The diffuse GeV-TeV $γ$-ray emission of the Cygnus region

Abstract

Recently the Milagro experiment observed diffuse multi-TeV gamma-ray emission in the Cygnus region, which is significantly stronger than what predicted by the Galactic cosmic ray model. However, the sub-GeV observation by EGRET shows no excess to the prediction based on the same model. This TeV excess implies possible high energy cosmic rays populated in the region with harder spectrum than that observed on the Earth. In the work we studied this theoretical speculation in detail. We find that, a diffuse proton source with power index $α_p\lesssim 2.3$, or a diffuse electron source with power index $α_e\lesssim2.6$ can reproduce the Milagro's observation without conflicting with the EGRET data. Further detections on neutrinos, a diagnostic of the hadronic model, and hard X-ray synchrontron radiation, a diagnostic of the lepton model, help to break this degeneracy. In combination with the gamma ray observations to several hundred GeV by Fermi, we will be able to understand the diffuse emission mechanisms in the Cygnus region better.