Search for steady emission of 10-TeV gamma rays from the Crab Nebula, Cygnus X-3, and Hercules X-1 using the Tibet air shower array

The Tibet air shower array at an altitude of 4300 m has been in operation since January 1990. We have searched for continual emission of 10-TeV gamma rays from the Crab Nebula, Cygnus X-3 and Hercules X-1 during the period from June 1990 through January 1992. No dc excess is found from these three sources. Flux upper limits above 10 TeV are obtained to be 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the Crab Nebula, 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for Cygnus X-3, and 0.58\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for Hercules X-1 at the 95% C.L. This is the first observation, with an air shower array, for these sources in the 10-TeV energy region.