An explanation about the flat radio spectrum for Mrk 421

It is well known that a flat radio spectrum is a common property in the spectral energy distribution of blazars.Although one-zone leptonic models are generally successful in explaining the multi-wave band emission,they are problematic in reproducing the radio spectrum.In the study of Mrk 421,one-zone models suggest that in order to avoid overproducing the radio flux,the minimum electron Lorentz factor should be larger than a few hundred at least,even considering the synchrotron self-absorption effect.This result suggests that the model predicted spectral index in the radio band of Mrk 421 should be-1/3.On the basis of this result,by assuming there is a neglected region that will also contribute the radio emission and its electron energy index naturally originates from the simplest first-order Fermi acceleration mechanism,we can get a superimposed flat radio spectrum.In this paper,a two-zone model is proposed to reproduce the quiescent state spectral energy distribution of Mrk 421.In addition to taking into account the emission from a conventional radiation zone,we further consider emission from the acceleration zone in which particles are accelerated at a shock front.With the present model,our fitting result suggests that the low frequency flat radio spectrum of Mrk 421 might be explained as a superposition of the synchrotron emission from acceleration zone and radiation zone.