Estimation of the TeV gamma-ray duty cycle of Mrk 421 with Milagro
  The blazar Markarian 421 (Mrk 421) is one of the brightest sources in the extragalactic X-ray/TeV sky. It is also one of the fastest varying TeV $\gamma$-ray sources, showing flaring activity on time scales as short as tens of minutes. To know the level of activity of this source, Tluczykont et al. (2007) calculated the fraction of time spent by Mrk 421 in flaring states with fluxes above 1 Crab at TeV energies (i.e., TeV - duty cycle). Here we present an alternative approach to calculate the TeV duty cycle of Mrk 421 taking advantage of the continuous monitoring of the source by the Milagro observatory. Milagro was a water Cherenkov detector sensitive at energies 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