Synthetic spectra from particle-in-cell simulations of relativistic jets containing an initial toroidal magnetic field

ABSTRACT The properties of relativistic jets, their interaction with the environment, and their emission of radiation can be self-consistently studied by using collisionless particle-in-cell (PIC) numerical simulations. Using three-dimensional relativistic PIC simulations, we present the first self-consistently calculated synthetic spectra of head-on and off-axis emission from electrons accelerated in cylindrical relativistic plasma jets containing an initial toroidal magnetic field. The jet particles are initially accelerated during the linear stage of growing plasma instabilities, which are the Weibel instability (WI), kinetic Kelvin–Helmholtz instability (kKHI), and mushroom instability (MI). In the non-linear stage, these instabilities are dissipated and generate turbulent magnetic fields, which accelerate particles further. We calculate the synthetic spectra by tracing a large number of jet electrons in the non-linear stage, near the jet head where the magnetic fields are turbulent. Our results show the basic properties of jitter-like radiation emitted by relativistic electrons when they travel through a magnetized plasma with the plasma waves driven by kinetic instabilities (WI, kKHI, and MI) growing into the non-linear regime. At low frequencies, the slope of the spectrum is $\sim 0.94$, which is similar to that of the jitter radiation, rather than that of the classical synchrotron radiation, which is $\sim 1/3$. Although we start with a weak magnetized plasma, the plasma magnetization increases locally in regions where the magnetic field becomes stronger due to kinetic instabilities. The results of this study may be relevant for probing photon emission from low energies up to, at least, low energies in the X-ray domain in active galactic nucleus/blazar and gamma-ray burst jets, as the peak frequency of synthetic spectra increases as the Lorentz factor of the jet increases from 15 to 100.