Estimation of the Upper Limit for the Amplitude of the Dipole Anisotropy of the Total Flux of 25-Gev to 1-Tev Cosmic-Ray
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EMENTARY PARTICLES AND FIELDS Experiment
Estimation of the Upper Limit for the Amplitude of the Dipole Anisotropy of the Total Flux of 25-Gev to 1-Tev Cosmic-Ray Electrons and Positrons A. V. Karelin1)* and S. A. Voronov1) (On behalf of the PAMELA Collaboration) Received April 2, 2019; revised April 2, 2019; accepted April 2, 2019
Abstract—Searches for the anisotropy of the total electron and positron fluxes in high-energy galactic cosmic rays are performed. Data collected by the PAMELA magnetic spectrometer from June 2006, when it was launched into Earth’s orbit, to 2016 were used for the respective analysis. The main event selection and a determination of particle arrival directions were accomplished by means of a position-sensitive microstrip calorimeter. A neutron detector was used to suppress additionally the background of protons and nuclei. An analysis of information obtained over a nearly ten-year period of measurements made it possible to set upper limits on the amplitude of the dipole anisotropy of the total electron and positron fluxes for two energy ranges, between 25 and 100 GeV and between 100 GeV and 1 TeV. These results supplement data from other satellite-borne experiments, AMS-02 and Fermi. DOI: 10.1134/S1063778819050090
1. INTRODUCTION The total flux of electrons and positrons of galactic cosmic rays that is detected near the Earth in the energy range between a few GeV units and 1 TeV is formed in sources situated in the Milky Way Galaxy relatively close to the Earth—most probably in supernova remnants. The proximity of the sources of this flux follows from the fact that cosmic-ray electrons and positrons propagating through an interstellar medium lose fast their energy via emitting synchrotron radiation, as well as via undergoing multiple inverse Compton scattering in collisions with lowenergy background photons. Because of the proximity of the sources, the measured distribution of the directions of the total electron and positron flux is expected show a moderately small dipole anisotropy, even though the propagation of these fluxes from the sources is thought to obey regularities prescribed by the diffusion model, which take into account the scattering of electrons and positrons in galactic magnetic fields (in view of this scattering, it is next to impossible to determine the initial direction of cosmic-ray fluxes in the vicinity of the Earth). Nevertheless, no such anisotropy was found in some recent measurements in the AMS-02 [1], Fermi [2], and HESS [3] experiments. However, a structure in the energy 1)
National Research Nuclear University MEPhI, Moscow, 115409 Russia. * E-mail: [email protected]
spectrum of electrons in the energy region around 1 TeV was discovered quite recently in the CALET [4] and DAMPE [5] experiments. This is indicative of the existence of a very close source or group of sources of the total electron and positron flux. At the same time, the AMS-02 group recently published [6] an energy spectrum of electrons that does not exhibit distinct features and which agrees well wit
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