## Abstract

We study the long term evolution of magnetic fields generated by an initially unmagnetized collisionless relativistic e^{+}e^{-} shock. Our 2D particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle distributions are approximately isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially, non-propagating, and decaying. Using linear kinetic theory, we find a simple analytic form for these damping rates. Our theory predicts that the overall magnetic energy decays as (ω_{p} t) ^{-q} with q ∼ 1, which compares favorably with simulations, but predicts overly rapid damping of short-wavelength modes. The magnetic trapping of particles within the magnetic structures may be the origin of this discrepancy. We conclude that initially unmagnetized relativistic shocks in electron-positron plasmas are unable to form persistent downstream magnetic fields. These results put interesting constraints on synchrotron models for the prompt and afterglow emission from GRBs.

Original language | English (US) |
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Pages (from-to) | 1769-1775 |

Number of pages | 7 |

Journal | International Journal of Modern Physics D |

Volume | 17 |

Issue number | 10 |

DOIs | |

State | Published - Sep 2008 |

## All Science Journal Classification (ASJC) codes

- Mathematical Physics
- Astronomy and Astrophysics
- Space and Planetary Science

## Keywords

- Gamma-ray bursts
- Plasmas
- Shock waves
- Turbulence