Multi-TeV Gamma-Ray Flares from Markarian 421 in 2000 and 2001 Observed with the Tibet Air Shower Array

Several strong TeV (gamma)-ray flares were detected from Mrk 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 (sigma). Mrk 421 was unprecedentedly active at X-ray and TeV (gamma)-ray energies during this period, and a positive correlation was found between the change of the all-sky monitor Rossi X-Ray Timing Explorer X-ray flux and the Tibet TeV (gamma)-ray flux. When a power-law energy spectrum for (gamma)-rays from this source is assumed, the spectral index is calculated to be -3.24 ± 0.69 at the most active phase in 2001. The spectral index observed by the Tibet air shower array is consistent with those obtained via imaging air Cerenkov telescopes.