First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle

TL;DR

The paper tackles measuring the Sun's halo gamma-ray emission produced by inverse-Compton scattering of galactic cosmic-ray electrons and positrons with sunlight over a full solar cycle. It develops a moving-source background modeling approach for 15 years of Fermi-LAT data, enabling robust detection of the halo from 31.6 MeV to 100 GeV out to 45° and enabling time, energy, and azimuthal studies of solar modulation. The authors show the halo is well described by simple ICS-based modulation models, with best-fit electron modulation around $\Phi_{0,e^-}\sim 475$–$500$ MV and negligible positron modulation, and they uncover time- and azimuthal-variation consistent with Earth-based CR measurements, while highlighting potential inner-Sun modulation effects. Collectively, these results constrain cosmic-ray transport in the inner heliosphere, validate heliospheric modulation theories, and establish a framework for solar-system gamma-ray studies with implications for space physics and beyond-Standard-Model searches near the Sun.

Abstract

We analyze 15 years of Fermi-LAT data and produce a detailed model of the Sun's inverse-Compton scattering emission (solar halo), which is powered by interactions between ambient cosmic-ray electrons and positrons with sunlight. By developing a novel analysis method to analyze moving sources, we robustly detect the solar halo at energies between 31.6 MeV and 100 GeV, and angular extensions up to 45$^\circ$ from the Sun, providing new insight into spatial regions where there are no direct measurements of the galactic cosmic-ray flux. The large statistical significance of our signal allows us to sub-divide the data and provide the first $γ$-ray probes into the time-variation and azimuthal asymmetry of the solar modulation potential, finding time-dependent changes in solar modulation both parallel and perpendicular to the ecliptic plane. Our results are consistent with (but with independent uncertainties from) local cosmic-ray measurements, unlocking new probes into both astrophysical and beyond-standard-model processes near the solar surface.

Figures (14)

• Figure 1: Spectrum (top) and morphology (bottom) of the ICS halo surrounding the Sun in one representative angular bin (top) and one representative energy bin (bottom), compared to simple solar halo models that employ a force-field potential that is equivalent for both electrons and positrons and are not fit to the data. Data are shown for our "low-background map". The solar halo is robustly detected between 30 MeV and 100 GeV and out to 45$^\circ$ from the Sun. We find a close match between our models (colored lines) and the data.
• Figure 2: The data, templates, and residuals in our analysis, evaluated at energies between 1 and 3.16 GeV in helioprojective coordinates. For this plot, we utilize the full dataset without point sources or the galactic plane masks ( high-statistics map), to convey the visual aspects of the plot more clearly. We show the Fermi-LAT data (top left) and the astrophysical background model (top right), noting the bright point sources that appear in each model. Arbitrarily normalized models for the solar disk (left) and solar halo (right) are shown on a log-scale in the second row, where we note the extension of the solar disk is driven by the $\sim$1$^\circ$ angular resolution in the 1--3.16 GeV bin. The third row shows the significance of the remaining residual after the best-fitting background and solar disk model are subtracted, which represents the true solar halo emission along with any mismodeled residuals. The bright emission in the center is the solar ICS halo. The true residual after the best-fit solar halo model is also subtracted is shown in the bottom right.
• Figure 3: Same as Fig. \ref{['fig:main']} (top), but showing the spectrum of the solar halo in eight different radial bins spanning between 0 and 45$^\circ$ from the Sun. The halo is robustly detected in the energy range from 31.6 MeV to 100 GeV. We show our theoretical models for ICS, including Model I (solid) and Model II (dashed). These models are not fit to the data, but instead follow a simple solar modulation potential that is fit to e$^+$e$^-$ data at the Earth position. This demonstrates the close match between theory and observation. We find that moderate modulation potentials near 500 MV provide the best fit to the $\gamma$-ray data.
• Figure 4: Same as Fig. \ref{['fig:main']} (bottom), but showing the radial properties of the solar halo in six different energy bins spanning between 31.6 MeV and 100 GeV. The halo is robustly detected in every energy bin, and out to the 45$^\circ$ limits of our analysis. We show two solar modulation models, Model I (solid) and Model II (dashed), which are again not fit to the data, and thus demonstrate the close match between theory and observation. The preference for a modulation potential near 500 MV is primarily driven by low-energy data, which dominates the total $\gamma$-ray count rate. At high energies, we note significant systematic errors that exceed the statistical fluctuations of our analysis.
• Figure 5: The relative log-likelihood fit to the $\gamma$-ray data in our ROI as a function of the solar modulation potential defined at 1 AU for electrons (x-axis) and positrons (y-axis). The best-fit models have electron modulation potentials of $\sim$475 MV, with negligible modulation for positrons. The uncertainty on the dominant electron contribution is much smaller than the uncertainty on the smaller positron contribution.