A search for ultrahigh-energy gamma rays from egret-detected active galactic nuclei using CASA-MIA

Extensive air shower data collected using the CASA-MIA detector are examined for evidence of gamma rays above 50 TeV from the directions of 32 active galactic nuclei. Gamma rays above 100 MeV from all of these objects have been detected previously by the Energetic Gamma Ray Experiment Telescope aboard the Compton Gamma Ray Observatory. One of these, Markarian 421, has also been observed by the Whipple Observatory Gamma Ray Telescope to emit TeV gamma rays. No evidence of continuous or short-term emission is found for data collected between 1990 March and 1994 November. The flux limits calculated from these data indicate a reduction in the signal extrapolated from the EGRET data, which is consistent with microwave attenuation of the gamma-ray spectra.