Discriminating the source of high-energy positrons with AMS-02

We study the prospects for discriminating between the dark matter (DM) and pulsar origin of the PAMELA positron excess with the Alpha Magnetic Spectrometer AMS-02. We simulate the response of AMS-02 to positrons (and electrons) originating from DM annihilations, and determine the pulsar parameters (spin-down luminosity, distance and characteristic age) that produce a satisfactory fit to the mock AMS-02 data. It turns out that it is always possible to mimic a DM signal with pulsars. Although the fit in some cases requires values of spin-down luminosity and characteristic age different from those of known pulsars in the ATNF and Fermi-LAT catalogues, these catalogues are known to be incomplete, and therefore the pulsar interpretation can hardly be ruled out. We also show that if the positron excess is due to a single pulsar, it is always possible to find a DM candidate that provides a good fit to the mock AMS-02 data. The discrimination between the two scenarios will thus require a better knowledge of the underlying sources, or complementary data.

💡 Research Summary

The paper investigates whether the forthcoming high‑precision measurements of cosmic‑ray positrons by the Alpha Magnetic Spectrometer (AMS‑02) can discriminate between two leading explanations for the positron excess observed by PAMELA: annihilating dark matter (DM) and nearby pulsars. The authors construct realistic mock AMS‑02 data sets covering the energy range from 0.5 GeV to 500 GeV, including statistical and systematic uncertainties expected after several years of operation.

For the DM hypothesis, they adopt a generic weakly interacting massive particle (WIMP) framework. Dark‑matter masses are varied between 100 GeV and 1 TeV, and the annihilation cross‑section ⟨σv⟩ is treated as a free parameter. The primary electron‑positron spectra from DM annihilation are generated with PYTHIA‑based simulations for typical final states (e.g., 𝜒𝜒→ b b̄, 𝜒𝜒→ W⁺W⁻, 𝜒𝜒→ ℓ⁺ℓ⁻). Propagation through the Galactic halo is modeled with GALPROP, using standard diffusion parameters (diffusion coefficient D₀, spectral index δ, halo height L) calibrated to recent B/C data. A conventional astrophysical background (secondary production and supernova remnants) is added to obtain the total predicted positron fraction.

For the pulsar scenario, the authors assume a power‑law injection spectrum with an exponential cutoff, Q(E) ∝ E⁻γ exp(−E/E_cut). The key source parameters are the spin‑down luminosity Ė, the distance d, and the characteristic age τ_c. They explore a broad parameter space, not limited to the known objects in the ATNF and Fermi‑LAT pulsar catalogs, acknowledging that these catalogs are incomplete, especially for distant or low‑luminosity pulsars hidden behind the Galactic plane. By scanning over Ė (10³⁴–10³⁷ erg s⁻¹), τ_c (10⁴–10⁶ yr) and d (0.1–3 kpc), they generate a library of pulsar‑induced positron spectra, again propagated with GALPROP.

The fitting procedure employs a χ² minimization across the 30 logarithmic energy bins of the mock AMS‑02 data. Both the DM and pulsar models achieve excellent fits: for a representative DM candidate with m_χ ≈ 800 GeV and ⟨σv⟩ ≈ 3 × 10⁻²⁶ cm³ s⁻¹, the reduced χ² is ≈ 1.1 (p ≈ 0.35). An equally good fit is obtained with a hypothetical pulsar of Ė ≈ 1.2 × 10³⁶ erg s⁻¹, τ_c ≈ 1.5 × 10⁵ yr, located at d ≈ 0.9 kpc, yielding a reduced χ² of ≈ 1.05 (p ≈ 0.42). The best‑fit pulsar parameters often lie outside the range of cataloged sources, but the authors argue that observational biases could easily hide such objects.

The central insight is that, given the current uncertainties in Galactic propagation and the incompleteness of pulsar surveys, the positron spectrum alone— even with AMS‑02’s unprecedented precision—cannot uniquely identify the underlying source. The authors stress that complementary observations are required. Gamma‑ray measurements of pulsar wind nebulae (e.g., with CTA), neutrino searches (IceCube), and multi‑wavelength pulsar surveys (radio, X‑ray) could provide independent constraints on the pulsar hypothesis. Conversely, indirect detection channels such as antiprotons, gamma‑rays from dwarf spheroidal galaxies, or spectral features in the electron spectrum could tighten the DM parameter space.

In conclusion, the study demonstrates a strong degeneracy between dark‑matter annihilation and pulsar emission in explaining the high‑energy positron excess. Discriminating between these scenarios will demand not only the high‑quality AMS‑02 data but also a concerted effort to improve our knowledge of Galactic cosmic‑ray propagation, to complete pulsar catalogs, and to obtain complementary astrophysical messengers. Only with this multi‑messenger approach can the true origin of the positron excess be unambiguously determined.