likely to be less pronounced than in the
Galaxy as a whole, providing a natural explanation for the harder spectrum which is closer to
the intrinsic CR-source spectra. The main uncertainty in estimating the flux of CRs in the GC
is the uncertainty in the amount of target material. Following [3] and using the mass estimate of
Tsuboi [11] we c an estimate the expected γ-ray flux from the region, assuming for the moment that
the GC cosmic-ray flux and spectrum are identical to those measured in the solar neighbourhood.
Fig. 3 shows the expected γ-ray flux as a grey band, together with the observed spectrum. Whilst
below 500 GeV there is reasonable agreement with this simple prediction, there is a clear excess
of high energy γ-rays over expectations. The γ-ray flux above 1 TeV is a factor 3 − 9 higher than
the expected flux. The implication is that the number density of CRs with multi-TeV e nergies
exceeds the local density by the same factor. The size of the e nhancement incre ases rapidly at
energies above 1 TeV.
The observation of correlation between target material and TeV γ-ray emission is unique and
provides a compelling case for an origin of the emission in the interactions of CR nuclei. In addition,
the key experimental facts of a harder than expected spectrum, and a higher than expected TeV
flux, imply that there is an additional component to the GC cosmic-ray population above the
CR ‘sea’ which fills the Galaxy. This is the first time that such direct ev idence for recently
accelerated (hadronic) CRs in any par t of our galaxy has been found. The e nergy required to
accelerate this additional component is estimated to be 10
49
erg in the energy ra nge 4-40 TeV
or ∼ 1 0
50
erg in total if the meas ured spe c trum extends from 10
9
− 10
15
eV. Given a typical
supe rnova explosion energy of 10
51
erg, the observed CR excess could have been produced in a
single SNR, assuming a 10% efficiency for CR acceleration. Following such a scenario, any epoch
of CR production must have occurred in the recent enough past that the CRs accelerated have
not yet diffused out of the GC region. Representing the diffusion of protons with energies of
several TeV in the form D = η10
30
cm
2
s
−1
, where 10
30
cm
2
s
−1
is the approximate value of the
diffusion coefficient in the Galactic Disk at TeV energies, we estimate the diffusion time-scale to
be t = R
2
/2D ≈ 3000(θ / 1
◦
)
2
/η years, where θ is the angular distance from the GC. Due to the
larger magnetic field and higher turbulence in the central region compared to more conventional
regions of the Galactic disk, the normalisation parameter η is likely ≤ 1 and a source or sources
of age ∼10 kyr could fill the region |l| < 1
◦
with CRs. Indeed, the observation of a deficit in VHE
emission at l = 1.3
◦
relative to the available target material (see Fig. 2) suggests that CRs, which
were recently acc elerated in a source or s ources in the GC region, have not yet diffused out beyond
|l| = 1
◦
.
The observed morphology and spectrum of the γ-ray e mission provide evidence that one or
more cosmic-ray accelerators have been active in the GC in the last 10,000 years. The fact that the
diffuse emission exhibits a photon index Γ which is the same - within errors - as that of the central
source HESS J1745−29 0 sugge sts that this o bject could be the source in question. Within the 1
arcminute error box of HESS J1745−290 are two compelling candidates for such a CR accelerator.
The first is the SNR Sgr A East [13] with its estimated age around 10 kyr [14] (younger ages have
been quoted for Sgr A East [15] r eflecting the sig nificant uncertainty in this estimate). The second
is the supermassive black hole Sgr A
⋆
[16, 17] which may have been more active in the past.
A distinct alter native possibility is that a population of electron accelerators produces the
observed γ-ray emissio n via IC scattering. Extended objects with pho ton indices close to the value
2.3 observed in the GC are observed elsewhere in the Galactic pla ne [10]. The parent population
of objects such as pulsar wind nebulae (i.e. massive sta rs) would likely follow approximately the
distribution o f molecular gas. However, in the intense photon fields and high magnetic fields
within and close to the GC molecular clouds [18, 19], TeV electrons would los e their energy
rapidly: t
rad
≈ 120 (B/100 µG)
−2
(E
e
/10 TeV)
−1
years. We would there fo re expect to see rather
compact source s (point-like for H.E.S.S.) which would also b e bright in the X-ray regime (as is
for example G 0.9+0.1). The existence of ∼ 10 such unknown sources in this small region a gain
seems unlikely. Any substantially extended IC source would most likely be a foreground source
along the line-of-sight towards the GC region, making any correlation with GC molecular clouds
entirely coincidental.
3
Acknowledgements
The 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 suppor t by the Ger-
man 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. Particle Physics and Astronomy Research Council (PPARC), the IPNP of the Charles
University, the South African Department of Science and Technology and National Research Foun-
dation, and by the University of Namibia. We would like to thank M. Tsuboi for providing the CS
survey data used here and Y. Moriguchi and Y. Fukui for helpful discussions on molecular tracers.
References
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4
![Figure 3: γ-ray flux per unit solid angle in the GC region (data points), in comparison with the expected flux assuming a cosmic-ray spectrum as measured in the solar neighbourhood (shaded band). The spectrum of the region −0.8◦ < l < 0.8◦, |b| < 0.3◦ is shown using full circles. These data can be described by a power law: dN/dE = k(E/TeV)−Γ, with k = (1.73 ± 0.13stat ± 0.35sys) × 10 −8 TeV−1 cm−2 s−1 sr−1 and a photon index Γ = 2.29 ± 0.07stat ± 0.20sys. The shaded box shows the range of expected π0-decay fluxes from this region assuming a CR spectrum identical to that found in the solar neighbourhood and a total mass of 1.7− 4.4× 107 solar masses in the region −0.8◦ < l < 0.8◦, |b| < 0.3◦, estimated from CS measurements. Above 1 TeV an enhancement by a factor 3 − 9 relative to this prediction is observed. Using independent mass estimates derived from sub-millimeter measurements [24], 5.3± 1.0× 107 solar masses, and from C18O measurements [25], 3+2 −1×10 7 solar masses, results in enhancement factors of 4−6 and 5−13, respectively. The strongest emission away from the bright central source HESS J1745−290 occurs close to the Sagittarius B complex of giant molecular clouds [26]. In a box covering this region (0.3◦ < l < 0.8◦, −0.3◦ < b < 0.2◦), integrated CS emission suggests a molecular target mass of 6 − 15 × 106 solar masses. The energy spectrum of this region is shown using open circles. The measured γ-ray flux (> 1 TeV) implies a high-energy cosmic-ray density which is 4 − 10 times higher than the local value. Standard γ-ray selection cuts are applied here, yielding a spectral analysis threshold of 170 GeV. The spectrum of the central source HESS J1745-290 is shown for comparison (using an integration radius of 0.14◦). All error bars show ±1 standard deviation.](/figures/figure-3-g-ray-flux-per-unit-solid-angle-in-the-gc-region-2p9k0w38.webp)
![Figure 1: VHE γ-ray images of the GC region. Top: γ-ray count map, bottom: the same map after subtraction of the two dominant point sources, showing an extended band of gamma-ray emission. White contour lines indicate the density of molecular gas, traced by its CS emission. The position and size of the composite SNR G0.9+0.1 is shown with a yellow circle. The position of Sgr A⋆ is marked with a black star. The 95% confidence region for the positions of the two unidentified EGRET sources in the region are shown as dashed green ellipses [20]. These smoothed and acceptance corrected images are derived from 55 hours of data consisting of dedicated observations of SgrA⋆, G 0.9+0.1 and a part of the data of the H.E.S.S. Galactic plane survey [21]. The excess observed along the Galactic plane consists of ≈3500 γ-ray photons and has a statistical significance of 14.6 standard deviations. The absence of any residual emission at the position of the point-like γ-ray source G0.9+0.1 demonstrates the validity of the subtraction technique. The energy threshold of the maps is 380 GeV due to the tight γ-ray selection cuts applied here to improve signal/noise and angular resolution. We note that the ability of H.E.S.S. to map extended γ-ray emission has been demonstrated for the shell-type SNRs RXJ1713.7 −3946 [22] and RXJ0852.0−4622 [23]. The white contours are evenly spaced and show velocity integrated CS line emission from Tsuboi et al. [11], and have been smoothed to match the angular resolution of H.E.S.S..](/figures/figure-1-vhe-g-ray-images-of-the-gc-region-top-g-ray-count-24n1tsfo.webp)