Northern Sky Survey for Gamma-ray Point Sources in 100 TeV Region with the Tibet Air Shower Array

We searched for steady 100 TeV gamma-ray point sources from −10° to +70° in declination based on six year data obtained from Nov. 1999 to Nov. 2005 by the Tibet air shower array. No new source was found and no significant excess of events could be detected from known TeV sources. We set flux upper limits above 100 TeV on several nearby objects located inside Our Galaxy. All of them are slightly higher than extrapolation from lower energies by a factor of 1 to 20. Our new project named Tibet MD will improve our sensitivity by a factor of ∼10 in this energy region. We would discover ∼10 new sources and be able to measure cutoff energy of known and unknown sources, which might lead to identification of the origin of cosmic rays.