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

TL;DR

This paper investigates the origin of the diffuse gamma-ray emission in the Cygnus region, proposing models involving cosmic rays with specific spectral indices to explain observations from Milagro and EGRET, and discusses future diagnostics to distinguish these models.

Contribution

The study provides detailed modeling of cosmic ray sources to explain gamma-ray excesses, and suggests observational strategies to differentiate between hadronic and leptonic origins.

Findings

A proton source with spectral index ≤ 2.3 can explain Milagro's TeV gamma-ray excess.

A electron source with spectral index ≤ 2.6 can also reproduce the observations.

Future neutrino and X-ray observations can help distinguish the emission mechanisms.

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 $\alpha_p\lesssim 2.3$, or a diffuse electron source with power index $\alpha_e\lesssim2.6$ 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.