Detection of the Small Magellanic Cloud in gamma-rays with Fermi/LAT

¹ Space Science DivisionNaval Research Laboratory, Washington, DC 20375, USA
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
e-mail: knodlseder@cesr.fr;jean@cesr.fr
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
e-mail: martinp@mpe.mpg.de
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

Received: 23 April 2010
Accepted: 1 August 2010

Abstract

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.