Fermi Constrains Dark Matter Origin of High Energy Positron Anomaly

TL;DR

Fermi gamma-ray data significantly limit the possibility that dark matter decay causes the high-energy positron excess, by constraining the positron energies and associated gamma-ray emissions.

Contribution

This work provides new constraints on dark matter decay models for the positron anomaly using gamma-ray background measurements and inverse-Compton scattering analysis.

Findings

Positrons from dark matter decay must have energies between 100-250 GeV.

Gamma-ray emissions from these positrons are constrained by Fermi observations.

Additional gamma-ray signals from decay modes are limited by observational data.

Abstract

Fermi measurements of the high-latitude gamma-ray background strongly constrain a decaying-dark-matter origin for the 1--100 GeV Galactic positron anomaly measured with PAMELA. Inverse-Compton scattering of the microwave background by the emergent positrons produces a bump in the diffuse 100-200 MeV gamma-ray background that would protrude from the observed background at these energies. The positrons are thus constrained to emerge from the decay process at a typical energy between ~100 GeV and ~250 GeV. By considering only gamma-ray emission of the excess positrons and electrons, we derive a minimum diffuse gamma-ray flux that, apart from the positron spectrum assumed, is independent of the actual decay modes. Any gamma-rays produced directly by the dark-matter decay leads to an additional signal that make the observational limits more severe. A similar constraint on the energy of emergent positrons from annihilation in dark-matter substructures is argued to exist, according to recent estimates of enhancement in low-mass dark-matter substructures, and improved simulations of such substructure will further sharpen this constraint.