Estimation of the TeV gamma-ray duty cycle of Mrk 421 with the Milagro   observatory
  Markarian 421 (Mrk 421) is one the brightest and closest (z=0.031) blazars known (de Vaucouleurs et al 1991). It is also one of the fastest varying TeV $\gamma$-ray sources, with a flaring activity on time scales as short as tens of minutes. The activity of Mrk 421 at different frequencies may reflect the radiation mechanisms involved. Tluczykont et al. (2007) estimated the TeV activity of Mrk 421 through calculating the fraction of time spent in flaring states at TeV energies (TeV duty cycle) by using data from several imaging atmospheric Cherenkov telescopes (IACTs). Since IACT observations are biased towards high flux states they overestimated the TeV duty cycle of Mrk 421. Here we propose an alternative approach to calculate the TeV duty cycle of Mrk 421 that takes advantage of the continuous monitoring of the source by the Milagro experiment, a water Cherenkov detector sensitive to primary $\gamma$-rays between 100 GeV and 100 TeV. We present our estimation of the TeV - duty cycle and study its robustness. 
B. Patricelli, M.M. Gonz\'alez, N. Fraija and A. Marinelli (for the   Milagro collaboration)
astro-ph.HE astro-ph.CO