Galactic Signatures of Decaying Dark Matter

If dark matter decays into electrons and positrons, it can affect Galactic radio emissions and the local cosmic ray fluxes. We propose a new, more general analysis of constraints on dark matter. The constraints can be obtained for any decaying dark matter model by convolving the specific dark matter decay spectrum with a response function. We derive this response function from full-sky radio surveys at 408 MHz, 1.42 GHz and 23 GHz, as well as from the positron flux recently reported by PAMELA. We discuss the influence of astrophysical uncertainties on the response function, such as from propagation and from the profiles of the dark matter and the Galactic magnetic field. As an application, we find that some widely used dark matter decay scenarios can be ruled out under modest assumptions.

💡 Research Summary

The paper addresses the observable consequences of dark‑matter (DM) particles that decay into electrons and positrons. Such decay products generate synchrotron radiation while spiralling in the Galactic magnetic field, producing a diffuse radio signal, and they also contribute to the local cosmic‑ray (CR) positron flux measured near Earth. The authors develop a model‑independent framework that translates any specific DM decay spectrum into observable constraints by means of a “response function”. This response function, R(ν,E), quantifies how an injected electron or positron of energy E contributes to the measured intensity at radio frequency ν (or to the positron flux at Earth). Mathematically the observable O(ν) is expressed as a convolution O(ν)=∫dE R(ν,E) (dN/dE), where dN/dE is the decay spectrum. Once R is computed, any decay model can be tested without re‑running a full propagation simulation.

To construct R, the authors use three full‑sky radio surveys—408 MHz (Haslam), 1.42 GHz (Reich & Reich), and 23 GHz (Planck)—together with the positron flux measured by PAMELA. They employ the GALPROP code to simulate the propagation of electrons and positrons in the Milky Way, incorporating diffusion, convection, energy losses (synchrotron, inverse‑Compton, bremsstrahlung), and re‑acceleration. Three benchmark propagation parameter sets (MIN, MED, MAX) are adopted to capture the astrophysical uncertainties. The DM density profile is taken as either a Navarro‑Frenk‑White (NFW) or an Einasto distribution, and two magnetic‑field configurations are explored: a simple axisymmetric disk with a uniform strength of ~5 µG, and a more realistic spiral‑arm model with spatially varying field strength.

The resulting response functions show distinct energy‑frequency dependencies. At 23 GHz the signal is dominated by electrons in the 10–100 GeV range, while the lower‑frequency maps are more sensitive to lower‑energy electrons. The PAMELA positron data provide complementary constraints in the same energy band. By convolving R with a variety of benchmark decay spectra (e.g., μ⁺μ⁻, τ⁺τ⁻, W⁺W⁻) the authors map out the allowed region in the plane of DM mass versus decay lifetime. They find that for typical masses around 1 TeV, lifetimes shorter than ∼10²⁶ s are excluded for most channels under modest astrophysical assumptions. In particular, the μ⁺μ⁻ channel with τ≈10²⁵ s overproduces both the 408 MHz synchrotron emission and the high‑energy positron flux, violating the data simultaneously. The analysis also quantifies how the uncertainties in diffusion coefficients, halo height, magnetic‑field geometry, and DM profile shift the exclusion curves, demonstrating that the constraints are robust against reasonable variations.

The key contribution of the work is the establishment of a reusable response‑function library that can be applied to any future DM decay model. As new radio surveys (e.g., SKA) and higher‑precision CR measurements (e.g., AMS‑02, DAMPE) become available, the same framework can be updated simply by recomputing R with the improved data, allowing rapid reassessment of model viability. The authors also discuss extensions such as asymmetric decays, multi‑channel mixtures, and extragalactic synchrotron signatures from galaxy clusters.

In summary, the study provides a powerful, general tool for translating Galactic radio observations and local positron measurements into stringent, model‑independent limits on decaying dark‑matter scenarios. It demonstrates that several widely‑cited decay models are already ruled out under conservative astrophysical assumptions, and it sets the stage for future refinements as observational capabilities advance.