Detection of Multi‐TeV Gamma Rays from Markarian 501 during an Unforeseen Flaring State in 1997 with the Tibet Air Shower Array

Abstract

In 1997, the BL Lac Object Mrk 501 entered a very active phase and was thebrightest source in the sky at TeV energies, showing strong and frequentflaring. Using the data obtained with a high density air shower array that hasbeen operating successfully at Yangbajing in Tibet since 1996, we searched forgamma-ray signals from this source during the period from February throughAugust in 1997. Our observation detected multi-TeV $\\gamma$-ray signals at the3.7-Sigma level during this period. The most rapid increase of the excesscounts was observed between April 7 and June 16 and the statisticalsignificance of the excess counts in this period was 4.7-Sigma. Among severalobservations of flaring TeV gamma-rays from Mrk 501 in 1997, this is the onlyobservation using a conventional air shower array. We present the energyspectrum of gamma-rays which will be worthy to compare with those obtained byimaging atmospheric Cerenkov telescopes.