Discovery of diffuse TeV gamma ray emission from the galactic plane using Milagro detector

Milagro is a water Cherenkov extensive air shower array that continuously views the entire overhead sky and is sensitive to gamma rays with energies near 1 TeV. Prior to this work, EGRET had observed the highest energy gamma rays from the Galactic plane. In the direction of the inner Galaxy and at energies between 1 GeV and 30 GeV, EGRET measured a gamma-ray spectral index of approximately α_γ = 2.4, harder than the cosmic-ray spectrum. The shape and intensity of the spectrum beyond EGRET energies is sensitive to the production and propagation of cosmic rays. Here we report the detection of the TeV gamma rays from the region of the Milagro inner Galaxy (±5° Galactic latitude and (40°, 100°) Galactic longitude) with a significance of 5σ. The integral flux is estimated to be Φ_γ(E > 1TeV) = (5.7 ± 2.0)·10 ^-10 (ster^-1cm^-2sec^-1) assuming a differential power law spectral index of 2.7 and an EGRET-like source distribution. Comparison with EGRET's data from the same part of the Galaxy may be used to constrain models of cosmic-ray production in the Galaxy.