Exciting Dark Matter and the INTEGRAL/SPI 511 keV signal

TL;DR

The paper proposes exciting dark matter (XDM) as a mechanism to generate Galactic positrons: inelastic DM–DM scattering excites DM to a MeV-scale state, which de-excites by emitting $e^+e^-$ pairs, converting kinetic energy into positrons in the Galactic center. A concrete pseudo-Dirac fermion model with a light mediator yields the observed relic density and predominantly annihilates to $e^+e^-$, while the inelastic cross section and velocity threshold produce a natural radial cutoff aligned with the INTEGRAL/SPI 511 keV map. The analysis connects particle physics to the observed signal via a line-of-sight integral over the DM distribution and velocity field, and discusses broader consequences, including cluster heating and possible impacts on early black hole growth and high-energy signatures. Overall, XDM provides a testable link between DM microphysics and Galactic-center gamma-ray observations, with distinctive morphology and cross-section requirements that distinguish it from standard DM scenarios.

Abstract

We propose a WIMP candidate with an ``excited state'' 1-2 MeV above the ground state, which may be collisionally excited and de-excites by e+e- pair emission. By converting its kinetic energy into pairs, such a particle could produce a substantial fraction of the 511 keV line observed by INTEGRAL/SPI in the inner Milky Way. Only a small fraction of the WIMPs have sufficient energy to excite, and that fraction drops sharply with galactocentric radius, naturally yielding a radial cutoff, as observed. Even if the scattering probability in the inner kpc is << 1% per Hubble time, enough power is available to produce the ~3x10^42 pairs per second observed in the Galactic bulge. We specify the parameters of a pseudo-Dirac fermion designed to explain the positron signal, and find that it annihilates chiefly to e+e- and freezes out with the correct relic density. We discuss possible observational consequences of this model.

Figures (6)

• Figure 1: The fraction of pairs with velocity above $v_{thresh}$ of 600, 800, or 1000 km/s, for $v_{rms}=200{\rm ~km/s}$ (solid) and $v_{rms}=180{\rm ~km/s}$ (dashed). The approximate half-max radius of the observed 511 keV emission (vertical dotted) is shown for reference.
• Figure 2: Model curves for the constant sigma model in Eq (\ref{['eqn:sigmavsimple']}), taking $\sigma_{mr}=1.3\times 10^{-26}{\rm ~cm}^2\left(\frac{0.3~{\rm GeV}/cm^3}{\rho_0}\right)^2= 2.4 \times 10^{-27} - 1.2 \times 10^{-25} {\rm ~cm}^2$, both unsmoothed (thin) and smoothed by a $3^\circ$ FWHM beam (thick) to approximate the spatial response of SPI. In all cases the solid red lines represent our fiducial model ($M=500~{\rm GeV}$, $v_{thresh}=850{\rm ~km/s}$, and halo parameters $v_{rms}=200{\rm ~km/s}$, and Merritt index $\alpha=0.2$), and dashed blue lines represent variations of one parameter. (a) $M=400, 500, 600~{\rm GeV}$ with $\delta$ held fixed at $1~{\rm MeV}$ so that $v_{thresh}=950, 850, 776{\rm ~km/s}$, respectively. (b) $v_{thresh}=800, 850, 900{\rm ~km/s}$, keeping $M=500~{\rm GeV}$ while $\delta$ now varies above and below $1~{\rm MeV}$. (c) The Merritt profile index $\alpha$ is varied (see Eq \ref{['eqn:merritt']}). (d) The RMS velocity in the inner Milky Way is varied. This is assumed to be constant with radius. The observed $6^\circ$ FWHM signal (lower dotted line) and that signal plus an arbitrary baseline of 0.002 (upper dotted line) are shown in (a). The SPI zero is set by measurements $10^\circ$ off the Galactic plane to correct for instrumental background.
• Figure 3: Same as Fig. \ref{['fig:spiflux4const']}, but using the full expression for the inelastic scattering cross section (Eq. \ref{['eqn:kitchensink']}) derived in Appendix A, setting the coupling $\lambda_+ = \lambda_- = 0.18$ and $\rho=0.4~{\rm GeV}/{\rm cm}^3$.
• Figure 4: Excitation diagrams for $\chi \chi \rightarrow \chi^* \chi$.
• Figure 5: Decay of the excited state into the ground state.