The Emissive Mechanism of Flat‐Spectrum Radio Sources

Abstract

An optically thin synchrotron emission model based on evolutionary relativistic electron spectra is developed to explain radio flat spectra of blazars. The basic physical processes of relativistic electron spectrum evolution for synchrotron emission are second-order Fermi acceleration of turbulent plasma waves, Fermi acceleration of shock waves, radiation loss, particle escape, and adiabatic deceleration in a flow of synchrotron plasma. When the timescale of turbulence acceleration is larger than the Alfven timescale, the evolutionary spectra of relativistic electrons are flattened in low energy and steepen rapidly in high energy. The shape of relativistic electron spectra depends on the level of plasma turbulence and shock wave acceleration. In the case of strong plasma turbulence, the resulting synchrotron emission has a flatter spectrum below a special frequency νc, which is inversely proportional to magnetic field, plasma density, and the square of emission region size. In the case of weak plasma turbulence, the flat spectrum is not formed by the physical processes in the plasma. Based on this model, the radio flat-spectrum sources are easily formed in compact active galactic nuclei. The flat radio sources with higher frequency extension appear in compact, active nuclei with lower plasma density and magnetic field.