LONG-TERM MONITORING OF MRK 501 FOR ITS VERY HIGH ENERGY gamma EMISSION AND A FLARE IN 2011 OCTOBER

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 jets from active galactic nuclei. The ARGO-YBJ experiment has monitored Mrk 501 for gamma-ray above 0.3 TeV since 2007 November. The largest flare since 2005 was observed from 2011 October and lasted until about 2012 April. In this paper, a detailed analysis of this event is reported. During the brightest gamma-ray flaring episodes from 2011 October 17 to November 22, an excess of the event rate over 6 sigma is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the gamma-ray flux above 1 TeV by a factor of 6.6 +/- 2.2 from its steady emission. In particular, the gamma-ray flux above 8 TeV is detected with a significance better than 4 sigma. Based on time-dependent synchrotron self-Compton (SSC) processes, the broadband energy spectrum is interpreted as the emission from an electron energy distribution parameterized with a single power-law function with an exponential cutoff at its high-energy end. The average spectral energy distribution for the steady emission is well described by this simple one-zone SSC model. However, the detection of gamma-ray above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and gamma-ray are also investigated.