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In-flight measurement of the absolute energy scale of the Fermi Large Area Telescope |
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| Institution: | a W.W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics, SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA b 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 c Department of Astronomy, Stockholm University, SE-106 91 Stockholm, Sweden d The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, SE-106 91 Stockholm, Sweden e Department of Physics, Royal Institute of Technology (KTH), AlbaNova, SE-106 91 Stockholm, Sweden f Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy g Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy h Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy i Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy j Dipartimento di Fisica “G. Galilei”, Università di Padova, I-35131 Padova, Italy k Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia, Italy l Dipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy m Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, I-70126 Bari, Italy n Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy o Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France p Institut de Ciències de l’Espai (IEEE-CSIC), Campus UAB, 08193 Barcelona, Spain q INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy r Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur Yvette, France s Artep Inc., 2922 Excelsior Springs Court, Ellicott City, MD 21042, USA t ASI Science Data Center, I-00044 Frascati (Roma), Italy u Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, Montpellier, France v Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy w Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA x Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan y NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA z INAF Istituto di Radioastronomia, 40129 Bologna, Italy aa Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899, USA ab Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA ac Science Institute, University of Iceland, IS-107 Reykjavik, Iceland ad National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA ae College of Science, Ibaraki University, 2-1-1, Bunkyo, Mito 310-8512, Japan af Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan ag CNRS, IRAP, F-31028 Toulouse cedex 4, France ah Université de Toulouse, UPS-OMP, IRAP, Toulouse, France ai Yukawa Institute for Theoretical Physics, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan aj Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA ak Center for Research and Exploration in Space Science and Technology (CRESST), NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA al Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, I-00133 Roma, Italy am Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA an Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan ao Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan ap Max-Planck Institut für extraterrestrische Physik, 85748 Garching, Germany aq Max-Planck-Institut für Physik, D-80805 München, Germany ar Center for Earth Observing and Space Research, College of Science, George Mason University, Fairfax, VA 22030, USA as Institut für Astro- und Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria at Université Bordeaux 1, CNRS/IN2p3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France au NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA av Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA aw Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy ax Dipartimento di Fisica, Università di Roma “Tor Vergata”, I-00133 Roma, Italy ay Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden |
| Abstract: | The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to survey the gamma-ray sky from 20 MeV to several hundreds of GeV. In this energy band there are no astronomical sources with sufficiently well known and sharp spectral features to allow an absolute calibration of the LAT energy scale. However, the geomagnetic cutoff in the cosmic ray electron-plus-positron (CRE) spectrum in low Earth orbit does provide such a spectral feature. The energy and spectral shape of this cutoff can be calculated with the aid of a numerical code tracing charged particles in the Earth’s magnetic field. By comparing the cutoff value with that measured by the LAT in different geomagnetic positions, we have obtained several calibration points between ∼6 and ∼13 GeV with an estimated uncertainty of ∼2%. An energy calibration with such high accuracy reduces the systematic uncertainty in LAT measurements of, for example, the spectral cutoff in the emission from gamma ray pulsars. |
| Keywords: | Cosmic-rays Geomagnetic cutoff Absolute energy scale Fermi Large Area Telescope |
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