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Published November 10, 2003 | Published + Submitted
Journal Article Open

Polarization of Prompt Gamma‐Ray Burst Emission: Evidence for Electromagnetically Dominated Outflow

Abstract

Observations by the RHESSI satellite of the large polarization of the prompt γ-ray emission from γ-ray burst (GRB) 021206 imply that the magnetic field coherence scale is larger than the size of the visible emitting region, ~R/Γ, where R is the radius of the flow and Γ is the associated Lorentz factor. Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs, in which large-scale, dynamically dominant, magnetic fields are present in the outflow from the very beginning, provide a natural explanation of this large reported linear polarization. We derive the Stokes parameters of the synchrotron emission of a relativistically moving plasma with a given magnetic field configuration and calculate the pulse-averaged polarization fraction of the emission from a relativistically expanding shell carrying a global toroidal magnetic field. For viewing angles larger than 1/Γ, the observed patch of the emitting shell has an almost homogeneous magnetic field, producing a large fractional polarization (56% for a power-law energy distribution of relativistic particles, dn/dϵ ∝ ϵ^(-3)). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in a uniform magnetic field because of relativistic kinematic effects.

Additional Information

© 2003 The American Astronomical Society. Received 2003 May 21; accepted 2003 July 17. V. P. acknowledges support from Department of Energy grant DE-FG02-00ER54600.

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Published - Lyutikov_2003_ApJ_597_998.pdf

Submitted - 0305410.pdf

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