Constraining dark matter by the 511 keV line

Abstract

In the past few decades, observations indicated that an unexplained high production rate of positrons (the strong 511 keV line) exists in the Milky Way centre. By using the fact that a large amount of high-density gas used to exist near theMilkyWay centre million years ago, we model the rate of positrons produced due to dark matter annihilation. We consider the effect of adiabatic contraction of dark matter density due to the supermassive black hole at the Milky Way centre and perform a detailed calculation to constrain the possible annihilation channel and dark matter mass range. We find that only three annihilation channels (μ+μ -, 4e, and 4μ) can provide the required positron production rate and satisfy the stringent constraint of gamma-ray observations. In particular, the constrained mass range for the μ+μ - channel is m ≈ 80-100 GeV, which is close to the mass range obtained for the dark matter interpretation of the GeV gamma-ray and positron excess. In other words, the proposed scenario can simultaneously provide the required positron production rate to explain the 511 keV emission, the positron excess, and the GeV gamma-ray excess in our MilkyWay, and it is compatible with the density spike due to adiabatic growth model of the supermassive black hole.