An explanation about the flat radio spectrum for Mrk 421

TL;DR

This paper proposes a two-zone model combining emission from a radiation zone and an acceleration zone to explain the flat radio spectrum of Mrk 421, addressing limitations of one-zone models.

Contribution

It introduces a two-zone model incorporating shock acceleration to better reproduce the flat radio spectrum of Mrk 421, improving upon previous one-zone models.

Findings

The superposition of emissions from two zones explains the flat radio spectrum.

The model fits the quiescent state spectral energy distribution of Mrk 421.

The acceleration zone's emission accounts for the low-frequency radio flatness.

Abstract

It is well known that the flat radio spectrum is a common property in the spectral energy distribution of blazars. Although the 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 is naturally originate 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 the 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.