Energy-dependent polarization of Gamma-Ray Bursts' prompt emission with the POLAR and POLAR-2 instruments

Abstract: Gamma-Ray Bursts are among the most powerful events in the Universe. Despite half a century of observations of these transient sources, many open questions remain about their nature. Polarization measurements of the GRB prompt emission have long been theorized to be able to answer most of these questions. With the aim of characterizing the polarization of these prompt emissions, a compact Compton polarimeter, called POLAR, has been launched to space in September 2016. Time integrated polarization analysis of the POLAR GRB catalog have shown that the prompt emission is lowly polarized or fully unpolarized. However, time resolved analysis depicted strong hints of an evolving polarization angle within single pulses, washing out the polarization degree in time integrated analyses. Here we will for the first time present energy resolved polarization measurements with the POLAR data. The novel analysis, performed on several GRBs, will provide new insights and alter our understanding of GRB polarization. The analysis was performed using the 3ML framework to fit polarization parameters versus energy in parallel to the spectral parameters. Although limited by statistics, the results could provide a very relevant input to disentangle between existing theoretical models. In order to gather more statistics per GRB and perform joint time and energy resolved analysis, a successor instrument, called POLAR-2, is under development with a launch window early 2025 to the CSS. After presenting the first energy resolved polarization results of the POLAR mission, we will present the prospects for such measurements with the upcoming POLAR-2 mission.

• The paper demonstrates that time-resolved analysis reveals transient polarization angle shifts in GRB emissions despite overall low polarization levels.
• It employs the 3ML framework for energy-dependent analysis, noting minor low-energy polarization variations amidst current statistical limits.
• The study highlights POLAR-2's enhanced sensitivity, which is expected to refine GRB polarization models and deepen our theoretical understanding.

Energy-dependent Polarization of Gamma-Ray Bursts

The paper entitled "Energy-dependent polarization of Gamma-Ray Bursts' prompt emission with the POLAR and POLAR-2 instruments" offers a comprehensive analysis of polarization measurements in Gamma-Ray Bursts (GRBs), using data from the POLAR instrument and prospects for the forthcoming POLAR-2 mission. A fundamental aspect of understanding GRBs, which are among the most intense and enigmatic events in the universe, rests upon dissecting their polarization properties. Despite extensive observations over several decades, the physical mechanisms driving these cosmic phenomena remain largely unresolved. This study seeks to address this knowledge gap by leveraging polarization measurements, potentially illuminating the intricate emission mechanics and magnetic structures of GRBs.

POLAR Instrument Analysis and Findings

The POLAR instrument embarked on its mission to measure the polarization degree and angle of prompt GRB emissions, using Compton scattering mechanisms for detection. Positioned on the Tiangong-2 space laboratory since 2016, POLAR delivered insightful results regarding the polarization properties of 14 detected GRBs. Initial findings suggested that on a time-integrated basis, GRB emissions generally exhibited low polarization levels. However, through time-resolved analysis, the emergence of a transient polarization angle during pulsations in emissions provided pivotal insight. This dynamic aspect potentially reconciles prior findings of low polarization degrees due to neutralizing the overall polarization upon temporal integration.

To investigate further, the authors conducted energy-dependent analyses using tools like the 3ML framework. Although statistical limitations constrain the current dataset, these analyses point toward potential differentiators among theoretical models of GRB polarization. The results reveal no significant energy-dependent changes in polarization degrees, though deviations, especially at low-energy, hint at minor variations that warrant follow-up with more robust datasets.

Implications and Future Prospects with POLAR-2

The advent of POLAR-2 represents a key advancement in GRB polarization research, promising enhanced sensitivity and data fidelity through innovations such as an expanded detector area and advanced readout technologies (SiPMs). POLAR-2 aims to extract more granular polarization data, enabling joint time and energy-resolved analysis. Its operations are poised to provide invaluable statistical clarity, possibly resolving ambiguities in current measurements and solidifying the comprehension of GRB polarization energy dependency.

The preliminary results underscore the need for more sophisticated instruments like POLAR-2, which holds the potential to refine theoretical models or even unveil new paradigms in our understanding of GRBs. Consequently, the success of POLAR-2 in broadening observational capabilities will be pivotal, potentially influencing theoretical frameworks explaining GRB emissions and their associated magnetic fields.

Concluding Insights

The effort to unravel the mysteries of GRBs through energy-dependent polarization studies marks a progressive step in high-energy astrophysics. While limited by current instrumentation, POLAR's results epitomize the invaluable initial directions for subsequent, more precise explorations. As POLAR-2 complements these endeavors with enhanced fidelity, it is likely to delineate clearer associations between polarization characteristics and GRB emissions, refining or revolutionizing existing theoretical foundations in the process.