Search for Gamma Ray Emission from Galactic Plane with Milagro

Abstract

The majority of galactic gamma rays are produced by interaction of cosmic rays with matter or radiation fields. This results in a diffuse radiation concentrated in the galactic plane where the flux of cosmic rays and the density of material (mostly atomic, molecular and ionized hydrogen) is high. The interactions producing gamma rays include, among others, the decay of pi^{0}&#39;s produced in spallation reactions. Gamma emission from the plane has indeed been detected in the energy range up to 30 GeV. Above 1 GeV, the observed intensity is notably higher than expected in simple models, possibly implying an enhancement at the TeV region as well. Observations at TeV energies made by Milagro, a large aperture water Cherenkov detector for extensive air showers, are from a solid angle of more than two steradians in the overhead sky. An excess has been observed from the region of the Milagro inner Galaxy defined by 20 < l < 100 and |b| < 5 degrees with the significance 2.3 10^{-4} using data from 2000-2001 that we attribute to the first evidence for the diffuse emission at TeV energies. The emission from the region of the Milagro outer galaxy defined by 140 < l < 220 and |b| < 5 degrees is not inconsistent with being only that from the cosmic ray background.