Particle Acceleration and Magnetic Field Generation in Electron‐Positron Relativistic Shocks

Abstract

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel, and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a three-dimensional relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic electron-positron jet front propagating into an ambient electron-positron plasma with and without initial magnetic fields. We find small differences in the results for no ambient and modest ambient magnetic fields. New simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. Furthermore, the nonlinear fluctuation amplitudes of densities, currents, and electric and magnetic fields in the electron-positron shock are larger than those found in the electron-ion shock studied in a previous paper at a comparable simulation time. This comes from the fact that both electrons and positrons contribute to generation of the Weibel instability. In addition, we have performed simulations with different electron skin depths. We find that growth times scale inversely with the plasma frequency, and the sizes of structures created by the Weibel instability scale proportionally to the electron skin depth. This is the expected result and indicates that the simulations have sufficient grid resolution. While some Fermi acceleration may occur at the jet front, the majority of electron and positron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying nonuniform, small-scale magnetic fields, which contribute to the electron's (positron's) transverse deflection behind the jet head. This smallscale magnetic field structure is appropriate to the generation of ''jitter'' radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation has different properties than synchrotron radiation calculated assuming a uniform magnetic field. The jitter radiation resulting from small-scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Diffusive Acceleration of Particles at Oblique, Relativistic, Magnetohydrodynamic Shocks

“…Relativistic PIC codes have blossomed to model shocks in applications such as GRBs and pulsar wind termination shocks, focusing largely, but not exclusively, on perpendicular shocks (e.g., Gallant et al 1992;Smolsky & Usov 1996;Silva et al 2003;Hededal et al 2004;Liang & Nishimura 2004;Medvedev et al 2005;Nishikawa et al 2005;Spitkovsky 2008). These works have explored pair shocks, ion-doped shocks, Poynting flux-dominated outflows, and low-field systems with dissipation driven by the Weibel instability.…”

Are Gamma‐Ray Burst Shocks Mediated by the Weibel Instability?

“…Many authors have therefore assumed that the shock somehow manufactures field energy to meet this requirement, but no convincing mechanism has been proposed to date. One mechanism discussed is the Weibel instability ( Medvedev & Loeb 1999;Silva et al 2003;Frederiksen et al 2004;Jaroschek et al 2005;Medvedev et al 2005;Kato 2005;Nishikawa et al 2005), which has the fastest growth rate and produces relatively strong small-scale magnetic field even in an initially nonmagnetized plasma. It is expected that the thermalization of the upstream flow could occur via scattering of particles on the magnetic fluctuations.…”

Topical Issues for Particle Acceleration Mechanisms in Astrophysical Shocks

“…This is an issue also for a growing number of particle-in-cell (PIC) simulations (e.g. Silva et al 2003;Hededal et al 2004;Nishikawa et al 2005; see Section 4 below) used to explore field enhancement via the Weibel instability in relativistic shocks; such developments are not that salient for the problem of amplifying Alfvén turbulence in non-relativistic shocks, and mostly probe the inertial scales of thermal ions and electrons defined by their plasma frequencies.…”

“…Rich in their turbulence information, these have been used extensively in non-relativistic, heliospheric shock applications, and more recently, relativistic PIC codes have blossomed to model shocks in various astrophysical systems. PIC simulation research has largely, but not exclusively, focused on perpendicular shocks, first with Gallant et al(1992), Hoshino et al (1992), and then Smolsky & Usov (1996), Shimada & Hoshino (2000), Silva et al (2003), Nishikawa et al (2003Nishikawa et al ( , 2005, Spitkovsky & Arons (2004), Hededal et al (2004), Liang & Nishimura (2004), Medvedev et al (2005) and Hededal & Nishikawa (2005). These works have explored pair shocks, ion-doped shocks, Poynting flux-dominated outflows, and low-field systems with dissipation driven by the Weibel instability, in applications such as GRBs and pulsar wind termination shocks.…”