The POLAR Gamma-Ray Burst Polarization Catalog

TL;DR

This paper presents the POLAR Gamma-Ray Burst Polarization Catalog, analyzing polarization data from 14 GRBs to understand their emission mechanisms, revealing mostly low polarization but some moderate, rapidly changing polarization signals within individual pulses.

Contribution

It provides the first detailed polarization measurements of GRBs in the 50-500 keV range using POLAR, including time-resolved analysis and a new joint spectral-polarimetric modeling approach.

Findings

Most GRBs show low or zero polarization in time-integrated analysis.

Time-resolved analysis reveals moderate polarization with changing angles within pulses.

Results suggest polarization signals may be washed out in time-integrated data.

Abstract

Despite over 50 years of research, many open questions remain about the origin and nature of GRBs. Polarization measurements of the prompt emission of these extreme phenomena have long been thought to be the key to answering a range of these questions. The POLAR detector was designed to produce the first set of detailed and reliable polarization measurements in an energy range of approximately 50-500 keV. During late 2016 and early 2017, POLAR detected a total of 55 GRBs. Analysis results of 5 of these GRBs have been reported in the past. The results were found to be consistent with a low or unpolarized flux. However, previous reports by other collaborations found high levels of polarization. We study the polarization for all the 14 GRBs observed by POLAR for which statistically robust inferences are possible. Additionally, time-resolved polarization studies are performed on GRBs with sufficient apparent flux. A publicly available polarization analysis tool, developed within the 3ML framework, was used to produce statistically robust results. The method allows to combine spectral and polarimetric data from POLAR with spectral data from the Fermi GBM and Swift-BAT to jointly model the spectral and polarimetric parameters. The time integrated analysis finds all results to be compatible with a low or zero polarization with the caveat that, when time-resolved analysis is possible within individual pulses, we observe moderate polarization with a rapidly changing polarization angle. Thus, time-integrated polarization results, while pointing to lower polarization are potentially an artifact of summing over the changing polarization signal and thus, washing out the true moderate polarization. Therefore, we caution against over interpretation of any time-integrated results and encourage one to wait for more detailed polarization measurements from forthcoming missions such as POLAR-2 and LEAP.