Variable polarization measured in the prompt emission of GRB 041219A using IBIS on board INTEGRAL

TL;DR

This study measures the variable polarization of GRB 041219A's prompt gamma-ray emission using INTEGRAL's IBIS instrument, providing insights into the emission mechanisms and magnetic field structure of gamma-ray bursts.

Contribution

First measurement of variable polarization in a GRB prompt emission using IBIS, supporting synchrotron radiation models with coherent magnetic fields.

Findings

Polarization varies from <4% to 43% during the burst.

High polarization levels are consistent with synchrotron emission.

Rapid changes in polarization angle suggest a structured magnetic field.

Abstract

Polarization measurements provide direct insight into the nature of astrophysical processes. Unfortunately, only a few instruments are available for this kind of measurements at gamma-ray energies, and the sources need to be very bright. Gamma-Ray Bursts (GRBs) are ideal candidates due to their large flux over limited time intervals, maximizing the available signal-to-noise ratio. To date a few polarization measurements have been reported, claiming of a high degree of polarization in the prompt emission of GRBs but with low statistical evidence. We used the IBIS telescope on board the INTEGRAL satellite to measure the polarization of the prompt gamma-ray emission of the long and bright GRB 041219A in the 200-800 keV energy band. We find a variable degree of polarization ranging from less than 4% over the first peak to 43+/-25% for the whole second peak. Time resolved analysis of both peaks indicates a high degree of polarization, and the null average polarization in the first peak can be explained by the rapid variations observed in the polarization angle and degree. Our results are consistent with different models for the prompt emission of GRBs at these energies, but they favor synchrotron radiation from a relativistic outflow with a magnetic field which is coherent on an angular size comparable with the angular size of the emitting region (~1/Gamma) . Indeed this model has the best capabilities to maintain a high polarization level, and to produce the observed variability.