Volume 523, November-December 2010
|Number of page(s)||14|
|Published online||16 November 2010|
Detection of the Small Magellanic Cloud in gamma-rays with Fermi/LAT
Space Science DivisionNaval Research Laboratory,
2 National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA
3 W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
4 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
5 Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur Yvette, France
6 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, 34127 Trieste, Italy
7 Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
8 Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
9 Dipartimento di Fisica “G. Galilei”, Università di Padova, 35131 Padova, Italy
10 Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
11 Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
12 Centre d’Étude Spatiale des Rayonnements, CNRS/UPS, BP 44346, 31028 Toulouse Cedex 4, France
13 Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
14 Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, 70126 Bari, Italy
15 Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
16 Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France
17 Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
18 INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, 20133 Milano, Italy
19 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
20 Center for Research and Exploration in Space Science and Technology (CRESST) and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
21 Department of Physics and Center for Space Sciences and Technology, University of Maryland Baltimore County, Baltimore, MD 21250, USA
22 George Mason University, Fairfax, VA 22030, USA
23 Laboratoire de Physique Théorique et Astroparticules, Université Montpellier 2, CNRS/IN2P3, Montpellier, France
24 Department of Physics, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
25 The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
26 Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg Foundation, Sweden
27 CNRS/IN2P3, Centre d’Études Nucléaires BordeauxGradignan, UMR 5797, Gradignan, 33175, France
28 Université de Bordeaux, Centre d’Études Nucléaires Bordeaux Gradignan, UMR 5797, Gradignan, 33175, France
29 Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
30 Agenzia Spaziale Italiana (ASI) Science Data Center, 00044 Frascati ( Roma), Italy
31 INAF Istituto di Radioastronomia, 40129 Bologna, Italy
32 Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USA
33 Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
34 Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo, 169-8555 Japan
35 Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
36 Max-Planck Institut für extraterrestrische Physik, 85748 Garching, Germany
37 Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
38 Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, 00133 Roma, Italy
39 Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
40 Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
41 Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
42 Institut für Astro- und Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
43 Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
44 Space Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA
45 NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA
46 Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA
48 Consorzio Interuniversitario per la Fisica Spaziale (CIFS), 10133 Torino, Italy
49 INTEGRAL Science Data Centre, 1290 Versoix, Switzerland
50 Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133 Roma, Italy
51 Department of Physics, Royal Institute of Technology (KTH), AlbaNova, 106 91 Stockholm, Sweden
52 School of Pure and Applied Natural Sciences, University of Kalmar, 391 82 Kalmar, Sweden
Accepted: 1 August 2010
Context. The flux of gamma rays with energies greater than 100 MeV is dominated by diffuse emission coming from cosmic-rays (CRs) illuminating the interstellar medium (ISM) of our Galaxy through the processes of Bremsstrahlung, pion production and decay, and inverse-Compton scattering. The study of this diffuse emission provides insight into the origin and transport of cosmic rays.
Aims. We searched for gamma-ray emission from the Small Magellanic Cloud (SMC) in order to derive constraints on the cosmic-ray population and transport in an external system with properties different from the Milky Way.
Methods. We analysed the first 17 months of continuous all-sky observations by the Large Area Telescope (LAT) of the Fermi mission to determine the spatial distribution, flux and spectrum of the gamma-ray emission from the SMC. We also used past radio synchrotron observations of the SMC to study the population of CR electrons specifically.
Results. We obtained the first detection of the SMC in high-energy gamma rays, with an integrated >100 MeV flux of (3.7 ± 0.7) × 10-8 ph cm-2 s-1, with additional systematic uncertainty of ≤16%. The emission is steady and from an extended source ~3° in size. It is not clearly correlated with the distribution of massive stars or neutral gas, nor with known pulsars or supernova remnants, but a certain correlation with supergiant shells is observed.
Conclusions. The observed flux implies an upper limit on the average CR nuclei density in the SMC of ~15% of the value measured locally in the Milky Way. The population of high-energy pulsars of the SMC may account for a substantial fraction of the gamma-ray flux, which would make the inferred CR nuclei density even lower. The average density of CR electrons derived from radio synchrotron observations is consistent with the same reduction factor but the uncertainties are large. From our current knowledge of the SMC, such a low CR density does not seem to be due to a lower rate of CR injection and rather indicates a smaller CR confinement volume characteristic size.
Key words: acceleration of particles / cosmic rays / Magellanic Clouds / gamma rays: general
© ESO, 2010
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