Positron production scenarios and the angular profile of the galactic center 511-keV line

The observed angular profile of the 511-keV photon excess from the Milky Way galactic center can allow us to select among combinations of various dark matter and other positron production mechanisms with various models for the dark matter distribution. We find that a relic decay scenario gives too flat an angular distribution for any dark matter distribution in our survey, but that a dark matter-dark matter collisional scenario, or a scenario that involves particles emitted from a localized central source producing positrons some distance out, can match the observed galactic center angular profile if the dark matter distribution is neither too flat nor too cuspy. Additionally, positron migration or diffusion before annihilation broadens the angular profile to an extent that an average migration of more than half a kiloparsec is not viable with most dark matter distributions. The observed angular profile is also consistent with the occurrence of transient events in the past, followed by isotropic positron diffusion.

💡 Research Summary

The paper investigates the angular distribution of the 511 keV photon excess observed toward the Milky Way’s Galactic Center (GC) and uses this spatial information to discriminate among several proposed positron‑production mechanisms. The authors consider both dark‑matter (DM)–related scenarios—namely long‑lived particle decay and DM–DM collisions (annihilation or semi‑annihilation)—and non‑DM mechanisms such as a localized central source that emits high‑energy particles which subsequently generate positrons at some distance from the source. For each mechanism they explore a suite of DM density profiles, including cuspy Navarro‑Frenk‑White (NFW), cored isothermal, Burkert, and more flattened distributions, thereby covering the range of plausible Galactic halo shapes inferred from rotation curves and N‑body simulations.

The methodology proceeds in three steps. First, the spatial emissivity of positrons is calculated for each production channel, taking into account the relevant particle physics parameters (decay lifetime τ, annihilation cross‑section ⟨σv⟩, branching ratios into e⁺e⁻, π⁰, μ⁺μ⁻, etc.). Second, the authors model the subsequent propagation of positrons before annihilation using a diffusion equation ∂n/∂t = D∇²n − n/τ_ann, where D is the diffusion coefficient and τ_ann the annihilation timescale in the dense interstellar medium of the GC. The characteristic migration length L ≈ √(D τ_ann) is treated as a free parameter and varied from 0.1 kpc up to 1 kpc. Third, the line‑of‑sight integration of the annihilation rate yields a predicted angular profile, which is directly compared to the measured 511 keV intensity as a function of angle from the GC (as reported by INTEGRAL/SPI).

The results are striking. Decay‑driven models produce a very flat angular profile for any of the DM density profiles considered. Because decay injects positrons throughout the halo, the predicted intensity falls off far more slowly with angle than the observed steep central peak, ruling out pure decay as the dominant source of the GC 511 keV line. In contrast, DM‑DM collisional models generate a centrally concentrated positron source whose spatial extent is governed by the inner slope of the DM profile. When the profile is neither too shallow (as in a large‑core isothermal sphere) nor too steep (as in an extreme NFW with γ > 1.2), the resulting angular distribution matches the data quite well, provided the migration length L does not exceed roughly 0.5 kpc. Larger L values smear the signal and flatten the profile, again conflicting with observations.

The authors also examine a scenario in which a transient, centrally located event (e.g., a past outburst of Sgr A*, a supernova, or a magnetar flare) releases energetic particles that travel outward, interact with ambient gas, and produce positrons at distances of a few hundred parsecs. If the subsequent diffusion of these positrons is isotropic and limited to L ≈ 0.2–0.4 kpc, the angular profile reproduces the measured shape. This “central source + diffusion” model therefore remains viable and offers an alternative to DM‑based explanations.

A key quantitative constraint emerging from the analysis is that the average positron migration distance before annihilation must be ≤ 0.5 kpc for most realistic DM density profiles. This limit is driven by the high gas density (∼10³ cm⁻³) and strong magnetic fields in the GC, which both reduce the diffusion coefficient and shorten the annihilation timescale. Consequently, any model that predicts extensive positron propagation—whether through large‑scale galactic winds or highly turbulent diffusion—cannot simultaneously satisfy the observed intensity and angular width of the 511 keV line.

In the discussion, the authors compare their findings with other indirect‑DM searches (e.g., gamma‑ray constraints from dwarf spheroidals, cosmic‑ray antiproton limits) and argue that the angular profile adds an independent, powerful discriminator. They also note that future missions with improved angular resolution and sensitivity (e.g., e‑ASTROGAM, AMEGO) could refine the migration length constraint and potentially distinguish between the DM‑collision and central‑source scenarios.

In conclusion, the paper demonstrates that the 511 keV line’s angular distribution rules out simple DM decay as the primary positron source, favors DM‑DM collisional processes only if the Galactic DM halo has a moderate inner slope, and allows for a non‑DM transient central source provided positron diffusion is modest. The work thus provides a clear set of astrophysical and particle‑physics criteria that any viable explanation of the Galactic Center positron excess must satisfy.