Detection of TeV Gamma Rays from Mrk 501 with the Tibet II/HD Air Shower Array during a High State of Activity

佐々木 孝雄


   A blazar Mrk 501, suddenly came into a very active phase from February in 1997, with several large flares and lasted for almost half a year. The maximum gamma-ray flux during this period reached about 10 times as high as the Crab Nebula. Using the data obtained with a Tibet HD Air Shower Array, we searched for gamma-ray signals from Mrk 501 during the period from February 15 to August 25 in 1997. We found our array detected multi-TeV gamma rays from this source with 3.7 σ level. The most rapid increase of the excess count of air shower events was observed between April 7 and June 16 and the statistical significance of the excess counts in this 70 days was 4.7 σ. According to a simulation using the angular resolution of our array, the distribution of arrival direction of air shower events is consistent with gamma-ray initiated air showers coming from the point source. The average flux of gamma rays from Mrk 501 is estimated as 4.50(±1.20) × 10-12 cm-2s-1 TeV-1 for energy above 3 TeV in the period from February 15 to August 25, and 9.09(±1.52) × 10-12 cm-2s-1 TeV-1 in the period from April 7 to June 16. This is a first detection of extragalactic gamma-ray source by conventional surface array. The observed gamma-ray energy spectrum suggests to be somewhat suffered from an absorption in the energy region above 6 TeV by intergalactic infrared photons.