Discovery of a point-like very-high-energy gamma-ray source in Monoceros
Astronomy & Astrophysics manuscript no. 7299 c© ESO 2018
May 31, 2018
Discovery of a point-like very-high-energy γ-ray source in
Monoceros
F. A. Aharonian1,13, A.G. Akhperjanian2, A.R. Bazer-Bachi3 , B. Behera14, M. Beilicke4 , W. Benbow1,
D. Berge1 ⋆, K. Bernlöhr1,5, C. Boisson6, O. Bolz1, V. Borrel3, I. Braun1, E. Brion7, A.M. Brown8,
R. Bühler1, I. Büsching9, T. Boutelier17, S. Carrigan1, P.M. Chadwick8, L.-M. Chounet10, G. Coignet11,
R. Cornils4, L. Costamante1,23, B. Degrange10, H.J. Dickinson8, A. Djannati-Atäı12 , W. Domainko1,
L.O’C. Drury13, G. Dubus10, K. Egberts1, D. Emmanoulopoulos14, P. Espigat12, C. Farnier15, F. Feinstein15,
A. Fiasson15, A. Förster1, G. Fontaine10, Seb. Funk5, S. Funk1, M. Füßling5, Y.A. Gallant15, B. Giebels10,
J.F. Glicenstein7, B. Glück16, P. Goret7, C. Hadjichristidis8, D. Hauser1, M. Hauser14, G. Heinzelmann4,
G. Henri17, G. Hermann1, J.A. Hinton1,14 ⋆⋆, A. Hoffmann18, W. Hofmann1, M. Holleran9, S. Hoppe1,
D. Horns18, A. Jacholkowska15, O.C. de Jager9, E. Kendziorra18, M. Kerschhaggl5, B. Khélifi10,1,
Nu. Komin15, K. Kosack1, G. Lamanna11, I.J. Latham8, R. Le Gallou8, A. Lemière12,
M. Lemoine-Goumard10, T. Lohse5, J.M. Martin6, O. Martineau-Huynh19, A. Marcowith3,15,
C. Masterson1,23, G. Maurin12, T.J.L. McComb8, E. Moulin15,7, M. de Naurois19, D. Nedbal20, S.J. Nolan8,
A. Noutsos8, J-P. Olive3, K.J. Orford8, J.L. Osborne8, M. Panter1, G. Pedaletti14, G. Pelletier17,
P.-O. Petrucci17, S. Pita12, G. Pühlhofer14, M. Punch12, S. Ranchon11, B.C. Raubenheimer9, M. Raue4,
S.M. Rayner8, O. Reimer ⋆⋆⋆, J. Ripken4, L. Rob20, L. Rolland7, S. Rosier-Lees11, G. Rowell1 †, J. Ruppel21,
V. Sahakian2, A. Santangelo18, L. Saugé17, S. Schlenker5, R. Schlickeiser21, R. Schröder21, U. Schwanke5,
S. Schwarzburg18, S. Schwemmer14, A. Shalchi21, H. Sol6, D. Spangler8, R. Steenkamp22, C. Stegmann16,
G. Superina10, P.H. Tam14, J.-P. Tavernet19, R. Terrier12, M. Tluczykont10,23 ‡, C. van Eldik1,
G. Vasileiadis15, C. Venter9, J.P. Vialle11, P. Vincent19, H.J. Völk1, S.J. Wagner14, M. Ward8,
Y. Moriguchi24, and Y. Fukui24,25
(Affiliations can be found after the references)
Preprint online version: May 31, 2018
ABSTRACT
Aims. The complex Monoceros Loop SNR/Rosette Nebula region contains several potential sources of very-high-energy (VHE) γ-
ray emission and two as yet unidentified high-energy EGRET sources. Sensitive VHE observations are required to probe acceleration
processes in this region.
Methods. The H.E.S.S. telescope array has been used to search for very high-energy γ-ray sources in this region. CO data from the
NANTEN telescope were used to map the molecular clouds in the region, which could act as target material for γ-ray production
via hadronic interactions.
Results. We announce the discovery of a new γ-ray source, HESSJ0632+058, located close to the rim of the Monoceros SNR. This
source is unresolved by H.E.S.S. and has no clear counterpart at other wavelengths but is possibly associated with the weak X-ray
source 1RXSJ063258.3+054857, the Be-star MWC148 and/or the lower energy γ-ray source 3EG J0634+0521. No evidence for an
associated molecular cloud was found in the CO data.
Key words. gamma rays: observations
Send offprint requests to: J.A.Hinton@leeds.ac.uk,
Armand.Fiasson@lpta.in2p3.fr
⋆ now at CERN, Geneva, Switzerland
⋆⋆ now at School of Physics & Astronomy, University of Leeds,
Leeds LS2 9JT, UK
⋆⋆⋆ now at Stanford University, HEPL & KIPAC, Stanford, CA
94305-4085, USA
† now at School of Chemistry & Physics, University of
Adelaide, Adelaide 5005, Australia
‡ now at DESY Zeuthen
1. Introduction
Shell-type supernova remnants (SNRs) have been identified
as particle accelerators via their very-high-energy (VHE;
E > 100 GeV) γ-ray and non-thermal X-ray emission (see
e.g. Aharonian et al. (2006a) and Koyama et al. (1997)). It
has been suggested that interactions of particles acceler-
ated in SNR with nearby molecular clouds should produce
detectable γ-ray emission (Aharonian et al. 1994). For this
reason the well-known Monoceros Loop SNR (G 205.5+0.5,
distance∼1.6 kpc (Graham et al. 1982; Leahy et al. 1986)),
with its apparent interaction with the Rosette Nebula (a
young stellar cluster/molecular cloud complex, distance
http://arxiv.org/abs/0704.0171v1
2 F. A. Aharonian et al.: A point-like γ-ray source in Monoceros
1.4± 0.1 kpc (Hensberge et al. 2000)) is a prime target for
observations with VHE γ-ray instruments.
For the case of hadronic cosmic rays (CRs) interact-
ing in the interstellar medium to produce pions and hence
γ-rays via π0 decay, a spatial correlation between γ-ray
emission and tracers of interstellar gas is expected. Such a
correlation was used to infer the presence of a population of
recently accelerated CR hadrons in the Galactic Centre re-
gion (Aharonian et al. 2006b). This discovery highlights the
importance of accurate mapping of available target material
for the interpretation of TeV γ-ray emission. The NANTEN
4 m diameter sub-mm telescope at Las Campanas observa-
tory, Chile, has been conducting a 12CO (J=1→0) survey
of the Galactic plane since 1996 (Mizuno & Fukui 2004).
The Monoceros region is covered by this survey and the
NANTEN data are used here to trace the target material
for interactions of accelerated hadrons.
2. H.E.S.S. Observations and Results
The observations described here took place between March
2004 and March 2006 and comprise 13.5 hours of data after
data quality selection and dead-time correction. The data
were taken over a wide range of zenith angles from 29 to
59 degrees, leading to a mean energy threshold of 400 GeV
with so-called standard cuts used here for spectral analysis
and 750 GeV with the hard cuts used here for the source
search and position fitting. These cuts are described in de-
tail in Aharonian et al. (2006c).
A search in this region for point-like emission was made
using a 0.11◦ On source region and a ring of mean radius
0.5◦ for Off source background estimation (see Berge et al.
(2006) for details). Fig. 1 shows the resulting significance
map, together with CO data from NANTEN, radio con-
tours and the positions of all Be-stars in this region. The
peak significance in the field is 7.1σ. The number of sta-
tistical trials associated with a search of the entire field of
view, in 0.01◦ steps along both axes, is≈ 105. The measured
peak significance corresponds to 5.3σ after accounting for
these trials. A completely independent analysis based on a
fit of camera images to a shower model (Model Analysis de-
scribed in de Naurois (2006)), yields a significance of 7.3σ
(5.6σ post-trials).
The best fit position of the new source is 6h32m58.3s,
+5◦48′20′′ (RA/Dec. J2000) with 28′′ statistical errors
on each axis, and is hence identified as HESSJ0632+057.
Systematic errors are estimated at 20′′ on each axis. There
is no evidence for intrinsic extension of the source and we
derive a limit on the rms size of the emission region of 2′
(at 95% confidence), under the assumption that the source
follows a Gaussian profile. This source size upper limit is
shown as a dashed circle in the bottom panel of Fig. 1. Fig. 2
demonstrates the point-like nature of the source. The an-
gular distribution of excess γ-ray-like events with respect
to the best fit position is shown together with the expected
distribution for a point-like source.
The reconstructed energy spectrum of the source is con-
sistent with a power-law: dN/dE = k(E/1TeV)−Γ with
photon index Γ = 2.53± 0.26stat ± 0.20sys and a flux nor-
malisation k = 9.1±1.7stat±3.0sys×10
−13 cm−2s−1TeV−1.
Fig. 3 shows the H.E.S.S. spectrum together with that
for the unidentified EGRET source 3EGJ0634+0521 (dis-
cussed below) and an upper limit derived for TeV emis-
sion from 3EGJ0634+0521 using the HEGRA telescope
s00m30h06s00m35h06
s00m30h06s00m35h06
G 205.5+0.5
Rosette Nebula
HESS J0632+057
SAX J0635.2+0533
3EG 0634+0521
5.6407
5.7507
5.8607
5.9707
s30m32h06s00m33h06s30m33h06
s30m32h06s00m33h06s30m33h06
H.E.S.S.
Fig. 1. Top: the Monoceros SNR / Rosette Nebula re-
gion. The grey-scale shows velocity integrated (0-30 km
s−1) 12CO (J=1→0) emission from the NANTEN Galactic
Plane Survey (white areas have highest flux). Yellow con-
tours show 4 and 6 σ levels for the statistical significance of
a point-like γ-ray excess. Radio observations at 8.35 GHz
from Langston et al. (2000) are overlaid as cyan contours,
and illustrate the extent of the Rosette Nebula. The nomi-
nal Green (2004) Catalogue position/size of the Monoceros
SNR is shown as an (incomplete) dashed circle. 95% and
99% confidence regions for the position of the EGRET
source 3EG0634+0521 are shown as dotted green contours.
The binary pulsar SAXJ0635.2+0533 is marked with a
square and Be-stars with pink stars. Bottom: an expanded
view of the centre of the top panel showing H.E.S.S. sig-
nificance as a colour scale. The rms size limit derived for
the TeV emission is shown as a dashed circle. The unidenti-
fied X-ray source 1RXSJ063258.3+054857 is marked with
a triangle and the Be-star MCW 148 with a star.
F. A. Aharonian et al.: A point-like γ-ray source in Monoceros 3
 (square degrees)2θ
0 0.005 0.01 0.015 0.02 0.025
Fig. 2. Distribution of excess (candidate γ-ray) events as a
function of squared angular distance from the best fit posi-
tion of HESS J0632+057 (points), compared to the expecta-
tion for this dataset from Monte-Carlo simulations (smooth
curve).
array (Aharonian et al. 2004), converted from an integral
to a differential flux using the spectral shape measured
by H.E.S.S. We find no evidence for flux variability of
HESS J0632+057 within our dataset. However, we note
that due to the weakness of the source and sparse sam-
pling of the light-curve, intrinsic variability of the source
is not strongly constrained. The bulk of the available data
was taken in two short periods in December 2004 (P1, 4.7
hours) and November/December 2005 (P2, 6.2 hours). The
integral fluxes (above 1 TeV) in these two periods were:
6.3±1.8×10−13 cm−2 s−1 (P1) and 6.4±1.5×10−13 cm−2
s−1 (P2).
Energy (GeV)
-110 1 10 210 310 410
-1010
HEGRA
HESS J0623+057
3EG J0634+0521
Fig. 3. Reconstructed VHE γ-ray spectrum of
HESS J0632+057 compared to the HE γ-ray source
3EGJ0634+0521. An upper limit derived for
3EGJ0634+0521 at TeV energies using the HEGRA
instrument is also shown.
Amongst the candidate VHE sources in this field is the
34 ms binary pulsar SAXJ0635.2+0533. There is no signif-
icant γ-ray emission at the position of this object and we
derive a 99% confidence upper limit on the integral flux,
F (> 1TeV), of 2.6 × 10−13 cm−2 s−1, assuming an E−2
type spectrum.
3. Possible Associations of HESS J0632+057
The new VHE source HESS J0632+057 lies in a complex
region and several associations with objects known at other
wavelengths seem plausible. We therefore consider each of
these potential counterparts in turn.
The Monoceros Loop SNR is rather old in
comparison to the known VHE γ-ray shell-type
SNRs RXJ1713.7−3946 (Aharonian et al. 2006a),
RXJ0852.0−4622 (Aharonian et al. 2005b) and Cas-
A (Aharonian et al. 2001). All these objects have estimated
ages less than ∼ 2000 years, in contrast the Monoceros
Loop SNR has an age of ∼ 3 × 104 years (Leahy et al.
1986). This supernova remnant therefore appears to be in
a different evolutionary phase (late Sedov or Radiative)
compared to these known VHE sources. However, CR
acceleration may occur even at this later evolutionary stage
(see for example Yamazaki et al. (2006)). The principal
challenge for a scenario involving the Monoceros Loop
is to explain the very localised VHE emission at only
one point on the SNR limb. The interaction of the SNR
with a compact molecular cloud is one possible solution.
In this scenario (and indeed any π0 decay scenario) for
the observed γ-ray emission, a correlation is expected
between the TeV emission and the distribution of target
material. An unresolved molecular cloud listed in a CO
survey at 115 GHz (Oliver et al. 1996) lies rather close to
HESS J0623+057, at l = 205.75 b = −1.31. The distance
estimate for this cloud (1.6 kpc) is consistent with that
for the Monoceros SNR, making it a potential target for
hadrons accelerated in the SNR. However, as can be seen
clearly in the NANTEN data in Fig. 1, the intensity peak
of this cloud is significantly shifted to the East of the
H.E.S.S. source. We find no evidence in the NANTEN
data for any clouds along the line of sight to the H.E.S.S.
source.
3EGJ0634+0521 is an unidentified EGRET source
(Hartman et al. 1999) with positional uncertainties such
that HESS J0632+057 lies close to the 99% confidence con-
tour. Given that this source is flagged as possibly extended
or confused, a positional coincidence of these two objects
seems plausible. Furthermore, the reported third EGRET
catalogue flux above 100 MeV ((25.5± 5.1)× 10−8 photons
cm−2 s−1 with a photon index of 2.03 ± 0.26, see Fig. 3),
is consistent with an extrapolation of the H.E.S.S. spec-
trum. A global fit of the two spectra gives a photon index
of 2.41±0.06.
1RXSJ063258.3+054857 is a faint ROSAT source
(Voges et al. 2000) which lies 36′′ from the H.E.S.S. source
with a positional uncertainty of 21′′ (see Fig. 1 bottom).
Given the uncertainties on the positions of both objects
this X-ray source can certainly be considered a potential
counterpart of HESS J0632+057. The chance probability of
the coincidence of a ROSAT Faint Source Catalogue source
within the H.E.S.S. error circle is estimated as 0.1% by
scaling the total number of sources in the field of view. The
ROSAT source is rather weak, with only 4 counts detected
above 0.9 keV, spectral comparison is therefore rather diffi-
cult. In the scenario where the γ-ray emission is interpreted
as inverse Compton emission from a population of energetic
electrons, the ROSAT source could be naturally ascribed to
the synchrotron emission of the same electron population.
However, the low level of the X-ray emission (∼ 10−13 erg
cm−2 s−1) in comparison with the TeV flux (∼ 10−12 erg
4 F. A. Aharonian et al.: A point-like γ-ray source in Monoceros
cm−2 s−1) implies a very low magnetic field (≪ 3µG) un-
less a strong radiation source exists in the neighbourhood
of the emission region and/or the X-ray emission suffers
from substantial absorption. Observations at > 4 keV are
required to resolve this absorption issue. In a π0 decay sce-
nario for the γ-ray source, secondary electron production
via muon decay is expected along with γ-ray emission. The
synchrotron emission of these secondary electrons would in
general produce a weaker X-ray source than the IC scenario,
probably compatible with the measured ROSAT flux.
MWC148 (HD 259440) is a massive emission-line star
of spectral type B0pe which lies within the H.E.S.S. error
circle. The chance probability of this coincidence is hard to
assess, as there was no a-priori selection of stellar objects
as potential γ-ray sources. However, given the presence of
only 3 Be-type stars in the field of view of the H.E.S.S.
observation (see Fig. 1) and the solid angle of the H.E.S.S.
error circle, the naive chance probability of the associa-
tion is 10−4. Stars of this spectral type have winds with
typical velocities and mass loss rates of 1000 km s−1 and
10−7M⊙/year, respectively. Plausible acceleration sites are
in strong internal or external shocks of the stellar wind.
We estimate that an efficiency of 1-10% in the conversion
of the kinetic energy of the wind into γ-ray emission would
be required to explain the H.E.S.S. flux (assuming this star
lies at the distance of the Rosette Nebula). However, as no
associations of similar stars with point-like γ-ray sources
were found in the H.E.S.S. survey of the inner Galaxy, this
scenario seems rather unlikely.
A related possibility is that MWC148 is part of a binary
system with an, as yet undetected, compact companion.
Such a system might then resemble the known VHE γ-ray
source PSRB1259-63/SS2883 (Aharonian et al. 2005a).
Further multi-wavelength observations are required to con-
firm or refute this scenario.
Acknowledgements. The support of the Namibian authorities and of
the University of Namibia in facilitating the construction and op-
eration 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. Particle Physics and Astronomy Research Council (PPARC),
the IPNP of the Charles University, 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 oper-
ation of the equipment. The NANTEN project is financially supported
from JSPS (Japan Society for the Promotion of Science) Core-to-Core
Program, MEXT Grant-in-Aid for Scientific Research on Priority
Areas, and SORST-JST (Solution Oriented Research for Science and
Technology: Japan Science and Technology Agency). We would also
like to thank Stan Owocki and James Urquhart for very useful dis-
cussions.
References
Aharonian, F., Akhperjanian, A., Barrio, J., et al. 2001, A&A, 370,
Aharonian, F., Akhperjanian, A. G., Aye, K.-M., et al. 2005a, A&A,
442, 1
Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., et al. 2006a,
A&A, 449, 223
Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., et al. 2006b,
Nature, 439, 695
Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., et al. 2006c,
Aharonian, F., Akhperjanian, A. G., Bazer-Bachi, A. R., et al. 2005b,
A&A, 437, L7
Aharonian, F. A., Akhperjanian, A. G., Beilicke, M., et al. 2004, A&A,
417, 973
Aharonian, F. A., Drury, L. O., & Voelk, H. J. 1994, A&A, 285, 645
Berge, D., Funk, S., & Hinton, J. 2006, astro-ph/0610959
de Naurois, M. 2006, astro-ph/0607247
Graham, D. A., Haslam, C. G. T., Salter, C. J., & Wilson, W. E.
1982, A&A, 109, 145
Green, D. A. 2004, Bulletin of the Astronomical Society of India, 32,
Hartman, R. C., Bertsch, D. L., Bloom, S. D., et al. 1999, ApJS, 123,
Hensberge, H., Pavlovski, K., & Verschueren, W. 2000, A&A, 358, 553
Koyama, K., Kinugasa, K., Matsuzaki, K., et al. 1997, PASJ, 49, L7
Langston, G., Minter, A., D’Addario, L., et al. 2000, AJ, 119, 2801
Leahy, D. A., Naranan, S., & Singh, K. P. 1986, MNRAS, 220, 501
Mizuno, A. & Fukui, Y. 2004, in ASP Conf. Ser. 317: Milky
Way Surveys: The Structure and Evolution of our Galaxy, ed.
D. Clemens, R. Shah, & T. Brainerd, 59
Oliver, R. J., Masheder, M. R. W., & Thaddeus, P. 1996, A&A, 315,
Voges, W., Aschenbach, B., Boller, T., et al. 2000, IAU Circ., 7432, 1
Yamazaki, R., Kohri, K., Bamba, A., et al. 2006, MNRAS, 371, 1975
1 Max-Planck-Institut für 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 Colonel Roche, BP 4346, F-31029 Toulouse Cedex 4,
France
4 Universität Hamburg, Institut für Experimentalphysik,
Luruper Chaussee 149, D 22761 Hamburg, Germany
5 Institut für Physik, Humboldt-Universität zu Berlin,
Newtonstr. 15, D 12489 Berlin, Germany
6 LUTH, UMR 8102 du CNRS, Observatoire de Paris,
Section de Meudon, F-92195 Meudon Cedex, France
7 DAPNIA/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette,
Cedex, France
8 University of Durham, Department of Physics, South
Road, Durham DH1 3LE, U.K.
9 Unit for Space Physics, North-West University,
Potchefstroom 2520, South Africa
10 Laboratoire Leprince-Ringuet, IN2P3/CNRS, Ecole
Polytechnique, F-91128 Palaiseau, France
11 Laboratoire d’Annecy-le-Vieux de Physique des Particules,
IN2P3/CNRS, 9 Chemin de Bellevue - BP 110 F-74941
Annecy-le-Vieux Cedex, France
12 APC, 11 Place Marcelin Berthelot, F-75231 Paris Cedex
05, 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, D
69117 Heidelberg, Germany
15 Laboratoire de Physique Théorique et Astroparticules,
IN2P3/CNRS, Université Montpellier II, CC 70, Place
Eugène Bataillon, F-34095 Montpellier Cedex 5, France
16 Universität Erlangen-Nürnberg, Physikalisches Institut,
Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany
17 Laboratoire d’Astrophysique de Grenoble, INSU/CNRS,
Université Joseph Fourier, BP 53, F-38041 Grenoble Cedex
9, France
18 Institut für Astronomie und Astrophysik, Universität
Tübingen, Sand 1, D 72076 Tübingen, Germany
19 Laboratoire de Physique Nucléaire et de Hautes Energies,
IN2P3/CNRS, Universités Paris VI & VII, 4 Place Jussieu,
F-75252 Paris Cedex 5, France
20 Institute of Particle and Nuclear Physics, Charles
University, V Holesovickach 2, 180 00 Prague 8, Czech
Republic
21 Institut für Theoretische Physik, Lehrstuhl IV: Weltraum
und Astrophysik, Ruhr-Universität Bochum, D 44780
F. A. Aharonian et al.: A point-like γ-ray source in Monoceros 5
Bochum, Germany
22 University of Namibia, Private Bag 13301, Windhoek,
Namibia
23 European Associated Laboratory for Gamma-Ray
Astronomy, jointly supported by CNRS and MPG
24 Department of Astrophysics, Nagoya University, Chikusa-
ku, Nagoya 464-8602, Japan
25 Nagoya University Southern Observatories, Nagoya 464-
8602, Japan
	Introduction
	H.E.S.S. Observations and Results
	Possible Associations of HESSJ0632+057
