TY - JOUR
T1 - Magnetized Reverse Shock
T2 - Density-fluctuation-induced Field Distortion, Polarization Degree Reduction, and Application to GRBs
AU - Deng, Wei
AU - Zhang, Bing
AU - Li, Hui
AU - Stone, James McLellan
N1 - Funding Information:
We are grateful to Christopher White, Xuening Bai, Gustavo Rocha da Silva, Zhaohuan Zhu, Jiming Shi, Yanfei Jiang, and Chang-Goo Kim for the important technical support during the usage of the “Athena++” MHD code, Haocheng Zhang for helpful instruction regarding the “3DPol” polarization calculation code, and Fan Guo, Haocheng Zhang, Xiangrong Fu, and Shangfei Liu for helpful discussions and suggestions. An anonymous referee is greatly appreciated for very helpful comments. This work is supported by NASA through grants NNX15AK85G and NNX14AF85G, and by the LANL/LDRD program and Institutional Computing Programs at LANL and by DOE/ Office of Fusion Energy Science through CMSO.
Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved..
PY - 2017/8/10
Y1 - 2017/8/10
N2 - The early optical afterglow emission of several gamma-ray bursts (GRBs) shows a high linear polarization degree (PD) of tens of percent, suggesting an ordered magnetic field in the emission region. The light curves are consistent with being of a reverse shock (RS) origin. However, the magnetization parameter, σ, of the outflow is unknown. If σ is too small, an ordered field in the RS may be quickly randomized due to turbulence driven by various perturbations so that the PD may not be as high as observed. Here we use the "Athena++" relativistic MHD code to simulate a relativistic jet with an ordered magnetic field propagating into a clumpy ambient medium, with a focus on how density fluctuations may distort the ordered magnetic field and reduce PD in the RS emission for different σ values. For a given density fluctuation, we discover a clear power-law relationship between the relative PD reduction and the σ value of the outflow. Such a relation may be applied to estimate σ of the GRB outflows using the polarization data of early afterglows.
AB - The early optical afterglow emission of several gamma-ray bursts (GRBs) shows a high linear polarization degree (PD) of tens of percent, suggesting an ordered magnetic field in the emission region. The light curves are consistent with being of a reverse shock (RS) origin. However, the magnetization parameter, σ, of the outflow is unknown. If σ is too small, an ordered field in the RS may be quickly randomized due to turbulence driven by various perturbations so that the PD may not be as high as observed. Here we use the "Athena++" relativistic MHD code to simulate a relativistic jet with an ordered magnetic field propagating into a clumpy ambient medium, with a focus on how density fluctuations may distort the ordered magnetic field and reduce PD in the RS emission for different σ values. For a given density fluctuation, we discover a clear power-law relationship between the relative PD reduction and the σ value of the outflow. Such a relation may be applied to estimate σ of the GRB outflows using the polarization data of early afterglows.
KW - galaxies: jets
KW - gamma-ray burst: general
KW - magnetic fields
KW - magnetohydrodynamics (MHD)
KW - polarization
KW - shock waves
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U2 - 10.3847/2041-8213/aa7d49
DO - 10.3847/2041-8213/aa7d49
M3 - Article
AN - SCOPUS:85028333615
VL - 845
JO - Astrophysical Journal Letters
JF - Astrophysical Journal Letters
SN - 2041-8205
IS - 1
M1 - L3
ER -