Deep view of the Large Magellanic Cloud with six years of Fermi-LAT observations^⋆

¹ Deutsches Elektronen Synchrotron DESY, 15738 Zeuthen, Germany
² 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
³ 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
⁴ Università di Pisa and 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 e Astronomia “G. Galilei”, Università di Padova, 35131 Padova, Italy
¹⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
¹¹ Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
¹² Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy
¹³ Dipartimento di Fisica Generale “Amadeo Avogadro” , Università degli Studi di Torino, 10125 Torino, Italy
¹⁴ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁵ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, Montpellier, France
¹⁶ Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France
¹⁷ Consorzio Interuniversitario per la Fisica Spaziale (CIFS), 10133 Torino, Italy
¹⁸ INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica, 20133 Milano, Italy
¹⁹ Agenzia Spaziale Italiana (ASI) Science Data Center, 00133 Roma, Italy
²⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy
²¹ Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
²² College of Science, George Mason University, Fairfax, VA 22030, resident at Naval Research Laboratory, Washington, DC 20375, USA
²³ INAF Osservatorio Astronomico di Roma, 00040 Monte Porzio Catone (Roma), Italy
²⁴ INAF Istituto di Radioastronomia, 40129 Bologna, Italy
²⁵ Dipartimento di Astronomia, Università di Bologna, 40127 Bologna, Italy
²⁶ Dipartimento di Fisica, Università di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Gruppo Collegato di Udine, 33100 Udine, Italy
²⁷ Università Telematica Pegaso, Piazza Trieste e Trento, 48, 80132 Napoli, Italy
²⁸ Università di Udine, 33100 Udine, Italy
²⁹ Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, 70126 Bari, Italy
³⁰ Centre d’Études Nucléaires de Bordeaux Gradignan, IN2P3/CNRS, Université Bordeaux 1, BP120, 33175 Gradignan Cedex, France
³¹ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace – Université d’Orléans/CNRS, 45071 Orléans Cedex 02, France
³² Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, 18330 Nançay, France
³³ NASA Postdoctoral Program Fellow, USA
³⁴ School of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
³⁵ Science Institute, University of Iceland, 107 Reykjavik, Iceland
³⁶ CNRS, IRAP, 31028 Toulouse Cedex 4, France
³⁷ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
³⁸ Department of Physics, KTH Royal Institute of Technology, AlbaNova, 106 91 Stockholm, Sweden
³⁹ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
⁴⁰ Institute of Space Sciences (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
⁴¹ Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, 739-8526 Hiroshima, Japan
⁴² Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, 00133 Roma, Italy
⁴³ Center for Cosmology, Physics and Astronomy Department, University of California, Irvine, CA 92697-2575, USA
⁴⁴ Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
⁴⁵ Max-Planck-Institut für Physik, 80805 München, Germany
⁴⁶ Institut für Astro- und Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
⁴⁷ Department of Physics, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
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 Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, 739-8526 Hiroshima, Japan
⁵¹ Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain
⁵² Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA

^⋆⋆ Corresponding author: Pierrick Martin, e-mail: pierrick.martin@irap.omp.eu

Received: 9 July 2015
Accepted: 28 September 2015

Abstract

Context. The nearby Large Magellanic Cloud (LMC) provides a rare opportunity of a spatially resolved view of an external star-forming galaxy in γ-rays. The LMC was detected at 0.1–100 GeV as an extended source with CGRO/EGRET and using early observations with the Fermi-LAT. The emission was found to correlate with massive star-forming regions and to be particularly bright towards 30 Doradus.

Aims. Studies of the origin and transport of cosmic rays (CRs) in the Milky Way are frequently hampered by line-of-sight confusion and poor distance determination. The LMC offers a complementary way to address these questions by revealing whether and how the γ-ray emission is connected to specific objects, populations of objects, and structures in the galaxy.

Methods. We revisited the γ-ray emission from the LMC using about 73 months of Fermi-LAT P7REP data in the 0.2–100 GeV range. We developed a complete spatial and spectral model of the LMC emission, for which we tested several approaches: a simple geometrical description, template-fitting, and a physically driven model for CR-induced interstellar emission.

Results. In addition to identifying PSR J0540−6919 through its pulsations, we find two hard sources positionally coincident with plerion N 157B and supernova remnant N 132D, which were also detected at TeV energies with H.E.S.S. We detect an additional soft source that is currently unidentified. Extended emission dominates the total flux from the LMC. It consists of an extended component of about the size of the galaxy and additional emission from three to four regions with degree-scale sizes. If it is interpreted as CRs interacting with interstellar gas, the large-scale emission implies a large-scale population of ~1–100 GeV CRs with a density of ~30% of the local Galactic value. On top of that, the three to four small-scale emission regions would correspond to enhancements of the CR density by factors 2 to 6 or higher, possibly more energetic and younger populations of CRs compared to the large-scale population. An alternative explanation is that this is emission from an unresolved population of at least two dozen objects, such as pulsars and their nebulae or supernova remnants. This small-scale extended emission has a spatial distribution that does not clearly correlate with known components of the LMC, except for a possible relation to cavities and supergiant shells.

Conclusions. The Fermi-LAT GeV observations allowed us to detect individual sources in the LMC. Three of the newly discovered sources are associated with rare and extreme objects. The 30 Doradus region is prominent in GeV γ-rays because PSR J0540−6919 and N 157B are strong emitters. The extended emission from the galaxy has an unexpected spatial distribution, and observations at higher energies and in radio may help to clarify its origin.