Linear polarization in gamma-ray burst prompt emission

Ramandeep Gill; Jonathan Granot; Pawan Kumar

doi:10.1093/mnras/stz2976

paper

Linear polarization in gamma-ray burst prompt emission

arXiv:1811.11555 · doi:10.1093/mnras/stz2976

Abstract

Despite being hard to measure, GRB prompt $γ$ -ray emission polarization is a valuable probe of the dominant emission mechanism and the outflow's composition and angular structure. During the prompt emission the outflow is ultra-relativistic with Lorentz factors $Γ ≫ 1$ . We describe in detail the linear polarization properties of various emission mechanisms: synchrotron radiation from different magnetic field structures (ordered: toroidal $B_{tor}$ or radial $B_{∥}$ , and random: normal to the radial direction $B_{⊥}$ ), Compton drag, and photospheric emission. We calculate the polarization for different GRB jet angular structures (e.g. top-hat, Gaussian, power-law) and viewing angles $θ_{obs}$ . Synchrotron with $B_{⊥}$ can produce large polarizations, up to $25% ≲ Π ≲ 45%$ , for a top-hat jet but only for lines of sight just outside the jet's sharp edge. The same also holds for Compton drag, albeit with a slightly higher overall $Π$ . Moreover, we demonstrate how $Γ$ -variations during the GRB or smoother jet edges would significantly reduce $Π$ . We construct a semi-analytic model for non-dissipative photospheric emission from structured jets. Such emission can produce up to $Π ≲ 15%$ with reasonably high fluences, but this requires steep gradients in $Γ (θ)$ . A polarization of $50% ≲ Π ≲ 65%$ can robustly be produced only by synchrotron emission from a transverse magnetic field ordered on angles $≳ 1/Γ$ around our line of sight (like a global toroidal field). Therefore, such a model would be strongly favored even by a single secure measurement within this range. We find that such a model would also be favored if $Π ≳ 20%$ is measured in most GRBs within a large enough sample, by deriving the polarization distribution for our different emission and jet models.

arXiv comment31 pages, 18 figures. Minor changes to the text; Accepted for publication in MNRAS

Linear polarization in gamma-ray burst prompt emission

Abstract

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