A SEARCH OF THE NORTHERN SKY FOR ULTRA HIGH-ENERGY POINT SOURCES

A search has been made for steady emission of ultra-high-energy radiation from individual point sources over most of the northern sky. Over 2 x 10(8) air showers induced by primary particles of energy greater than about 10 TeV were recorded from 1986 April until 1991 May by the CYGNUS air shower array. No statistically significant excess above the background from the isotropic flux of cosmic rays was found for any direction in the sky from 0-degrees to 80-degrees declination. In addition, 49 specific potential sources were examined, and none showed a statistically significant excess of events. 90% confidence level flux upper limits are established for the continuous emission from these sources; for a source that transits the zenith, a representative upper limit corresponds to an integral flux above 40 TeV of about 1.7 x 10(-13) cm-2 s-1 (0.5% of the cosmic-ray flux within an angular area of 1.8 x 10(-3) sr) assuming a power-law energy spectrum with a differential spectral index of -2.7.