Polarization Evolution of Early Optical Afterglows of Gamma-Ray Bursts

Abstract

The central engine and jet composition of gamma-ray bursts (GRBs) remain mysterious. Here we suggest that observations on polarization evolution of early optical afterglows may shed light on these questions. We first study the dynamics of a reverse shock and a forward shock that are generated during the interaction of a relativistic jet and its ambient medium. The jet is likely magnetized with a globally large-scale magnetic field from the central engine. The existence of the reverse shock requires that the magnetization degree of the jet should not be high ($\sigma \le 1$), so that the jet is mainly composed of baryons and leptons. We then calculate the light curve and polarization evolution of an early optical afterglow, and find that when the polarization position angle changes by $90^{\circ }$ during the early afterglow, the polarization degree is zero for a toroidal magnetic field but is very likely to be non-zero for an aligned magnetic field. This result would be expected to provide a probe for the central engine of GRBs, because an aligned field configuration could originate from a magnetar central engine and a toroidal field configuration could be produced from a black hole via the Blandford-Znajek mechanism. Finally, for such two kinds of magnetic field configurations, we fit the observed data of the early optical afterglow of GRB 120308A equally well.

10 pages, 12 figures, accepted for publication in ApJ

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• Interpreting time-integrated polarization data of gamma-ray burst prompt emission
• An energy-conserving dynamical model of GRB afterglows from magnetized forward and reverse shocks
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• Testing Models for the Shallow Decay Phase of Gamma-Ray Burst Afterglows with Polarization Observations
• Afterglow polarizations in a stratified medium with effect of the equal arrival time surface
• Polarization of Astrophysical Events with Precessing Jets
• A Pulsar Wind Nebula Model Applied to Short GRB 050724
• On the Polarized Absorption Lines in Gamma-ray Burst Optical Afterglows
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• A semi-analytical solution to the forward-reverse shock hydrodynamics of the gamma-ray burst afterglow