Long-term monitoring of mrk 501 for its very high energy γ emission and a flare in 2011 october

Bartoli, B.; Bernardini, P.; Bi, X.; Bleve, C.; Bolognino, I.; Branchini, P.; Budano, A.; Melcarne, A. C.; Camarri, P.; Cao, Z.; Cardarelli, R.; Catalanotti, S.; Cattaneo, C.; Chen, S.; Chen, T.; Chen, Y.; Creti, P.; Cui, S.; Dai, B.; Staiti, G. D.; Danzengluobu,; Dattoli, M.; Mitri, I. D.; Piazzoli, B. D.; Girolamo, T. D.; Ding, X.; Sciascio, G. D.; Feng, C.; Feng, Z.; Feng, Z.; Galeazzi, F.; Giroletti, E.; Gou, Q.; Guo, Y.; He, H.; Hu, H.; Hu, H.; Huang, Q.; Iacovacci, M.; Iuppa, R.; James, I.; Jia, H.; Labaciren,; Li, H.; Li, J.; Li, X.; Liguori, G.; Liu, C.; Liu, C.; Liu, J.; Liu, M.; Lu, H.; Ma, L.; Ma, X.; Mancarella, G.; Mari, S.; Marsella, G.; Martello, D.; Mastroianni, S.; Montini, P.; Ning, C.; Pagliaro, A.; Panareo, M.; Panico, B.; Perrone, L.; Pistilli, P.; Ruggieri, F.; Salvini, P.; Santonico, R.; Shen, P.; Sheng, X.; Shi, F.; Stanescu, C.; Surdo, A.; Tan, Y.; Vallania, P.; Vernetto, S.; Vigorito, Carlo Francesco; Wang, B.; Wang, H.; Wu, C.; Wu, H.; Xu, B.; Xue, L.; Yang, Q.; Yang, X.; Yao, Z.; Yuan, A.; Zha, M.; Zhang, H.; Zhang, J.; Zhang, J.; Zhang, L.; Zhang, P.; Zhang, X.; Zhang, Y.; Zhao, J.; Zhaxiciren, Zhaxisangzhu; Zhou, X.; Zhu, F.; Zhu, Q.; Zizzi, A.

doi:10.1088/0004-637X/758/1/2

As one of the brightest active blazars in both X-ray and very high energy γ-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 γ-rays 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 γ-ray flaring episodes from 2011 October 17 to November 22, an excess of the event rate over 6σ is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the γ-ray flux above 1 TeV by a factor of 6.6 ± 2.2 from its steady emission. In particular, the γ-ray flux above 8 TeV is detected with a significance better than 4σ. 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 γ-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and γ-rays are also investigated. © 2012. The American Astronomical Society. All rights reserved.