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Measurement of the high-energy gamma-ray emission from the Moon with the Fermi Large Area Telescope

M. Ackermann et al. (Fermi LAT Collaboration)
Phys. Rev. D 93, 082001 – Published 8 April 2016
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  • INTRODUCTION
  • LUNAR GAMMA-RAY EMISSION SPECTRUM
  • DATA SELECTION
  • DATA ANALYSIS AND RESULTS
  • TIME EVOLUTION STUDIES
  • MONTE CARLO SIMULATION OF CR…
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • References

    Abstract

    We have measured the gamma-ray emission spectrum of the Moon using the data collected by the Large Area Telescope onboard the Fermi satellite during its first seven years of operation, in the energy range from 30 MeV up to a few GeV. We have also studied the time evolution of the flux, finding a correlation with the solar activity. We have developed a full Monte Carlo simulation describing the interactions of cosmic rays with the lunar surface. The results of the present analysis can be explained in the framework of this model, where the production of gamma rays is due to the interactions of cosmic-ray proton and helium nuclei with the surface of the Moon. Finally, we have used our simulation to derive the cosmic-ray proton and helium spectra near Earth from the Moon gamma-ray data.

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    • Received 23 December 2015

    DOI:https://doi.org/10.1103/PhysRevD.93.082001

    This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Cosmic ray sourcesCosmic rays & astroparticlesGamma ray astronomy
    1. Physical Systems
    Solar cycles
    Gravitation, Cosmology & Astrophysics

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    Issue

    Vol. 93, Iss. 8 — 15 April 2016

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    Images

    • Figure 1
      Figure 1

      Significance map of the Moon as a function of right ascension and declination relative to the instantaneous Moon position for photons in the energy range from 30 MeV to 10 GeV. The map is built using a HEALPix [16] pixelization of the sky with Nside=256 (each pixel corresponds to a solid angle ≈1.6×10−5  sr). The significance is evaluated following the prescriptions of Ref. [15].

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    • Figure 2
      Figure 2

      Count distributions as a function of gamma-ray energy for the signal (black circles) and background (red circles) regions. Blue symbols represent the net signal count spectrum, evaluated by the method described in Ref. [17]. Circles and associated error bars represent the average values and the rms values of the corresponding PDFs. Arrows represent upper limits at 95% confidence level.

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    • Figure 3
      Figure 3

      Gamma-ray energy spectrum of the Moon. The flux values ϕγ(E) in each bin are multiplied by E2=E1E2, where E1 and E2 are the lower and upper energy edges of each bin. The results from the present analysis (black points) are compared with those published in Ref. [3]. Only statistical error bars are shown. The central values of each bin represent the mean flux values, while the error bars represent the rmss of the corresponding PDFs.

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    • Figure 4
      Figure 4

      (a) Time evolution of the gamma-ray intensity from the Moon. The red, green, blue, and purple symbols represent the intensites above 56, 75, 100, and 178 MeV respectively. The dashed lines indicate the average values calculated over the whole data-taking period. (b) Time evolution of the corrected count rates registered by the neutron monitors of McMurdo (red), Newark (green), South Pole (blue), and Thule (purple). The data of the neutron monitors correspond to the good time intervals selected for the Moon data analysis. Each point of the plot corresponds to an average value taken over a 6 month period.

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    • Figure 5
      Figure 5

      Comparison between the gamma-ray integral intensities from the Moon above 56 (red), 75 (green), 100 (blue), and 178 MeV (purple) and the count rate registered by the McMurdo neutron monitor. The dashed lines represent the linear regression curves of each series. The values reported in brackets are the correlation coefficients.

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    • Figure 6
      Figure 6

      Yields of gamma rays produced by the interactions of protons (top) and He4 nuclei (bottom) on the Moon. The yields have been evaluated assuming the MP composition model.

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    • Figure 7
      Figure 7

      Average number of gamma rays per primary particle (in units of photons/particle) produced by primary protons (black) and He4 nuclei (red) as a function of the primary particle kinetic energy per nucleon. The calculations have been performed for both the MP (continuous lines) and the TUR (dashed lines) composition models.

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    • Figure 8
      Figure 8

      Differential photon energy flux from the Moon produced by the interactions of protons (top) and He4 nuclei (bottom) with the Moon surface. The photon intensities have been evaluated by folding the gamma-ray yields with the CR proton and helium intensity spectra measured by AMS-02 [5, 6]. The calculation has been performed with the Moon surface composition model in Ref. [35].

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    • Figure 9
      Figure 9

      Gamma-ray flux from the Moon as a function of energy in the period May 2011–November 2013. The results from the LAT data analysis (black points) are compared with the expected fluxes obtained after folding the CR proton and helium spectra measured by AMS-02 in 2011-13 with the gamma-ray yields evaluated in Sec. 6a with our simulation. The calculations were perfomed using the lunar surface composition models in Refs. [35] (left) and [36] (right). The continuous red lines indicate the total flux, while the dashed blue and purple lines represent the contributions to the lunar gamma-ray spectrum from protons and helium nuclei respectively.

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    • Figure 10
      Figure 10

      Left panel: CR proton and helium spectra obtained from the best fit of the Fermi LAT Moon gamma-ray data. The fit was performed using the MP lunar surface model. The results of the fit (continuous black and red lines) are compared with the proton measurements taken by PAMELA [39] in 2008 (blue points) and 2009 (purple points) and with the AMS-02 proton [5] (cyan points) and helium data [6] (violet points). The plot shows also the proton and helium LIS (dashed black and red lines) and the Voyager 1 proton (light green points) and helium (dark green) data [44]. Right panel: Gamma-ray flux from the Moon as a function of energy. The results from our analysis are compared with those of the fit. The continuous red line represents the average gamma-ray spectrum obtained from the fit, assuming that the Moon-LAT distance is equal to its average value during the whole data-taking period.

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    • Figure 11
      Figure 11

      Time evolution of the solar modulation potential, evaluated from a fit of the lunar gamma-ray emission. The central band corresponds to the average value of the solar modulation potential during the whole data-taking period.

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