Observations of the Large Magellanic Cloud with Fermi

¹ Space Science Division, Naval 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 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 e-mail: tporter@scipp.ucsc.edu
⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
⁶ Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, 34127 Trieste, Italy
⁸ Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Padova, 35131 Padova, Italy
¹⁰ Dipartimento di Fisica “G. Galilei”, Università di Padova, 35131 Padova, Italy
¹¹ Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
¹² Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
¹³ Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
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 Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
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), NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²¹ 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 and Astronomy, Sonoma State University, Rohnert Park, CA 94928-3609, USA
²⁵ Department of Physics, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
²⁶ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden

²⁷ Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg Foundation
²⁸ Agenzia Spaziale Italiana (ASI) Science Data Center, 00044 Frascati (Roma), Italy
²⁹ Dipartimento di Fisica, Università di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Gruppo Collegato di Udine, 33100 Udine, Italy
³⁰ Université de Bordeaux, Centre d'Études Nucléaires Bordeaux Gradignan, UMR 5797, 33175 Gradignan, France
³¹ CNRS/IN2P3, Centre d'Études Nucléaires Bordeaux Gradignan, UMR 5797, 33175 Gradignan, France
³² Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³³ University of Maryland, College Park, MD 20742, USA
³⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, and Università di Trieste, 34127 Trieste, Italy
³⁵ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
³⁶ University of Alabama in Huntsville, Huntsville, AL 35899, USA
³⁷ Department of Physics, Royal Institute of Technology (KTH), AlbaNova, 106 91 Stockholm, Sweden
³⁸ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, BP 44346, 31028 Toulouse Cedex 4, France e-mail: [pierre.jean;jurgen.knodlseder]@cesr.fr
³⁹ Department of Physics, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551, Japan
⁴⁰ Waseda University, 1-104 Totsukamachi, Shinjuku-ku, Tokyo, 169-8050, Japan
⁴¹ Cosmic Radiation Laboratory, Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
⁴² Max-Planck Institut für extraterrestrische Physik, 85748 Garching, Germany
⁴³ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, 00133 Roma, Italy
⁴⁴ Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
⁴⁵ Institut für Astro- und Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
⁴⁶ Institut de Ciencies de l'Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
⁴⁷ Space Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA
⁴⁸ NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA
⁴⁹ Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA
⁵⁰ Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain
⁵¹ Consorzio Interuniversitario per la Fisica Spaziale (CIFS), 10133 Torino, Italy
⁵² Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
⁵³ North-West University, Potchefstroom Campus, Potchefstroom 2520, South Africa
⁵⁴ Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133 Roma, Italy
⁵⁵ Australia Telescope National Facility, CSIRO, Epping NSW 1710, Australia
⁵⁶ School of Pure and Applied Natural Sciences, University of Kalmar, 391 82 Kalmar, Sweden

Received: 15 October 2009
Accepted: 22 December 2009

Abstract

Context. The Large Magellanic Cloud (LMC) is to date the only normal external galaxy that has been detected in high-energy gamma rays. High-energy gamma rays trace particle acceleration processes and gamma-ray observations allow the nature and sites of acceleration to be studied.

Aims. We characterise the distribution and sources of cosmic rays in the LMC from analysis of gamma-ray observations.

Methods. We analyse 11 months of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-Ray Space Telescope and compare it to tracers of the interstellar medium and models of the gamma-ray sources in the LMC.

Results. The LMC is detected at 33σ significance. The integrated >100 MeV photon flux of the LMC amounts to (2.6 ± 0.2) × 10^-7 ph cm^-2 s^-1 which corresponds to an energy flux of (1.6 ± 0.1) × 10^-10 erg cm^-2 s^-1, with additional systematic uncertainties of 16%. The analysis reveals the massive star forming region 30 Doradus as a bright source of gamma-ray emission in the LMC in addition to fainter emission regions found in the northern part of the galaxy. The gamma-ray emission from the LMC shows very little correlation with gas density and is rather correlated to tracers of massive star forming regions. The close confinement of gamma-ray emission to star forming regions suggests a relatively short GeV cosmic-ray proton diffusion length.

Conclusions. The close correlation between cosmic-ray density and massive star tracers supports the idea that cosmic rays are accelerated in massive star forming regions as a result of the large amounts of kinetic energy that are input by the stellar winds and supernova explosions of massive stars into the interstellar medium.