Evidence for TeV Gamma-Ray Emission from a Region of the Galactic Plane

Gamma-ray emission from a narrow band at the galactic equator has previously been detected up to 30 GeV. We report evidence for a TeV gamma-ray signal from a region of the galactic plane by Milagro, a large-field-of-view water Cherenkov detector for extensive air showers. An excess with a significance of 4.5 standard deviations has been observed from the region of galactic longitude $l\in (40°,100°)$ and latitude $|b|<5°$. Under the assumption of a simple power law spectrum, with no cutoff in the EGRET-Milagro energy range, the measured integral flux is ${\varphi }_{\gamma }(>3.5\mathrm{TeV})=(6.4\pm 1.4\pm 2.1)\times {10}^{-11}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$. This flux is consistent with an extrapolation of the EGRET spectrum between 1 and 30 GeV in this galactic region.