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TeV flaring activity with time scales as short as tens of minutes and anorphan TeV flare have been observed from the blazar Markarian 421 (Mrk 421).The TeV emission from Mrk 421 is believed to be produced by leptonicsynchrotron self-Compton (SSC) emission. In this scenario, correlations betweenthe X-ray and the TeV fluxes are expected, TeV orphan flares are hardlyexplained and the activity (measured as duty cycle) of the source at TeVenergies is expected to be equal or less than that observed in X-rays if onlySSC is considered. To estimate the TeV duty cycle of Mrk 421 and to establishlimits on its variability at different time scales, we continuously observedMrk 421 with the Milagro observatory. Mrk 421 was detected by Milagro with astatistical significance of 7.1 standard deviations between 2005 September 21and 2008 March 15. The observed spectrum is consistent with previousobservations by VERITAS. We estimate the duty cycle of Mrk 421 for energiesabove 1 TeV for different hypothesis of the baseline flux and for differentflare selections and we compare our results with the X-ray duty cycle estimatedby Resconi et al. 2009. The robustness of the results is discussed.

The Study of TeV Variability and Duty Cycle of Mrk 421 from 3 Years of Observations with the Milagro Observatory