A Search for Cosmic-Ray Proton Anisotropy with the Fermi Large Area Telescope

Ajello M, Baldini L, Barbiellini G, Bastieri D, Bechtol K, Bellazzini R, Bissaldi E, Blandford RD, Bonino R, Bottacini E, Brandt TJ, Bruel P, Buson S, Cameron RA, Caputo R, Cavazzuti E, Chen S, Chiaro G, Ciprini S, Cohen-Tanugi J, Costantin D, Cuoco A, Cutini S, D'Ammando F, Luque PDLT, De Palma F, Desai A, Digel SW, Di Lalla N, Di Venere L, Dominguez A, Fegan SJ, Fukazawa Y, Funk S, Fusco P, Gargano F, Gasparrini D, Giglietto N, Giordano F, Giroletti M, Green D, Grenier IA, Guiriec S, Hayashi K, Hays E, Hewitt JW, Horan D, Johannesson G, Kuss M, Latronico L, Li J, Liodakis , Longo F, Loparco F, Lubrano P, Maldera S, Manfreda A, Martí-Devesa G, Mazziotta MN, Meehan M, Mereu , Meyer M, Michelson PF, Mirabal N, Mitthumsiri W, Mizuno T, Morselli A, Negro M, Nuss E, Omodei N, Orienti M, Orlando E, Paliya VS, Paneque D, Persic M, Pesce-Rollins M, Piron F, Porter TA, Principe G, Raino S, Rando R, Razzano M, Razzaque S, Reimer A, Reimer O, Serini D, Sgro C, Siskind EJ, Spandre G, Spinelli P, Suson DJ, Tajima H, Thayer JB, Torres DF, Troja E, Vandenbroucke J, Yassine M, Zimmer S (2019)

Publication Type: Journal article

Publication year: 2019


Book Volume: 883

Journal Issue: 1

DOI: 10.3847/1538-4357/ab3a2e


The Fermi Large Area Telescope (LAT) has amassed a large data set of primary cosmic-ray protons throughout its mission. In fact, it is the largest set of identified cosmic-ray protons ever collected at this energy. The LAT' s wide field of view and full-sky survey capabilities make it an excellent instrument for studying cosmic-ray anisotropy. As a space-based survey instrument, the LAT is sensitive to anisotropy in both R.A. and decl., while ground-based observations only measure the anisotropy in R.A. We present the results of the first-ever proton anisotropy search using Fermi LAT. The data set was collected over eight years and consists of approximately 179 million protons above 78 GeV, enabling it to probe dipole anisotropy below an amplitude of 10(-3), resulting in the most stringent limits on the decl. dependence of the dipole to date. We measure a dipole amplitude delta = 3.9 +/- 1.5 x 10(-4) with a p-value of 0.01 (pretrials) for protons with energy greater than 78 GeV. We discuss various systematic effects that could give rise to a dipole excess and calculate upper limits on the dipole amplitude as a function of minimum energy. The 95% confidence level upper limit on the dipole amplitude is delta(UL) = 1.3 x 10(-3) for protons with energy greater than 78 GeV and delta(UL )= 1.2 x 10(-3) for protons with energy greater than 251 GeV.

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Involved external institutions

Clemson University US United States (USA) (US) University of Pisa / Università di Pisa (UniPi) IT Italy (IT) National Institute for Nuclear Physics / Istituto Nazionale di Fisica Nucleare (INFN) IT Italy (IT) University of Wisconsin - Madison US United States (USA) (US) Politecnico di Bari IT Italy (IT) Stanford University US United States (USA) (US) National Aeronautics and Space Administration (NASA) US United States (USA) (US) École Polytechnique - Université Paris-Saclay FR France (FR) Julius-Maximilians-Universität Würzburg DE Germany (DE) Italian Space Agency / Agenzia Spaziale Italiana (ASI) IT Italy (IT) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) IT Italy (IT) University of Montpellier / Université Montpellier FR France (FR) University of Padua / Universita degli Studi di Padova IT Italy (IT) Universidad Complutense de Madrid (UCM) ES Spain (ES) Hiroshima University / 広島大学 JP Japan (JP) Max-Planck-Institut für Physik (Werner-Heisenberg-Institut, MPP) / Max Planck Institute for Physics DE Germany (DE) University of Paris 7 - Denis Diderot / Université Paris VII Denis Diderot FR France (FR) Nagoya University / 名古屋大学 JP Japan (JP) University of North Florida (UNF) US United States (USA) (US) University of Iceland (UI) / Háskóli Íslands IS Iceland (IS) Deutsches Elektronen-Synchrotron DESY DE Germany (DE) Leopold-Franzens-Universität Innsbruck / University of Innsbruck AT Austria (AT) Università degli Studi di Perugia IT Italy (IT) Mahidol University / มหาวิทยาลัยมหิดล TH Thailand (TH) Purdue University Northwest / Purdue University Calumet US United States (USA) (US) Institute of Space Sciences (IEEC-CSIC) ES Spain (ES) University of Johannesburg ZA South Africa (ZA) NYCB Real-Time Computing, Inc. US United States (USA) (US)

How to cite


Ajello, M., Baldini, L., Barbiellini, G., Bastieri, D., Bechtol, K., Bellazzini, R.,... Zimmer, S. (2019). A Search for Cosmic-Ray Proton Anisotropy with the Fermi Large Area Telescope. Astrophysical Journal, 883(1). https://doi.org/10.3847/1538-4357/ab3a2e


Ajello, M., et al. "A Search for Cosmic-Ray Proton Anisotropy with the Fermi Large Area Telescope." Astrophysical Journal 883.1 (2019).

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