Detection of TeV gamma rays from Mrk 501 at high flaring state of activity in 1997 with the Tibet air shower array

The BL Lac object Mrk 501 came into a high flaring state in 1997, brightest in the sky at TeV energies. Using the air shower data obtained with a high density part of the Tibet II array, being operating at Yangbajing since 1996, we searched for γ-ray signals from this source during the period from February to August in 1997. The result shows a 3.7σ excess for showers with its mode energy 3 TeV for the whole period and 4.7σ excess for the period from April 7 to June 16. The estimated energy spectrum of γ rays is consistent with those obtained by imaging atmospheric Cherenkov detectors.