Host: Gwenael Giacinti
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Abstract:
The afterglows of gamma-ray bursts (GRB) are synchrotron emissions from electrons accelerated by relativistic shocks. Comparison of the light curves and the theoretical model suggests that the magnetic field downstream of the forward shocks is amplified at least two orders of magnitude stronger than the shock-compressed magnetic field of the interstellar medium. The amplification mechanism of such a strong magnetic field is a major mystery in the radiation mechanism of the afterglow, and proving it is important for constraining the physical quantities and radiation mechanism of GRB jets and the acceleration mechanism of high- energy cosmic rays. Two field amplification mechanisms, plasma kinetic instabilities and magnetohydrodynamic instabilities, have been discussed so far. However, it is difficult to verify either mechanism by simulations in a realistic setting, and the origin of the strong magnetic field suggested by observations is still unknown. In this work, we focus on the fact that the coherence length of the turbulent magnetic field amplified by the two amplification mechanisms differs by about 10 orders of magnitude, and attempt to settle this problem using synchrotron polarization, which is the only way to verify the turbulence scale observationally. Theoretical works of polarization
have been extensively studied and compared with observations in the microscopic-scale turbulent magnetic field amplified by plasma instabilities, but they are inconsistent with some observations. In this work, we conduct the first quantitative study of the large-scale turbulent magnetic field amplified by magnetohydrodynamic instabilities, and construct a semi-analytic model of the polarization. In the constructed model, we examine the effects of the magnetic field anisotropy, the observer viewing angle, and the ordered magnetic field component. The results indicate that our model could explain all the polarimetric observational data to date that
seem to be forward shock emission. We also discuss the observational predictions of our model and how to verify the turbulence models in future observations.
Biography:
I received my Ph.D. in Science in March 2025 from the Astronomical Institute at Tohoku University, Japan. My Ph.D. supervisor was Prof. Kenji Toma. Since April 2025, I have been working as a postdoctoral researcher at the Frontier Research Institute for Interdisciplinary Sciences (FRIS) at Tohoku University. My scientific interests include high-energy astrophysical phenomena, especially gamma-ray bursts and their afterglows. I work on the theoretical modeling of afterglow polarization to understand the energy dissipation mechanisms in relativistic shock waves.
