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As one of the brightest active blazars in both X-ray and very high energy$\gamma$-ray bands, Mrk 501 is very useful for physics associated with jetsfrom AGNs. The ARGO-YBJ experiment is monitoring it for $\gamma$-rays above 0.3TeV since November 2007. Starting from October 2011 the largest flare since2005 is observed, which lasts to about April 2012. In this paper, a detailedanalysis is reported. During the brightest $\gamma$-rays flaring episodes fromOctober 17 to November 22, 2011, an excess of the event rate over 6 $\sigma$ isdetected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increaseof the $\gamma$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steadyemission. In particular, the $\gamma$-ray flux above 8 TeV is detected with asignificance better than 4 $\sigma$. Based on time-dependent synchrotronself-Compton (SSC) processes, the broad-band energy spectrum is interpreted asthe emission from an electron energy distribution parameterized with a singlepower-law function with an exponential cutoff at its high energy end. Theaverage spectral energy distribution for the steady emission is well describedby this simple one-zone SSC model. However, the detection of $\gamma$-raysabove 8 TeV during the flare challenges this model due to the hardness of thespectra. Correlations between X-rays and $\gamma$-rays are also investigated.

Long-term Monitoring on Mrk 501 for Its VHE gamma Emission and a Flare in October 2011