Expanding one-zone model for blazar emission

Abstract

Context. As a subcategory of active galactic nuclei, blazars are distinguished by their non-thermal variable emission. This emission extends over the whole electromagnetic spectrum and is a consequence of particle acceleration inside their relativistic jets. However, an open question remains regarding the relation of radio emission compared to that of higher frequencies. Aims. Observations of blazar emission show that the location of radio might be very different from the one where the rest of the spectrum is produced and, thus, often requires separate modeling. We aim to produce both types of emission within the context of a single model. Methods. We constructed a self-consistent one-zone expanding leptonic model for studying the connection between the radio emission and the emission at higher frequencies. We then applied it to the flaring states of blazars. Assuming an accelerating episode as the source moves down the jet and expands, we numerically studied the electron evolution as they lose energy due to adiabatic expansion and synchrotron or inverse Compton radiation. Results. We find that high-frequency radiation mimics the electron injection and is mainly produced close to the acceleration site where cooling is strong. In contrast, the radio emission is produced further down the jet when the emitting region has become optically thin with regard to synchrotron self-absorption due to expansion. We briefly present the role of the initial parameters, such as the magnetic field strength, the electron luminosity, and expansion velocity, on the localization of the radio emission site. We show that the expanding one-zone model is inherently different from the non-expanding one and, in addition, it requires more parameters. For example, we apply our approach to the observational data of a Mrk 421 γ-ray and radio flare observed in 2013.