## Abstract

Afterglow observations are commonly used to determine the parameters of GRB explosions, the energy E, surrounding density n, postshock magnetic field equipartition fraction ε_{B}, and electron equipartition fraction ε_{e}, under the frequently made assumption that the efficiency of electron "injection" into relativistic shock acceleration is high, i.e., that the fraction f of electrons that undergo acceleration is f ≈ 1. We show that the value of f cannot be determined by current observations, since currently testable model predictions for a parameter choice {E′ = E/f, n′ = n/f, ε′_{B} =f ε_{B}, ε′_{e} =f ε_{e}} are independent of the value of f for m_{e}/m_{p} ≤ f ≤ 1. Current observations imply that the efficiency f is similar for highly relativistic and subrelativistic shocks and plausibly suggest that f ∼ 1, quite unlike the situation in the Crab Nebula. However, values m_{e}/m_{p} ≤ f ≪ 1 cannot be ruled out, implying a factor m_{e}/m_{p} uncertainty in determination of model parameters. We show that early, ≤10 hr, radio afterglow observations, which will be far more accessible in the Swift era, may provide constraints on f. Such observations will therefore provide a powerful diagnostic of GRB explosions and of the physics of particle acceleration in collisionless shocks.

Original language | English |
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Pages (from-to) | 861-867 |

Number of pages | 7 |

Journal | Astrophysical Journal |

Volume | 627 |

Issue number | 2 I |

DOIs | |

State | Published - 10 Jul 2005 |

## Keywords

- Acceleration of particles
- Gamma rays: bursts
- Gamma rays: theory
- Radiation mechanisms: nonthermal
- Shock waves