Cosmic-Ray Physics with the Milagro Gamma-Ray Observatory

Abstract

The Milagro gamma-ray observatory is a water Cherenkov detector with an energy response between 100 GeV and 100 TeV. While the major scientific goals of Milagro were to detect and study cosmic sources of TeV gamma rays, Milagro has made measurements important to furthering our understanding of the cosmic radiation that pervades our Galaxy. Milagro has made the first measurement of the Galactic diffuse emission in the TeV energy band. In the Cygnus Region we measure a flux ∼2.7 times that predicted by GALPROP. Milagro has also made measurements of the anisotropy of the arrival directions of the local cosmic radiation. On large scales the measurements made by Milagro agree with those previously reported by the Tibet ASγ array. However, we have also discovered a time dependence to this anisotropy, perhaps due to solar modulation. On smaller scales, ∼10 degrees, we have detected two regions of excess. These excesses have a spectrum that is inconsistent with the local cosmic-ray spectrum.