APASTRA ProceedingsAPASTRA Proc.2199-3963Copernicus GmbHGöttingen, Germany10.5194/ap-2-17-2015Search for a positron anisotropy with PAMELA experimentPanicoB.beatrice.panico@na.infn.itAdrianiO.BarbarinoG. C.BazilevskayaG. A.BellottiR.BoezioM.BogomolovE. A.BongiM.BonviciniV.BottaiS.BrunoA.CafagnaF.CampanaD.CarlsonP.CasolinoM.CastelliniG.De DonatoC.De SantisC.De SimoneN.Di FeliceV.FormatoV.GalperA. M.GiaccariU.KarelinA. V.KoldashovS. V.KoldobskiyS.KrutkovS. Y.KvashninA. N.LeonovA.MalakhovV.MarcelliL.MartucciM.MayorovA. G.MennW.MergéM.MikhailovV. V.https://orcid.org/0000-0003-3851-2901MocchiuttiE.MonacoA.MoriN.MuniniR.OsteriaG.PalmaF.PearceM.PicozzaP.RicciM.RicciariniS. B.SarkarR.ScottiV.SimonM.SparvoliR.SpillantiniP.StozhkovY. I.VacchiA.VannucciniE.VasilyevG. I.VoronovS. A.YurkinY. T.ZampaG.ZampaN.INFN, Sezione di Napoli, 80126 Naples, ItalyUniversity of Naples “Federico II”, Department of Physics, 80126 Naples, ItalyUniversity of Florence, 50019 Sesto Fiorentino, Florence, ItalyINFN, Sezione di Firenze, 50019 Sesto Fiorentino, Florence, ItalyLebedev Physical Institute, 119991, Moscow, RussiaUniversity of Bari, Department of Physics, 70126 Bari, ItalyINFN, Sezione di Bari, 70126 Bari, ItalyINFN, Sezione di Trieste, 34149 Trieste, ItalyIoffe Physical Technical Institute, 194021 St. Petersburg, RussiaKTH, Department of Physics, and the Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 10691 Stockholm, SwedenINFN, Sezione di Roma Tor Vergata, 00133 Rome, ItalyRIKEN, Advanced Science Institute, Wako-shi, Saitama, JapanIFAC, 50019 Sesto Fiorentino, Florence, ItalyUniversity of Rome Tor Vergata, Department of Physics, 00133 Rome, ItalyAgenzia Spaziale Italiana (ASI) Science Data Center, Via del Politecnico snc 00133 Rome, ItalyUniversity of Trieste, Department of Physics, 34147 Trieste, ItalyNational Research Nuclear University MEPhI, 115409 MoscowINFN, Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044 Frascati, ItalyUniversitat Siegen, Department of Physics, 57068 Siegen, Germanynow at: Universidade Federal do Rio de Janeiro, Instituto de Fisica, Rio de Janeiro, RJ, Brazilformerly at: INFN, Sezione di Trieste, 34149 Trieste, ItalyB. Panico (beatrice.panico@na.infn.it)9September201522172018May201517July201520August2015This 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/2/17/2015/ap-2-17-2015.htmlThe full text article is available as a PDF file from https://ap.copernicus.org/articles/2/17/2015/ap-2-17-2015.pdf
The PAMELA experiment has been collecting data since 2006; its results
indicate a rise in the positron fraction with respect to the sum of electrons
and positrons in the cosmic-ray (CR) spectrum above 10 GeV. This excess can
be due to additional sources, as SNRs or pulsars, which can lead to an
anisotropy in the local CR positron, detectable from current experiments. We
report on the analysis on spatial distributions of positron events collected
by PAMELA, taking into account also the geomagnetic field effects. No
significant deviation from the isotropy has been observed.
Event maps for positrons (left) and protons (right).
Significance maps for 10 GV ≤R≤ 200 GV, over the
following angular scales: 10, 30, 60, 90∘.
Angular power spectra C(l) as a function of the multipole l.
Grey area represents the systematic effects.
Introduction
In the last years, different experiments like PAMELA (),
Fermi () and AMS-02
() showed a rise of the positron fraction for energies
greater than 10 GeV. Diffusive models which consider only a purely secondary
positron production, as by and ,
cannot explain these results. Therefore, an additional source of CR positrons
is required; the most favoured hypotheses are related to nearby astrophysical
sources, like supernova remnants or pulsars, or to a contribution from dark
matter decay or annihilation. In both cases, the presence of an additional
source can produce structure with definite angular width in the collected
data, leading to an anisotropy in the positron flux direction.
Fermi results () are compatible with an
isotropic distribution of CR arrival directions, but refer to the total
sample of electron and positron events. Latest results on positron
anisotropies are reported in , which set an upper limit on
the amplitude of the dipole anisotropy δ≤0.030 at the 95 % C.L
for E>16 GeV. We studied the distribution of a sample of positrons
measured by PAMELA, considering the effect of the geomagnetic field and
providing an analysis of the systematic effects which could affect the
results. Also a multipole analysis of the power spectrum has been carried
out. A detailed description of the method and the results of the performed
analysis is reported in .
The PAMELA detector
PAMELA is a space-based CR detector, installed on the upward side of the
Russian Resurs-DK1 satellite; it was launched 15 June 2006 and it is still
taking data. PAMELA orbit is elliptical with an inclination of
∼ 70∘ and an altitude ranging between 350 and 610 km. In
September 2010, the orbit was changed to a nearby circular one, at an
altitude of ∼ 570 km. PAMELA is composed of different detectors: a
time of flight system, a magnetic spectrometer, an electromagnetic
calorimeter, an anticoincidence system, a bottom scintillator counter and a
neutron detector. All instruments are described in detail by
. The particle arrival direction is reconstructed using the
trajectory inside the instrument and the satellite position on the orbit,
with an accuracy of about 2∘ over the whole energy range.
Analysis of positron anisotropy
The first step of the analysis is the selection of positrons in the rigidity
range from 10 to 200 GV. The dataset refers to the period June
2006–January 2010, to be comparable with data used in .
Also the track and event quality selection are preserved, leading to a
negligible amount of proton contamination, as described by .
To represent the isotropic background used as reference, we also select a
sample of protons in the same period of time and rigidity range, taking care
to preserve the instrument exposure, the dead times and any other detector
effect. About ∼2×103 positrons and ∼4.5×105
protons have been obtained. To consider the effects of the Earth's magnetic
field, which could smear a weak anisotropy, particles were backtraced up to
∼ 25 Earth radii on the basis of numerical integration methods
(). Backtraced positrons and protons are reported in
Fig. , respectively on the left and right side, by using the
Healpix software (). The angular pixel extension is ∼7∘ and the Galactic reference system is used. In a blind search, the
size of the anisotropy signal is not known. Therefore, to increase
sensitivity, we integrate signal and background maps on four different radii,
10, 30, 60, 90∘, which represent the angular scale at which the
anisotropy could be expected. Finally, we compare the integrated maps with
two different techniques: the statistical significance test introduced by
and the spherical harmonic analysis. In the first analysis, the
significance is evaluated for each integration radius, applying the
likelihood functions on signal and background maps. The results are shown in
Fig. . PAMELA has an uniform exposure over the entire sky,
therefore we can expand the CR intensity over the celestial sphere in
spherical harmonics (). In Fig. the
power spectrum is reported from modes l=1 (dipole) up to l=20; the
dotted lines represent the 5σ bounds of the expected power spectrum
from an isotropic sky. Also systematic effects are considered taking into
account the energy and angular resolution of the instrument. The resulting
estimates are reported as grey bands in Fig. .
Conclusions
The analysis on the arrival direction of positrons detected by PAMELA has
been carried out. Results are consistent with an isotropic distribution at
all angular scales considered. Also the power spectrum is compatible with the
prediction of an isotropic sky. The results of this analysis are in
agreeement with those published by and ,
taking into account the differences previously described.Edited by: K. Scherer Reviewed by: P. Kuhl and one anonymous referee
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