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 ∼0.94 , which is similar to that of the jitter radiation, rather than that of the classical synchrotron radiation, which is ∼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.The authors would like to thank the collaborators Jacek Niemiec and Martin Pohl for valuable discussions during the development of this work. We are grateful to the anonymous reviewer for their thoughtful questions and suggestions that subsequently improved the quality of the paper. The simulations presented in this report have been performed on the Frontera supercomputer at the Texas Advanced Computing Center under the AST 23035 award: PIC Simulations of Relativistic Jets with Toroidal Magnetic Fields (PI: Athina Meli) through the NSF grant No 2302075; and the AST21038 award: Computational Study of Astrophysical Plasmas; through the NASA grant: Nature Of Hard X-rays From A TeV-detected RadioGalaxy (PI: Ka Wah Wong at SUNY Brockport) issued by the NuSTAR Guest Observer Cycle 6 2019; and using the Pleiades facilities at the NASA Advanced Supercomputing (NAS: s2004 and s2349), which is supported by the NSF; as well as Ares supercomputer at Cyfronet AGH (PI: Oleh Kobzar) through the grant PLG/2024/017211. ID acknowledges support from the Romanian Ministry of Research, Innovation and Digitalization under the Romanian National Core Program LAPLAS VII – contract no. 30N/2023. KN and AM acknowledge support from the NSF Excellence in Research Award No. (FAIN): 2302075. OK is supported by the Polish NSC (grant 2016/22/E/ST9/00061). CK has received funding from the Independent Research Fund Denmark (grant 1054-00104). YM is supported by the ERC Synergy Grant ‘BlackHoleCam: Imaging the Event Horizon of Black Holes’ (Grant No. 610058). JLG acknowledges the support of the Spanish Spanish Ministerio de Ciencia, Innovación y Universidades (grants PID2019-108995GB-C21 and PID2022-140888NB-C21) and the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofísica de Andalucía (CEX2021-001131-S).With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).Peer reviewe