Polarization of prompt GRB emission: evidence for electromagnetically-dominated outflow

Maxim Lyutikov; V. I. Pariev; Roger Blandford

doi:10.1086/378497

paper

Polarization of prompt GRB emission: evidence for electromagnetically-dominated outflow

arXiv:astro-ph/0305410 · doi:10.1086/378497

Abstract

Observations by the {\RHESSI} satellite of large polarization of the prompt $γ$ -ray emission from the Gamma Ray Burst GRB021206 \citep{coburn03} imply that the magnetic field coherence scale is larger than the size of the visible emitting region, $\sim R /Γ$ , where $R$ is the radius of the flow, $Γ$ is the associated Lorentz factor. Such fields cannot be generated in a causally disconnected, hydrodynamically dominated outflow. Electromagnetic models of GRBs \citep{lyutikov02}, 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 Stokes parameters of 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 global toroidal magnetic field. For viewing angles larger than $1/Γ$ the observed patch of the emitting shell has almost homogeneous magnetic field, producing a large fractional polarization ( $56$ for a power-law energy distribution of relativistic particles $d n / d ε \propto ε^{- 3}$ ). The maximum polarization is smaller than the theoretical upper limit for a stationary plasma in uniform magnetic field due to relativistic kinematic effects.

23 pages, 5 figure; submitted to ApJ

Polarization of prompt GRB emission: evidence for electromagnetically-dominated outflow

Abstract

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