Unstable Relics as a Source of Galactic Positrons

TL;DR

This paper investigates whether unstable relics, especially MeV-scale sterile neutrinos and light scalar/pseudoscalar particles, can explain the Galactic 511 keV positron annihilation line observed by INTEGRAL/SPI. It derives sterile-neutrino decay widths to $e^+e^-$ and neutrinos, connects their cosmological abundance to the decay rate, and computes the resulting 511 keV flux via line-of-sight integration through realistic dark matter halos. The results show that a large portion of the sterile-neutrino parameter space is excluded because it would overproduce the 511 keV flux, but an allowed region exists for $m_s$ in the MeV range with small to moderate abundances $\Omega_s$; the analysis also connects to the diffuse gamma-background constraints. Beyond sterile neutrinos, the work demonstrates that generic decaying relics with weak couplings can yield observable 511 keV fluxes, and highlights how halo-profile assumptions and potential photon lines (e.g., at $E_\gamma = m_s/2$) can provide discriminants between decaying relics and annihilating dark matter. The study emphasizes that lifetime scales around $10^{17}$ years render these relics testable with current gamma-ray observations and motivates further multi-messenger and collider searches to distinguish decay-origin positrons from alternative dark-matter scenarios.

Abstract

We calculate the fluxes of 511 KeV photons from the Galactic bulge caused by positrons produced in the decays of relic particles with masses less than 100 MeV. In particular, we tighten the constraints on sterile neutrinos over a large domain of the mass--mixing angle parameter space, where the resulting photon flux would significantly exceed the experimental data. At the same time, the observed photon fluxes can be easily caused by decaying sterile neutrinos in the mass range 1 MeV < m_sterile < 50 MeV with the cosmological abundance typically within 10^{-9} < Omega_sterile < 10^{-5}, assuming that Omega_sterile comes entirely from the conversion of active neutrinos in the early Universe. Other candidates for decaying relics such as neutral (pseudo)scalar particles coupled to leptons with the gravitational strength can be compatible with the photon flux, and can constitute the main component of cold dark matter.

Figures (3)

• Figure 1: Mixing angle -- sterile neutrino mass parameter space with the contours of sterile neutrino abundance for different halo profiles, $\alpha = 0.1$ and 0.2, and for different mixing angle $|V| = 0$ and 1. The dark shaded region covers the combination of parameters that creates too large a flux of 511 KeV photons. The boundary of the shaded region gives the same flux of photons, as observed by INTEGRAL/SPI. The area bounded by a thick colid curve is excluded by the diffuse cosmological background of gamma photons.
• Figure 2: The exclusion plot for sterile neutrino parameters based on conservative assumptions about the dark matter profile near the center of the Galaxy. The unshaded area corresponds to $|V|=1$ and the hatched area to $|V|$=0. The solid area is the most conservative model-independent result, corresponding to a dark matter density which is constant inside 3 kpc.
• Figure 3: The angular distribution of 511 KeV photons for two different inner halo profiles, $\alpha =0.1$ and $0.2$, averaged over the 2$^{{\rm {o}}}$ resolution of the SPI spectrometer. The experimental data were fit with a gaussian of full width at half maximum at a 9$^{{\rm {o}}}$ diameter circle (shown as a vertical dashed line), with a $2\sigma$ confidence interval of $6^{{\rm {o}}}-18^{{\rm {o}}}$ (shown as vertical dashed-dotted lines). As can be seen, the distribution for $\alpha =0.2$ falls well within the 2$\sigma$ confidence interval.