Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy

The diffuse gamma radiation arising from the interaction of cosmic-ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Data from the Milagro gamma-ray observatory—a water Cerenkov detector that continuously views ~2 sr of the overhead sky—shows that the brightest extended region in the entire northern sky is the Cygnus region of the Galactic plane. The TeV image of the Cygnus region contains at least one new source, MGRO J2019+37, which is 10.9 σ above the isotropic background, as well as correlations with the matter density in the region. However, the gamma-ray flux from the Cygnus region (after excluding MGRO J2019+37) as measured at ~12 TeV exceeds that predicted from a model of cosmic-ray production and propagation. This observation indicates the existence of either hard-spectrum cosmic-ray sources and/or unresolved sources of TeV gamma rays in the region.