TeV Gamma-Ray Survey of the Northern Hemisphere Sky Using the Milagro Observatory

Milagro is a water Cherenkov extensive air shower array that continuously monitors the entire overhead sky in the TeV energy band. The results from an analysis of $\sim$3 years of data (December 2000 through November 2003) are presented. The data has been searched for steady point sources of TeV gamma rays between declinations of 1.1 degrees and 80 degrees. Two sources are detected, the Crab Nebula and the active galaxy Mrk 421. For the remainder of the Northern hemisphere we set 95% C.L. upper limits between 275 and 600 mCrab (4.8-10.5$\times 10^{-12}$ cm$^{-2}$ s$^{-1}$) above 1 TeV for source declinations between 5 degrees and 70 degrees. Since the sensitivity of Milagro depends upon the spectrum of the source at the top of the atmosphere, the dependence of the limits on the spectrum of a candidate source is presented. Because high-energy gamma rays from extragalactic sources are absorbed by interactions with the extragalactic background light the dependence of the flux limits on the redshift of a candidate source are given. The upper limits presented here are over an order of magnitude more stringent than previously published limits from TeV gamma-ray all-sky surveys.