Search for Emission of Ultra--High-Energy Radiation from Active Galactic Nuclei

The CYGNUS air-shower array has been used to search for emission of ultra-high-energy gamma radiation from 13 active galactic nuclei (AGNs) that were detected by EGRET. The data set spans the period 1986 April 2 to 1993 January 1. The data set has been searched for continuous emission, for emission on the time scale of 1 week, and for emission on the time scale of 1 day. No evidence for emission from any of the AGNs on any of the time scales examined was found. The 90% Confidence Level (CL) upper limit to the continuous flux from Mrk 421 above 50 TeV is 7.5 × 10<SUP>-14</SUP> cm<SUP>-2</SUP> s<SUP>-1</SUP>.