Possible observation of 100 TeV gamma rays from the active galaxy Centaurus a

We have searched for evidence of ultra-high energy gamma rays from the active galaxy Centaurus A in the data set of the JANZOS experiment during the period 13 October 1987 to 18 January 1992. No significant DC excess was found from this source. Excess events were found, however, in the period 14 April - 3 June 1990. The duration of the excess, 48 days, and equivalent luminosity, ~ 10⁴³ erg s^-1, are both similar to those measured previously for X-ray outbursts of Centaurus A. Furthermore, the events appear to exhibit the expected absorption feature at 200 TeV due to interactions with the 2.7 K background radiation, but the chance probability for the excess is at the 2% level. The equivalent time averaged flux during this period was (5.5 ± 1.5) × 10^-12 photons cm^-2s^-1 at energies ≥ 110 TeV, about 20 times as large as the upper lim of steady emission, assuming a differential spectral index of -2.0. Further observations are required to confirm the result.