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The blazar Markarian 421 (Mrk 421) is one of the brightest sources in theextragalactic 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 tensof 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 withfluxes above 1 Crab at TeV energies (i.e., TeV - duty cycle). Here we presentan alternative approach to calculate the TeV duty cycle of Mrk 421 takingadvantage of the continuous monitoring of the source by the Milagroobservatory. Milagro was a water Cherenkov detector sensitive at energiesbetween 100 GeV and 100 TeV. We present our estimation of the TeV - duty cycleand study its robustness.

Estimation of the TeV gamma-ray duty cycle of Mrk 421 with Milagro