Work

Large-scale Magnetic Field Model of GRB Afterglow Polarization: Effects of Field Anisotropy, Off-axis Viewing Angle, and Ordered Field

Year: 2024

Type: article

Abstract: The afterglows of gamma-ray bursts are nonthermal electron synchrotron emissions from relativistic shocks. The origin of strong magnetic field in the emission region remains elusive, and two field amplification mechanisms via the plasma kinetic and magnetohydrodynamic instabilities have been discussed. The polarimetric observations are a powerful probe to distinguish these two mechanisms. So far, most theoretical works have focused on the former mechanism and constructed afterglow polarization models with microscopic-scale turbulence whose coherence length is much smaller than the thickness of the blast wave. In this work, focusing on the latter mechanism, we utilize our semianalytic model of the synchrotron polarization with large-scale turbulence whose coherence length is comparable to the thickness of the blast wave to investigate the effect of magnetic field anisotropy and the observer viewing angle. We find that the polarization in our large-scale turbulence model can exhibit both behaviors characteristic of the microscopic-scale turbulence model and those not seen in the microscopic-scale model. Then we find that the large-scale model could explain all the polarimetric observational data to date that seem to be forward shock emission. We also examine the effect of ordered-field components and find that polarization degree and polarization angle constant in time are realized only when the energy density ratio of the ordered and fluctuated components is ≳50. In this case, however, the polarization degree is much higher than the observed values.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/ad900b

Authors: Asuka Kuwata, Kenji Toma, Sara Tomita, Jiro Shimoda

Institutions: Tohoku University, Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Chiba University, Institute for Cosmic Ray Research, The University of Tokyo

Language: English

Topic: Gamma-ray bursts and supernovae

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $4,499

Funders: National Astronomical Observatory of Japan, Science and Technology Facilities Council, Japan Society for the Promotion of Science, Japan Science and Technology Agency