The impact of gamma-ray propagation effects on indirect dark matter searches

Abstract

In this work, we investigate dark matter (DM) detection in the context of weakly interacting massive particles (WIMPs). Upon annihilation, WIMPs generate cascades of secondary particles through various channels, many of which culminate in the production of gamma rays. As these gamma rays travel toward Earth, their spectra are reshaped by interactions with the intervening medium. While current models typically account for attenuation via pair production on the extragalactic background light, they often neglect the fate of the resulting electrons and positrons, specifically subsequent inverse Compton scattering of these secondary particles, which can regenerate high-energy gamma rays. Here, we revisit the predicted gamma-ray fluxes from WIMP annihilation by performing a more detailed treatment of propagation effects. We show that for distant sources and annihilation channels such as $τ^+τ^-$, the full treatments can significantly alter the observed gamma-ray flux, by up to a factor of three orders of magnitude for heavy WIMPs. This has an impact on current dark matter limits derived without taking into account propagation effects, depending on the considered WIMP mass and annihilation channel. Our study demonstrates the importance of a detailed propagation treatment for indirect dark matter searches, and the need to account for such effects in order to obtain accurate, more reliable dark matter signal predictions and exclusion limits.

Figures (8)

• Figure 1: Spectrum of gamma rays produced through the $b\bar{b}$ (left) and $\tau^{+}\tau^{-}$ (right panel) annihilation channels ($dN_\gamma / dE_\gamma$), as a function of the energy, for various DM masses.
• Figure 2: Inverse mean free path ($\lambda^{-1}$) of gamma rays as a function of energy. The purple curve represents interactions with the CMB) The red shaded band represents the uncertainty range of the EBL model Saldana_Lopez_2021, corresponding to the lower and upper limits derived from observational constraints. The central line indicates the best-fit model, and the contribution from the CRB is also shown in orange. Data from ref NITU2021102532. The blue dashed line indicates the inverse distance corresponding to the redshift of the Perseus Cluster ($z \sim 0.017$) kang2024_perseus. This distance is highlighted due to it's role in later sections, where it is used to constrain WIMP properties.
• Figure 3: Same as Figure \ref{['fig: free path PP']}, but for inverse Compton scattering between high-energy electrons and background photons from the CMB, EBL, and CRB. The inverse distance to the Perseus cluster is included for reference, as it serves as a benchmark in the analysis of WIMP-induced gamma-ray fluxes presented in later sections.
• Figure 4: Simulated dimensionless gamma-ray flux, expressed as $E \frac{dN}{dE}$, as a function of energy (in GeV) for the $b\bar{b}$ annihilation channel (left column) and the $\tau^{+}\tau^{-}$ channel (right column). The panels correspond to DM masses of 1 TeV (upper row), 10 TeV (middle row), and 100 TeV (lower row). Dashed gray lines denote the annihilation spectra at the source position, while black, purple, red and yellow solid lines correspond to different source distances, 0.5, 5, 100, and 600 Mpc, respectively. All relevant interactions during propagation are taken into account—including, and more importantly, inverse Compton scattering—which progressively alters the spectral shape as the distance increases.
• Figure 5: Ratio between the expected fluxes at Earth considering the effects of inverse Compton scattering (ICS) and ignoring it (nICS) ($F_{ICS} / F_{nICS}$) as a function of energy (in GeV) for $b\bar{b}$ (left) and $\tau^{+}\tau^{-}$ (right panels) channels, for different DM masses: 1 TeV (upper panels), 10 TeV (middle), and 100 TeV (lower panels). The black, purple, red and yellow lines correspond to different source distances, 0.5, 5, 100, and 600 Mpc, respectively.