Diffuse galactic gamma rays are from shock-accelerated cosmic rays

Abstract

Cosmic rays are thought to be accelerated at supernova remnant (SNR) shocks, but observation of the π0 bump that would confirm this expectation is hampered by weak γ-ray fluxes of SNRs, instrumental difficulties in observing lowenergy (≲ 100 MeV) emissions, background γrays made from cosmic-ray interactions with gas and dust in the vicinity of SNRs, leptonic γ rays, and γ-ray point sources. By contrast, cosmic-ray bombardment of nearby high-latitude gas located away from known molecular cloud complexes and with small ionized hydrogen column density along the line of sight provides a less confused emission region for study. The Large Area Telescope on the Fermi Gamma-ray Space Telescope has measured a γ-ray spectrum of high-latitude gas that reveals a low-energy cutoff corresponding to a π0 → 2γfeature [1]. A shock-accelerated particle flux ∝ p−s, where p is the particle momentum, follows from simple theoretical considerations of cosmic-ray acceleration at nonrelativistic shocks followed by rigidity-dependent escape into the Galactic halo. A flux of shock-accelerated cosmic-ray protons with s ≈ 2.8 provides an adequate fit to the Fermi-LAT gamma-ray emission spectrum of high-latitude gas when uncertainties in secondary nuclear production models are taken into acount. The cosmicray spectrum inferred from the γ-ray emissivity spectrum is in accord with the SNR hypothesis for the origin of the cosmic rays [2].