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A Solar Probe of Dark Matter Decay in the Galaxy

Maximilian Detering, Shyam Balaji

Abstract

Dark matter (DM) particles decaying in the Galactic halo can inject energetic $e^\pm$ that inverse-Compton scatter (ICS) solar photons into $γ$-rays, producing a diffuse and extended halo of emission around the Sun. We present the first quantitative study of this signal as an indirect probe of decaying DM. The intense solar photon field in the inner heliosphere amplifies the inverse-Compton emissivity by many orders of magnitude relative to the interstellar radiation field, making the Sun an unusually sensitive local converter of sunlight into $γ$-rays via scattering with injected $e^\pm$. Using 15 years of Fermi-LAT solar-halo data, we derive stringent limits on the DM lifetime for 10 GeV-10TeV masses at the level of $τ_χ\sim 10^{27}\,\mathrm{s}$ in leptonic decay channels. The predicted surface brightness rises steeply toward the Sun, and the $γ$-ray flux falls off at high energy due to Klein-Nishina suppression. Solar ICS $γ$-rays measured with degree scale angular resolution therefore provide a novel and complementary probe of DM decays, adding a local $γ$-ray search channel that is systematically distinct from both Galactic diffuse analyses and direct charged-particle measurements.

A Solar Probe of Dark Matter Decay in the Galaxy

Abstract

Dark matter (DM) particles decaying in the Galactic halo can inject energetic that inverse-Compton scatter (ICS) solar photons into -rays, producing a diffuse and extended halo of emission around the Sun. We present the first quantitative study of this signal as an indirect probe of decaying DM. The intense solar photon field in the inner heliosphere amplifies the inverse-Compton emissivity by many orders of magnitude relative to the interstellar radiation field, making the Sun an unusually sensitive local converter of sunlight into -rays via scattering with injected . Using 15 years of Fermi-LAT solar-halo data, we derive stringent limits on the DM lifetime for 10 GeV-10TeV masses at the level of in leptonic decay channels. The predicted surface brightness rises steeply toward the Sun, and the -ray flux falls off at high energy due to Klein-Nishina suppression. Solar ICS -rays measured with degree scale angular resolution therefore provide a novel and complementary probe of DM decays, adding a local -ray search channel that is systematically distinct from both Galactic diffuse analyses and direct charged-particle measurements.

Paper Structure

This paper contains 32 sections, 39 equations, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Solar γ-rays from inverse Compton scattering
  3. Emissivity
  4. Solar photon field
  5. Cosmic-ray electron and positron fluxes
  6. Cosmic-ray electron and positron background flux
  7. DM-induced flux
  8. Solar modulation
  9. Compton scattering cross section
  10. Line-of-sight intensity and observed γ-ray flux
  11. Analysis
  12. Data
  13. Background and bias
  14. Signal templates
  15. Likelihood
  16. ...and 17 more sections

Figures (6)

  • Figure 1: Geometry and kinematics for solar ICS in the SRF.
  • Figure 2: $\gamma$-ray flux and IC photon flux from background electron and positron fluxes. Top: Measured gamma ray flux (black) compared to the background IC $\gamma$-ray flux (blue, green, red) with an assumed constant relative systematic uncertainty of $\sim 30\%$. Bottom: Residuals between the data and the background flux for different values of the solar modulation potential, including a systematic uncertainty on the background shown as error bars. The IC photon flux is largely unaffected by solar modulation at high energies (gray-shaded).
  • Figure 3: Example of the DM-induced flux from decay via $\chi \to e^+ e^-$ compared to the data for a benchmark template with $m_{\chi}=100GeV$ and $\tau_\chi=e25s$. Top: Signal spectrum for the innermost annulus within $0.26^\circ < \theta < 2.5^\circ$ of the Sun. Bottom: Signal morphology in the energy bin centered at $0.2GeV$.
  • Figure 4: Upper limit on the lifetime $\tau_\chi$ of DM decaying into $e^+e^-$ (left panel) and $\mu^+\mu^-$ (right panel), based on $\gamma$-ray emission in the solar halo.
  • Figure 5: Comparison of the most recent $\gamma$-ray and cosmological constraints (gray) on decaying DM in the $e^{+}e^{-}$ channel with the PCL limits at 95 % CL from the solar $\gamma$-ray halo measurement (blue-shaded). Prospective limits on decaying DM from solar $\gamma$-ray halo measurements are indicated by the dashed blue line.
  • ...and 1 more figures