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

TL;DR

This study reports on long-term VHE gamma-ray monitoring of Mrk 501, highlighting a major flare in late 2011 with significant flux increase and spectral analysis challenging existing emission models.

Contribution

It provides the first detailed analysis of the largest flare since 2005, including detection of gamma rays above 8 TeV and implications for SSC models.

Findings

Detected a 6.6-fold increase in gamma-ray flux during the flare.

Observed gamma rays above 8 TeV with >4 sigma significance.

Challenged the simple one-zone SSC model due to spectral hardness.

Abstract

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 AGNs. The ARGO-YBJ experiment is monitoring it for $\gamma$-rays above 0.3 TeV since November 2007. Starting from October 2011 the largest flare since 2005 is observed, which lasts to about April 2012. In this paper, a detailed analysis is reported. During the brightest $\gamma$-rays flaring episodes from October 17 to November 22, 2011, 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$\pm$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 broad-band 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 $\gamma$-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and $\gamma$-rays are also investigated.