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

Abstract

Several strong TeV gamma-ray flares were detected from Markarian 421 in the years 2000 and 2001 with the Tibet III air shower array at the statistical significance of 5.5 sigma level. Markarian 421 was unprecedentedly active at X-ray and TeV gamma-ray energies during this period, and positive correlation was found between change of the ASM/RXTE X-ray flux and the Tibet TeV gamma-ray flux. It seems to be in favor of a linear correlation than quadratic one. When a power-law energy spectrum for gamma-rays from this source is assumed, the spectral index is calculated to be -3.80+/-0.43 at the most active phase in 2001 at energies above 1 TeV. The spectrum observed with the Tibet air shower array is consistent with those obtained with imaging air Cherenkov telescopes.