Study of TeV variability of Mrk 421 from 3 years of monitoring with the   Milagro Observatory
  The Milagro experiment was a TeV gamma-ray observatory designed to continuously monitor the overhead sky in the 0.1-100 TeV energy range. It operated from 2000 and 2008 and was characterized by a large field of view ($\sim$ 2 sr) and a high duty cycle ($\geq$ 90$\%$). Here we report on the long-term monitoring of the blazar Mrk 421 with Milagro over the period from September 21, 2005 to March 15, 2008. We present a study of the TeV variability of the source and provide upper limits for the measured flux for different time scales, ranging from one week up to one year. 
B. Patricelli, M.M. Gonz\'alez, N. Fraija and A. Marinelli (for the   Milagro collaboration)
astro-ph.HE astro-ph.CO