This project is the first study of important three-dimensional effects in relativistic strongly-magnetized outflows. It will lay a firm basis for the realistic modeling of jets from black hole binary star systems, known as microquasars, and from active galactic nuclei, and of pulsar wind outflows and gamma-ray bursts. It will add substantially to our understanding of relativistic magnetohydrodynamic processes. Numerical simulations and theoretical development will provide an understanding of the role of strong magnetic fields in relativistic jets and shocks. The results will be used with observations of jets in very different environments to make predictions about the acceleration and collimation process and accompanying magnetization, and means of energy transport, in different astrophysical systems and at different spatial scales. This research directly addresses the relationship between jet dynamics and magnetic processes, adding insight into the physics of jet acceleration, collimation, propagation and particle acceleration.
This study also enhances the partnership between scientists at two campuses of the University of Alabama, at the National Space Science and Technology Center (Huntsville, Alabama), at Ben Gurion University (Israel), and at other national and international facilities. Student training in mathematical and computational techniques, their involvement in forefront research, and their participation in scientific meetings, will help prepare them for a range of science-based careers. The researchers are severally involved in public outreach at a variety of levels.
Using our 3-D RPIC code parallelized with MPI, we have investigated long-term particle acceleration associated with a relativistic electron-positron jet (Nishikawa et al. 2009a, 2011a). Cold jet electrons are thermalized and ambient electrons are accelerated in the forward and reverse shocks. Behind the forward (bow) shock in the reverse (jet) shock strong electromagnetic fields are generated. Synthetic Spectra from Shock Accelerated Particles in Turbulent Magnetic Fields Figure 1a shows the synthetic spectra for cold (thin lines) and warm (thick lines) electron jets with γ = 10, 20, 50, 100, and 300 (Nishikawa et al. 2012c,d). The low frequency slope in νFν ∼ 1 is indicated by the red lines. The low frequency slopes in our synthetic spectra are very similar to those of the spectra in Fig. 1b in Science (Abdo et a. 2009). However, further investigation is necessary to follow the spectral evolution observed by Fermi and will be one of our research efforts. Kinetic Kelvin-Helmholtz Instability Velocity shears must be considered when studying particle acceleration scenarios, since these trigger the kinetic Kelvin-Helmholtz instability (KKHI). Figure 2a shows our simulation model where the sheath plasma can be stationary or moving in the same direction as the jet core. In this simulation the sheath velocity is zero. Figure 2b shows the magnitude of By is plotted in the x − z plane (jet flow in the +x-direction indicated by the large arrow) at the center of the simulation box, at simulation time t = 70 ωpe−1 for the case of γj = 15 and mi/me = 1836. Figure 3a shows the magnitude of Ez plotted in the y − z plane (jet flow is out of the page) at the center of the simulation box, at t = 30ωpe−1. Figure 3b shows Ez (blue), Ex (black), and Ey (red) electric field components at at time t = 30 ωpe−1. The z component of electric field, Ez grows quickly, this DC electric field is shown in Figs. 3a and 3b. As expected, the y component of magnetic field, By is very small at this earlier time. The magnetic field grows later. Figure 3c shows By (red), Bx (black), and Bz (blue) magnetic field components at t = 70ωpe−1. Current Driven Instability & Structure In outflows from an accretion disk around a black hole, and/or involves the extraction of energy from a rotating black hole, a toroidal magnetic field is wound up and in the far zone becomes dominant because the poloidal magnetic field falls off faster with expansion and distance. The most dangerous instability in configurations with toroidal magnetic field is the current driven (CD) kink mode. This instability excites large-scale helical motions that may disrupt the system. Figure 4 shows the time evolution of a density isosurface for a static plasma column with Ω0 = 0 and α = 1, where the time, t , is in units of tc ≡ R0/c (Mizuno et al. 2012). The initial conditions are similar to those used in Mizuno et al. (2009) but now with two times stronger initial magnetic field strength and a radially decreasing gas pressure profile. As seen in Mizuno et al. (2009), displacement of the initial force-free helical magnetic field by growth of the CD kink instability leads to a helically twisted magnetic filament wound around the density isosurface associated with the n = 1 kink mode wavelength. In the nonlinear phase, helically distorted density structure shows rapid transverse growth and disruption of the high-density plasma column. Relativistic Shocks in Turbulence We have investigated via two-dimensional relativistic MHD simulations the long-term evolution of turbulence created by a relativistic shock propagating through an inhomogeneous medium (Mizuno et al. 2011, 2013). In the postshock region, magnetic field is strongly amplified by turbulent motions triggered by preshock density inhomogeneities. Using a long-simulation box we have followed the magnetic-field amplification until it is fully developed and saturated. The turbulent velocity is sub-relativistic even for a strong shock. Magnetic-field amplification is controlled by the turbulent motion and saturation occurs when the magnetic energy is comparable to the turbulent kinetic energy. Magnetic-field amplification and saturation depend on the initial strength and direction of the magnetic field in the preshock medium, and on the shock strength. If the initial magnetic field is perpendicular to the shock normal, the magnetic field is first compressed at the shock and then can be amplified by turbulent motion in the postshock region. Saturation occurs when the magnetic energy becomes comparable to the turbulent kinetic energy in the postshock region. If the initial magnetic field in the preshock medium is strong, the postshock region becomes turbulent but significant field amplification does not occur. If the magnetic energy after shock compression is larger than the turbulent kinetic energy in the postshock region, significant field amplification does not occur.
