http://arxiv.org/abs/1407.1651
The high polarization observed in the prompt phase of some gamma-ray bursts (GRBs) arouses extensive studies on the emission mechanism. In this paper, we investigate the polarization properties of synchrotron-self-Compton (SSC) process from a highly relativistic jet. A magnetic-dominated, baryon-loaded jet ejected from central engine travels with a large Lorentz factor. Shells with slightly different velocities collides with each other and produce shocks. The shocks accelerate electrons to power-law distribution, and at the same time, magnify the magnetic field. Electrons move in the magnetic field and produce synchrotron photons. The synchrotron photons suffer from Compton scattering process and then escape from the jet and are detected by an observer locating slightly off-axis. We derive analytical formulae of polarization in the SSC process, based on the cross section of an initially polarized photon scattered by an unpolarized electron with any momentum. We show that photons induced by the SSC process can be highly polarized, with the maximum polarization $\Pi \sim 24\%$ in the energy band $[0.5,5]$ MeV. The polarization depends on the viewing angles, peaking in the plane perpendicular to the magnetic field. In the energy band $[0.05,0.5]$ MeV, in which most $\gamma$-ray polarimeters are active, the polarization is about twice of that in the Thomson limit, reaching to $\Pi\sim 20\%$. This implies that the Klein-Nishina effect, which is often neglected in literatures, should be carefully considered.
Z. Chang and H. Lin
Tue, 8 Jul 14
12/66
Comments: 15 pages, 5 figures
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