APASTRA ProceedingsAPASTRA Proc.2199-3963Copernicus GmbHGöttingen, Germany10.5194/ap-2-35-2015The large-scale anisotropy with the PAMELA calorimeterKarelinA.karelin@hotbox.ruAdrianiO.BarbarinoG.BazilevskayaG.BellottiR.BoezioM.BogomolovE.BongiM.BonviciniV.BottaiS.BrunoA.CafagnaF.CampanaD.CarboneR.CarlsonP.CasolinoM.CastelliniG.De DonatoC.De SantisC.De SimoneN.Di FeliceV.FormatoV.GalperA.KoldashovS.KoldobskiyS.Krut'kovS.KvashninA.LeonovA.MalakhovV.MarcelliL.MartucciM.MayorovA.MennW.MergéM.MikhailovV.MocchiuttiE.MonacoA.MoriN.MuniniR.OsteriaG.PalmaF.PanicoB.PapiniP.PearceM.PicozzaP.RicciM.RicciariniS.SarkarR.SimonM.ScottiV.SparvoliR.SpillantiniP.StozhkovY.VacchiA.VannucciniE.VasilyevG.VoronovS.YurkinY.ZampaG.ZampaN.National research nuclear university MEPhI, 115409, Moscow, RussiaUniversity of Florence, 50019 Sesto Fiorentiono, Florence, ItalyINFN, Sezione di Florence, 50019 Sesto Fiorentiono, Florence, ItalyUniversity of Naples “Federico II”, 80126 Naples, ItalyINFN, Sezione di Naples, 80126 Naples, ItalyLebedev Physical Institute, 119991 Moscow, RussiaUniversity of Bari, 70126 Bari, ItalyINFN, Sezione di Bari, 70126 Bari, ItalyINFN, Sezione di Trieste, 34149 Trieste, ItalyIoffe Physical Technical Institute, 194021 St. Petersburg, RussiaINFN, Sezione di Rome “Tor Vergata”, 00133 Rome, ItalyUniversity of Rome “Tor Vergata”, 00133 Rome, ItalyKTH, Department of Physics, and the Oskar Klein Centre for Cosmoparticle Physics AlbaNova University Centre, 10691 Stockholm, SwedenIFAC, 50019 Sesto Fiorentino, Florence, ItalyUniversity of Trieste, 34147 Trieste, ItalyUniversity of Siegen, 57068 Siegen, GermanyINFN, Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044 Frascati, ItalyA. Karelin (karelin@hotbox.ru)2October20152235377May201524September2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://ap.copernicus.org/articles/2/35/2015/ap-2-35-2015.htmlThe full text article is available as a PDF file from https://ap.copernicus.org/articles/2/35/2015/ap-2-35-2015.pdf
The large-scale anisotropy (or the so-called star-diurnal wave) has been
studied using the calorimeter of the space-born experiment PAMELA. The cosmic
ray anisotropy has been obtained for the Southern and Northern hemispheres
simultaneously in the equatorial coordinate system for the time period
2006–2014. The dipole amplitude and phase have been measured for energies
1–20 TeVn-1.
Introduction
Despite the assumption that the arrival direction of cosmic
rays should be distributed isotropically due to the different processes that
they undergo during their propagation, scientists have been searching for any
anisotropies since the discovery of cosmic rays in 1912. Till recent years,
searches for anisotropies were conducted with ground-based experiments.
Moreover, until the 1990s the obtained results were strongly affected by
atmospheric conditions and imperfections on experimental equipment. A further
limitation was that the experiments were carried out in a single (mostly in
the Northern) hemisphere, restricting the angular range of the anisotropy
measurement. The first results of large-scale anisotropy measurements
obtained from satellite data are presented in this paper. The dipole phase
and amplitude were measured in a two-dimensional scale in an equatorial
coordinate system.
The PAMELA experiment
The PAMELA magnetic spectrometer is a satellite experiment that was launched
in the summer of 2006 and has been operating since then . The main
scientific goals of the experiment are the study of particle and antiparticle
fluxes in a wide energy range. The PAMELA apparatus consist of several
various detectors positioned around a magnetic spectrometer (tracker). While
the tracker is able to measure the deflection of particles in the magnetic
field up to energies of about 1 TeV, another PAMELA subdetector –
a calorimeter, can be used to extend the measured energy range. Furthermore
the calorimeter allows us to measure particle direction over a wide range of
angles. The calorimeter consists of 44 silicon planes, with 96 strip
detectors in each one; interleaved with 22 tungsten layers. In neighboring
silicon planes, strips are orthogonal, providing topological and longitudinal
information of the shower development.
The particle selection and reconstruction of their arrival
To measure the particle direction, the shower axis inside the calorimeter was
used. The iterative procedure was used to restore the axis along the primary
particle track throughout the 44 planes . This procedure is a fit of
the center of gravity of energy released in each plane of each view. The axis
reconstruction is possible when inclination of the particle direction respect
to the vertical of the calorimeter is less than 15∘. Events for which
the shower axis was reconstructed were further selected based on their total
energy deposition in the calorimeter. A cut threshold was set at the level of
180 000 mip corresponding to particles with energy
1–20 TeVnuc-1. The obtained statistics allowed the study of the
anisotropy in a one-dimensional map as a function of right ascension – RA.
The isotropy map creation
To create an isotropy map for comparison with the experimental one a
shuffling method was used (for details see ). The idea of this
method is to randomize the reconstructed directions of events. A set of
isotropic simulated events can be built by randomly coupling the times and
the directions of real events in local instrument coordinates. The
randomization is implemented starting with the position of a given event in
the PAMELA frame and exchanging it with the direction of another event, which
was selected randomly from the data set with a uniform probability. Since
this method was designed mostly to study point sources, especially in
gamma-ray astronomy, its reliability for dipole anisotropy studies with the
PAMELA calorimeter had to be verified. A data subset from the experimental
data was used to construct the dipole anisotropy, which looks as expected
from previous measurements, but with an amplitude of order 10 %, see
Fig. 1. The shuffling method was applied to this handmade dipole anisotropy.
As it is seen in Fig. 2, the features of the simulated anisotropy
disappeared, and the obtained distribution looks similar to the isotropic
case (they are different because the distribution with the dipole is part of
the experimental distribution). The results of this test proves that the
method can be used with the PAMELA calorimeter experimental data for the
dipole anisotropy search.
The simulated dipole anisotropy with an amplitude of 10 %.
The comparison of the two data sets after applying the shuffling
method, red dots – the initial data set and blue dots –the data with the
dipole anisotropy.
The results
In Fig. 3, the obtained dipole anisotropy is shown.
The dipole is obtained by integrating events within 180∘ (right
accession) with shift of 5∘ of each bin. The data set covers the time
period 2006–2014. The anisotropy is measured in the equatorial coordinate
system in terms of relative intensity . The phase is 27±8, and
the amplitude is 0.0011±0.0001. The amplitude is in excellent agreement
with HAWC , Ice-Cube and Bacsan results , while
the phase is also in agreement with Bacsan one and Super-Kamiokande
.
The Ir / Is-1 depending on RA. Ir – the real intensity, Is –
the simulated isotropic intensity. The red line is a fit by a sine wave.
Acknowledgements
We acknowledge support from The Italian Space Agency (ASI), Deutsches Zentrum
fur Luftund Raumfahrt (DLR), The Swedish National Space Board, The Swedish
Research Council, The Russian Space Agency (Roscosmos). This work has been
done with support of the Russian Science Foundation grant no. 14-12-00373 and
the RF President grant MK-4599.2014.2.
Edited by: K. Scherer Reviewed by: D. Strauss and H. Fichtner
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