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Register a new userPIC methods in astrophysics: Simulations of relativistic jets and kinetic physics in astrophysical systems
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The Particle-In-Cell (PIC) method has been developed by Oscar Buneman, Charles Birdsall, Roger W. Hockney, and John Dawson in the 1950s and, with the advances of computing power, has been further developed for several fields such as astrophysical, magnetospheric as well as solar plasmas and recently also for atmospheric and laser-plasma physics. Currently more than 15 semi-public PIC codes are available which we discuss in this review. Its applications have grown extensively with increasing computing power available on high performance computing facilities around the world. These systems allow the study of various topics of astrophysical plasmas, such as magnetic reconnection, pulsars and black hole magnetosphere, non-relativistic and relativistic shocks, relativistic jets, and laser-plasma physics. We review a plethora of astrophysical phenomena such as relativistic jets, instabilities, magnetic reconnection, pulsars, as well as PIC simulations of laser-plasma physics (until 2021) emphasizing the physics involved in the simulations. Finally, we give an outlook of the future simulations of jets associated to neutron stars, black holes and their merging and discuss the future of PIC simulations in the light of petascale and exascale computing.
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Broadband emission from relativistic outflows (jets) of active galactic nuclei (AGN) and gamma-ray bursts (GRBs) contains valuable information about the nature of the jet itself, and about the central engine which launches it. Using special relativistic hydrodynamics and magnetohydronamics simulations we study the dynamics of the jet and its interaction with the surrounding medium. The observational signature of the simulated jets is computed using a radiative transfer code developed specifically for the purpose of computing multi-wavelength, time-dependent, non-thermal emission from astrophysical plasmas. We present results of a series of long-term projects devoted to understanding the dynamics and emission of jets in parsec-scale AGN jets, blazars and the afterglow phase of the GRBs.
The properties of relativistic jets, their interaction with the ambient environment, and particle acceleration due to kinetic instabilities are studied self-consistently with Particle-in-Cell simulations. An important key issue is how a toroidal magnetic field affects the evolution of an e$^{pm}$ and an e$^{-}$ - p$^{+}$ jet, how kinetic instabilities such as the Weibel instability (WI), the mushroom instability (MI) and the kinetic Kelvin-Helmholtz instability (kKHI) are excited, and how such instabilities contribute to particle acceleration. We show that WI, MI and kKHI excited at the linear stage, generate a quasi-steady $x$-component of electric field which accelerates and decelerates electrons. In this work, we use a new jet injection scheme where an electric current is self-consistently generated at the jet orifice by the jet particles. We inject both e$^{pm}$ and e$^{-}$ - p$^{+}$ jets with a toroidal magnetic field (with a top-hat jet density profile) and for a sufficiently long time in order to examine the non-linear effects of the jet evolution. Despite the weakness of the initial magnetic field, we observe significant differences in the structure of the strong electromagnetic fields that are driven by the kinetic instabilities. We find that different jet compositions present different strongly excited instability modes. The magnetic field in the non-linear stage generated by different instabilities becomes dissipated and reorganized into a new topology. The 3-dimensional magnetic field topology indicates possible reconnection sites and the accelerated particles are significantly accelerated in the non-linear stage by the dissipation of the magnetic field and/or reconnection. This study will shed further light on the nature of astrophysical relativistic magnetized jet phenomena.
Weakly magnetized, relativistic collisionless shock waves are not only the natural offsprings of relativistic jets in high-energy astrophysical sources, they are also associated with some of the most outstanding displays of energy dissipation through particle acceleration and radiation. Perhaps their most peculiar and exciting feature is that the magnetized turbulence that sustains the acceleration process, and (possibly) the secondary radiation itself, is self-excited by the accelerated particles themselves, so that the phenomenology of these shock waves hinges strongly on the microphysics of the shock. In this review, we draw a status report of this microphysics, benchmarking analytical arguments with particle-in-cell simulations, and extract consequences of direct interest to the phenomenology, regarding in particular the so-called microphysical parameters used in phenomenological studies.
We present the results of three-dimensional kinetic particle-in-cell (PIC) simulations of isotropic periodic relativistically magnetized pair-plasma equilibria known as the ABC fields. We performed several simulations for initial wavenumbers k_ini = 2 or k_ini = 4, different efficiencies of radiative cooling (including radiation reaction from synchrotron and inverse Compton processes), and different mean magnetization values. These equilibria evolve by means of ideal coalescence instability, the saturation of which generates ab initio localized kinetically-thin current layers -- sites of magnetic reconnection and non-thermal particle acceleration -- eventually relaxing to a state of lower magnetic energy at conserved total magnetic helicity. We demonstrate that magnetic relaxation involves in addition localized collapses of magnetic minima and bulk mergers of current layer pairs, which represents a novel scenario of spontaneous magnetic dissipation with application to the rapid gamma-ray flares of blazars and of the Crab Nebula. Particle acceleration under strong radiative losses leads to formation of power-law indices N(gamma) ~ gamma^(-p) up to p ~= -2.3 at mean hot magnetization values of <sigma_hot> ~ 6. Individual energetic particles can be accelerated within one light-crossing time by electric fields that are largely perpendicular to the local magnetic fields. The energetic particles are highly anisotropic due to the kinetic beaming effect, implying complex patterns of rapid variability. A significant fraction of the initial total energy can be radiated away in the overall process of magnetoluminescence.
High resolution radio imaging of AGN have revealed that some sources present motion of superluminal knots and transverse stratification of their jet. Recent observational projects have provided new observational constraints on the central region of rotating black holes in AGN, suggesting that there is an inner- or spine-jet surrounded by a disk wind. This relativistic spine-jet is likely to be composed of electron - positron pairs extracting energy from the black hole. In this article we present an extension and generalization to relativistic jets in Kerr metric of the meridional self similar mechanism. We aim at modeling the inner spine-jet of AGN as the relativistic light outflow emerging from a spherical corona surrounding a Kerr black hole. The model is built by expanding the metric and the forces with colatitude to first order in the magnetic flux function. Conversely to previous models, effects of the light cylinder are not neglected. Solutions with high Lorentz factor are obtained and provide spine-jet models up to the polar axis. As in previous publications, we calculate the magnetic collimation efficiency parameter, which measures the variation of the available energy across the field lines. This collimation efficiency is an integral of the model, generalizing to Kerr metric the classical magnetic rotator efficiency criterion. We study the variation of the magnetic efficiency and acceleration with the spin of the black hole and show their high sensitivity to this integral. These new solutions model collimated or radial, relativistic or ultra-relativistic outflows. We discuss the relevance of our solutions to model the M87 spine-jet. We study the efficiency of the central black hole spin to collimate a spine-jet and show that the jet power is of the same order with that determined by numerical simulations.
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