Anisotropy of Cosmic Ray Fluxes measured with AMS-02 on the ISS M. - - PowerPoint PPT Presentation

Anisotropy of Cosmic Ray Fluxes measured with AMS-02 on the ISS

• M. A. Velasco, CIEMAT, Madrid (Spain)
• n behalf of the AMS Collaboration

ORIGIN OF THE POSITRON EXCESS

Positron spectrum shows a significant excess above 25 GeV that is not consistent with only the secondary production of positrons The observation requires the inclusion of primary sources whether from a particle physics or an astrophysical origin Astrophysical point sources of cosmic ray positrons may induce some degree of anisotropy on the measured positron flux

• M. A. Velasco – TAUP 2019

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[Phys. Rev. Lett. 122, 041102 (2019)]

The cutoff energy (~800 GeV) is established with a significance of more than 4σ

ORIGIN OF THE ELECTRON EXCESS

Electron spectrum shows a significant excess above 42 GeV that is not consistent with lower energy trends The electron flux does not have an energy cutoff below 1.9 TeV, i.e. high energy electrons originate from different sources that positrons Astrophysical nearby sources of cosmic ray electrons may induce some degree of anisotropy on the measured electron flux

• M. A. Velasco – TAUP 2019

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[Phys. Rev. Lett. 122, 101101 (2019)]

ORIGIN OF PROTON & LIGHT NUCLEI FLUX DEVIATION

Proton and light nuclei fluxes measured by AMS show a deviation from a single power law above 200 GeV This observation may require modification of cosmic ray transport models

• r the inclusion of local sources of high rigidity events

A nearby source of cosmic ray protons or light nuclei may induce some degree of anisotropy in the high rigidity sample

• M. A. Velasco – TAUP 2019

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AMS Preliminary data Refer to the upcoming AMS publication

[Phys. Rev. Lett. 119, 251101 (2017)]

Galactic center Solar System Forward-Backward direction North-South direction East-West direction

ANALYSIS OF THE ANISOTROPY

Measurement of the cosmic ray fluxes as function of the arrival direction in Galactic Coordinates

• M. A. Velasco – TAUP 2019

5 X Z Y

SPHERICAL HARMONIC EXPANSION OF CR FLUXES

• M. A. Velasco – TAUP 2019

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Multipolar components Real spherical harmonics basis

The directional dependence of the CR flux is described in terms of an expansion in spherical harmonics Dipole anisotropy (ℓ=1)

Dipole amplitude Dipole components

East-West North-South Forward-Backward

AMS SKY COVERAGE

• M. A. Velasco – TAUP 2019

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Position Direction

Geographic coordinates Galactic coordinates

12º

E/p

e+ p

AMS-02 Data

POSITRON ANISOTROPY

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Proton background is reduced below the percent level with a selection based on cuts on E/p and the TRD and ECAL estimators Sample selection For the anisotropy analysis, selected events are grouped into 5 cumulative energy ranges: E > 16, 25, 40, 65, and 100 GeV

Event Sample: AMS 6.5 years 9.9x104 e+ (16 < E < 350 GeV)

POSITRON ANISOTROPY

• M. A. Velasco – TAUP 2019

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The arrival directions of positron events are compared to the expected map for an isotropic flux in Galactic coordinates Isotropic map 16 < E < 350 GeV 9.9 × 104 positrons

• M. A. Velasco – TAUP 2019

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Computation of isotropic map requires detailed understanding of detector efficiencies at different geographical locations

Geographical Coordinates Galactic Coordinates

2%

TRD Efficiency TRD Efficiency

cos(θM)

16 < E < 350 GeV

16 < E < 350 GeV

POSITRON ANISOTROPY: DETECTOR EFFICIENCIES

POSITRON ANISOTROPY

• M. A. Velasco – TAUP 2019

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Sky map of the relative fluctuation of the positron arrival directions in galactic coordinates The observed sky map shows no evident pattern 16 < E < 350 GeV

POSITRON ANISOTROPY: DIPOLE COMPONENTS

• M. A. Velasco – TAUP 2019

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Galactic Coordinates

ρEW ρNS ρFB

Results consistent with isotropy in all the dipole components and energy ranges

• M. A. Velasco – TAUP 2019

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Amplitude of the dipole anisotropy on e+ for 16 < E < 350 GeV δ < 1.9% at the 95% C.I.

[Phys. Rev. Lett. 122, 041102 (2019)]

Upper limits are set for each energy range

POSITRON ANISOTROPY: DIPOLE UPPER LIMITS

• M. A. Velasco – TAUP 2019

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In addition to the sensitivity to nearby astrophysical sources, the measurement of electron anisotropy provides a test of systematics for the positron analysis Electron sample AMS 6.5 years: 1.3 × 106 events (16 < E < 350 GeV)

ρEW ρNS ρFB

Results consistent with isotropy in all the dipole components and energy ranges

ELECTRON ANISOTROPY

Dipole components – Galactic Coordinates

• M. A. Velasco – TAUP 2019

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Amplitude of the dipole anisotropy on e- for 16 < E < 350 GeV δ < 0.5% at the 95% C.I.

[Phys. Rev. Lett. 122, 101101 (2019)]

Upper limits are set for each energy range

ELECTRON ANISOTROPY: DIPOLE UPPER LIMITS

PROTON ANISOTROPY

• M. A. Velasco – TAUP 2019

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The arrival directions of proton events collected in the first 7.5 years are compared to the expected map for an isotropic flux in Galactic coordinates Isotropic map R > 18 GV 1.3 × 108 protons Selected events are grouped into 9 cumulative rigidity ranges with R > 18, 30, 45, 80, 150, 200, 300, 500 and 1000 GV

• M. A. Velasco – TAUP 2019

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Computation of the isotropic map requires detailed understanding of detector efficiencies at different geographical locations

3% 2% R > 18 GV R > 18 GV

Geographical Coordinates Galactic Coordinates

cos(θM)

PROTON ANISOTROPY: DETECTOR EFFICIENCIES

• M. A. Velasco – TAUP 2019

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Galactic Coordinates Results consistent with isotropy in all the dipole components and rigidity ranges

ρEW ρNS ρFB

PROTON ANISOTROPY: DIPOLE COMPONENTS

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Amplitude of the dipole anisotropy on protons for R > 200 GV (2×106 events) δ < 0.38% at the 95% C.I.

AMS Preliminary data Refer to the upcoming AMS publication

Upper limits are set for each rigidity range

PROTON ANISOTROPY: DIPOLE UPPER LIMITS

HELIUM, CARBON & OXYGEN ANISOTROPY

• M. A. Velasco – TAUP 2019

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cos(θM) Helium R > 18 GV

► Similar analysis applied to the He, C and O

samples collected in 7.5 years

► Reduced amplitude of the geographical

dependence of the detector efficiencies allows to use extended detector acceptance

0.5%

• M. A. Velasco – TAUP 2019

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AMS Preliminary data Refer to the upcoming AMS publication AMS Preliminary data Refer to the upcoming AMS publication AMS Preliminary data Refer to the upcoming AMS publication

Helium Carbon Oxygen

HELIUM, CARBON & OXYGEN ANISOTROPY

All measurements found compatible with isotropy and upper limits to the amplitude

• f the dipole component are set

Helium: δ < 0.36% for R > 200 GV (2.2 × 106 He) Carbon: δ < 1.9% for R > 200 GV (6.1 × 104 C) Oxygen: δ < 1.7% for R > 200 GV (6.3 × 104 O)

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SUMMARY

1. The precise measurements performed by AMS on positron, electron, protons and nuclei fluxes show unexpected features that challenge the traditional paradigm of cosmic rays 2. The study of the directionality of cosmic rays, i.e. the anisotropy, provides complementary information to the spectra and may help to understand the origin of these features 3. A measurement of the anisotropy in the arrival directions of cosmic ray electrons, positrons, protons, helium, carbon and oxygen has been performed in galactic coordinates

► No deviation from isotropy has been observed and upper limits to

the dipole amplitude have been quoted 4. AMS will continue taking data until the end of ISS operation, currently 2024. By that time positron statistics will allow us to reach the 1% level predicted by pulsars models