Bartoli, B.B.BartoliBernardini, P.P.BernardiniBi, X. J.X. J.BiCao, Z.Z.CaoCatalanotti, S.S.CatalanottiChen, S. Z.S. Z.ChenChen, T. L.T. L.ChenCui, S. W.S. W.CuiDai, B. Z.B. Z.DaiD'Amone, A.A.D'AmoneDanzengluobuDe Mitri, I.I.De MitriD'Ettorre Piazzoli, B.B.D'Ettorre PiazzoliDi Girolamo, T.T.Di GirolamoDi Sciascio, G.G.Di SciascioFeng, C. F.C. F.FengFeng, ZhaoyangZhaoyangFengFeng, ZhenyongZhenyongFengGou, Q. B.Q. B.GouGuo, Y. Q.Y. Q.GuoHe, H. H.H. H.HeHu, HaibingHaibingHuHu, HongboHongboHuIacovacci, M.M.IacovacciIuppa, R.R.IuppaJia, H. Y.H. Y.JiaLabacirenLi, H. J.H. J.LiLiu, C.C.LiuLiu, J.J.Liu0000-0001-7680-2139Liu, M. Y.M. Y.LiuLu, H.H.LuMa, L. L.L. L.MaMa, X. H.X. H.MaMancarella, G.G.MancarellaMari, S. M.S. M.MariMarsella, G.G.MarsellaMastroianni, S.S.MastroianniMontini, P.P.MontiniNing, C. C.C. C.NingPerrone, L.L.PerronePistilli, P.P.PistilliSalvini, P.P.SalviniSantonico, R.R.SantonicoShen, P. R.P. R.ShenSheng, X. D.X. D.ShengShi, F.F.ShiSurdo, A.A.SurdoTan, Y. H.Y. H.TanVALLANIA, PIEROPIEROVALLANIA0000-0001-9089-7875VERNETTO, Silvia TeresaSilvia TeresaVERNETTO0000-0001-7270-7556Vigorito, C.C.VigoritoWang, H.H.WangWu, C. Y.C. Y.WuWu, H. R.H. R.WuXue, L.L.XueYang, Q. Y.Q. Y.YangYang, X. C.X. C.YangYao, Z. G.Z. G.YaoYuan, A. F.A. F.YuanZha, M.M.ZhaZhang, H. M.H. M.ZhangZhang, L.L.ZhangZhang, X. Y.X. Y.ZhangZhang, Y.Y.ZhangZhao, J.J.ZhaoZhaxicirenZhaxisangzhuZhou, X. X.X. X.ZhouZhu, F. R.F. R.ZhuZhu, Q. Q.Q. Q.ZhuARGO-YBJ Collaboration2021-01-082021-01-0820160067-0049http://hdl.handle.net/20.500.12386/29614We report on the extensive multi-wavelength observations of the blazar Markarian 421 (Mrk 421) covering radio to γ-rays, during the 4.5 year period of ARGO-YBJ and Fermi common operation time, from 2008 August to 2013 February. These long-term observations, extending over an energy range of 18 orders of magnitude, provide a unique chance to study the variable emission of Mrk 421. In particular, due to the ARGO-YBJ and Fermi data, the entire energy range from 100 MeV to 10 TeV is covered without any gap. In the observation period, Mrk 421 showed both low- and high-activity states at all wavebands. The correlations among flux variations in different wavebands were analyzed. The X-ray flux is clearly correlated with the TeV γ-ray flux, while the GeV γ-rays only show a partial correlation with the TeV γ-rays. Radio and UV fluxes seem to be weakly or not correlated with the X-ray and γ-ray fluxes. Seven large flares, including five X-ray flares and two GeV γ-ray flares with variable durations (3-58 days), and one X-ray outburst phase were identified and used to investigate the variation of the spectral energy distribution with respect to a relative quiescent phase. During the outburst phase and the seven flaring episodes, the peak energy in X-rays is observed to increase from sub-keV to a few keV. The TeV γ-ray flux increases up to 0.9-7.2 times the flux of the Crab Nebula. The behavior of GeV γ-rays is found to vary depending on the flare, a feature that leads us to classify flares into three groups according to the GeV flux variation. Finally, the one-zone synchrotron self-Compton model was adopted to describe the emission spectra. Two out of three groups can be satisfactorily described using injected electrons with a power-law spectral index around 2.2, as expected from relativistic diffuse shock acceleration, whereas the remaining group requires a harder injected spectrum. The underlying physical mechanisms responsible for different groups may be related to the acceleration process or to the environment properties.STAMPAenArticle10.3847/0067-0049/222/1/62-s2.0-84955476638000368982300006https://iopscience.iop.org/article/10.3847/0067-0049/222/1/6/meta2016ApJS..222....6BFIS/05 - ASTRONOMIA E ASTROFISICA