Connecting steady emission and Very High Energy flaring states in blazars: the case of Mrk 421

TL;DR

This paper develops a time-dependent radiative model to connect steady and flaring emission in blazars, applying it to Mrk 421's 2010 VHE flare, and finds a two-zone model with Fermi acceleration mechanisms best explains the observations.

Contribution

The study introduces a versatile, time-dependent radiative code and demonstrates that a two-zone model with Fermi acceleration mechanisms better explains blazar flaring behavior than a one-zone model.

Findings

A one-zone model is disfavored for connecting low and high states.

A two-zone model successfully reproduces MWL light curves and spectra.

Fermi-I and Fermi-II acceleration mechanisms are involved in different emission states.

Abstract

Various attempts have been made in the literature at describing the origin and the physical mechanisms behind flaring events in blazars with radiative emission models, but detailed properties of multi-wavelength (MWL) light curves still remain difficult to reproduce. We have developed a versatile radiative code, based on a time-dependent treatment of particle acceleration, escape and radiative cooling, allowing us to test different scenarios to connect the continuous low-state emission self-consistently with that during flaring states. We consider flares as weak perturbations of the quiescent state and apply this description to the February 2010 MWL flare of Mrk 421, the brightest Very High Energy (VHE) flare ever detected from this archetypal blazar, focusing on interpretations with a minimum number of free parameters. A general criterion is obtained, which disfavours a one-zone model connecting low and high state under our assumptions. A two-zone model combining physically connected acceleration and emission regions yields a satisfactory interpretation of the available time-dependent MWL light curves and spectra of Mrk 421, although certain details remain difficult to reproduce. The two-zone scenario finally proposed for the complex quiescent and flaring VHE emitting region involves both Fermi-I and Fermi-II acceleration mechanisms, respectively at the origin of the quiescent and flaring emission.