4.5-year simultaneous multi-wavelength observation of Mrk 421 in the ARGO-YBJ and Fermi overlap era

As one of the most active blazars, Mrk421 is an excellent candidate for the study of the physical processes within the jets of AGN. Here we report on the extensive multi-wavelength observations of Mrk 421 over 4.5 years, from 2008 August to 2013 February. This source was simultaneously monitored by several experiments at different wavelengths: ARGO-YBJ in TeV $\gamma$-rays, $Fermi$-LAT in GeV $\gamma$-rays, $Swift$-BAT in hard X-rays, $RXTE$-ASM, $MAXI$ and $Swift$-XRT in soft X-rays, $Swift$-UVOT in ultraviolet, and OVRO in radio frequencies. In particular, thanks to the ARGO-YBJ and $Fermi$ data, the whole energy range from 100 MeV to 10 TeV is covered without any gap. According to the observed light curves, ten states (including seven large flares, two quiescent phases and one outburst) were selected. For the first time, the multi-wavelength spectral evolutions of Mrk 421 during different states were systematically analyzed. During the outburst phase and the seven flaring episodes, the peak energy in X-rays is observed to increase from sub-keV to few keV. The TeV $\gamma$-ray flux increases up to 0.9$-$7.2 times the flux of the Crab Nebula. The behavior of GeV $\gamma$-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 observed radiation spectra above 0.3 keV of different states can be reasonably described by a simple one-zone synchrotron self-Compton model. The underlying physical mechanisms responsible for different states may be related to the acceleration process or to variations of the ambient medium.