Prompt GRB Polarization from Non-Axisymmetric Jets | Monthly Notices of the Royal Astronomical Society

Unlocking the Mystery of Gamma-Ray Burst Emission Through Polarization Measurements

In the quest to unravel the secrets of gamma-ray bursts (GRBs), scientists have turned to time-resolved linear polarization (Π) measurements of the prompt emission. By studying the polarization properties, researchers hope to gain insights into the dominant radiation mechanism responsible for these powerful cosmic events. One widely considered mechanism is synchrotron radiation, which involves the acceleration of charged particles in a magnetic field. The linear polarization can offer valuable clues about the structure of the jet and its composition.

Traditionally, axisymmetric jet models predict that the polarization angle (PA) can only change by 90°, causing a temporary disappearance of polarization (Π). However, recent time-resolved measurements have revealed a continuously changing PA, suggesting the presence of non-axisymmetric features in the jet. This non-axisymmetry could arise from variations in the jet’s emissivity, bulk Lorentz factor, or magnetic field.

In a groundbreaking study, researchers have now explored synchrotron emission in non-axisymmetric jets using an ultrarelativistic thin shell model. This model incorporates the concept of multiple radially-expanding mini-jets (MJs) or emissivity patches within the global jet, allowing for a continuously changing PA. The study investigates various scenarios with different magnetic field configurations and jet angular structures, considering both single and multiple overlapping pulses of emission.

The findings reveal that emission from multiple incoherent MJs or patches can lead to a reduction in the net polarization due to partial cancellation in the Stokes plane. Interestingly, when these MJs or patches contain a large-scale ordered field in the plane transverse to the radial direction, the polarization initially starts at maximum and then declines over the course of a single pulse. In the case of multiple pulses, the observations may exhibit multiple highly polarized peaks.

To distinguish between different magnetic field scenarios, researchers propose looking for integrated polarization levels of |$Π ≤ 40%$| (15%) over one (several) pulse(s). Such measurements would favor the presence of a shock-produced small-scale field, either ordered in the radial direction or tangled in the transverse plane.

This study opens up new avenues for understanding the complex nature of GRB emission and the underlying physics behind these cosmic fireworks. By combining advanced polarization measurements with sophisticated models, scientists aim to unlock the mysteries of these enigmatic phenomena and gain deeper insights into the workings of the universe.

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© The Author(s) 2023. Published by Oxford University Press on behalf of the Royal Astronomical Society.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.



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