Fermi Constrains Dark Matter Origin of High Energy Positron Anomaly

Understand

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.