Prompt GRB Polarization from Non-Axisymmetric Jets

TL;DR

This paper investigates how non-axisymmetric jet structures in gamma-ray bursts can produce continuously changing polarization angles, providing insights into the jet's magnetic field and composition through polarization measurements.

Contribution

It introduces a model of non-axisymmetric jets with multiple mini-jets or patches, explaining variable polarization angles and their dependence on magnetic field configurations.

Findings

Multiple mini-jets reduce net polarization due to partial cancellation.

Ordered transverse magnetic fields produce high initial polarization that declines over time.

Low observed polarization favors small-scale, shock-produced magnetic fields.

Abstract

Time-resolved linear polarization ($\Pi$) measurements of the prompt gamma-ray burst emission can reveal its dominant radiation mechanism. A widely considered mechanism is synchrotron radiation, for which linear polarization can be used to probe the jet's magnetic-field structure, and in turn its composition. In axisymmetric jet models the polarization angle (PA) can only change by $90^\circ$, as $\Pi$ temporarily vanishes. However, some time-resolved measurements find a continuously changing PA, which requires the flow to be non-axisymmetric in at least one out of its emissivity, bulk Lorentz factor or magnetic field. Here we consider synchrotron emission in non-axisymmetric jets, from an ultrarelativistic thin shell, comprising multiple radially-expanding mini-jets (MJs) or emissivity patches within the global jet, that yield a continuously changing PA. We explore a wide variety of possibilities with emission consisting of a single pulse or multiple overlapping pulses, presenting time-resolved and integrated polarization from different magnetic field configurations and jet angular structures. We find that emission from multiple incoherent MJs/patches reduces the net polarization due to partial cancellation in the Stokes plane. When these contain a large-scale ordered field in the plane transverse to the radial direction, $\Pi$ always starts near maximal and then declines over the single pulse or shows multiple highly polarized peaks due to multiple pulses. Observing $\Pi\lesssim40\%$ ($15\%$) integrated over one (several) pulse(s) will instead favor a shock-produced small-scale field either ordered in the radial direction or tangled in the plane transverse to it.