*5.1. Time-Integrated Polarization Measurements*

To date, the *γ*-ray polarization of a total of 31 GRBs has been published. For several GRBs, different analyses have been published, either by different groups using the same data or, in one case, using data from two different instruments. The time- and energyintegrated polarization parameters from these measurements are shown in Table 1, together with the energy range in which they were performed. It is important to note that the energy ranges mentioned here are those stated in the respective publications but that their definitions differ between experiments. The energy ranges stated by SPI for example come from an event selection based on the deposited energy, whereas, for POLAR, which cannot perform measurements of the incoming photon energy directly, the stated range is based on the energy-dependent effective area to polarization.

As can be seen from Table 1, especially for the earliest measurements, at the bottom of the table, the results indicate typically high levels of polarization, although, as explained earlier, this can in some cases be attributed to an error in the analysis. Additionally, publications of GRB polarization measurements have focused on those measurements for which a non-zero PD was found. At least several GRB measurements exist, such as some detected by GAP, for which the PD was found to be compatible with 0%; however, these were not published but only presented at conferences (https: //ttt.astro.su.se/groups/head/cost14/talks/Yonetoku.pdf talk accessed on 25 August 2014). This causes an additional bias towards higher PD values found in the list.


**Table 1.** The list of all GRBs for which a measurement has been published to date. † For GRB 160821A, several analyses were published by members of the AstroSAT collaboration. For this GRB, a time-resolved analysis found high levels of polarization with varying PA as well.


**Table 1.** *Cont.*

In recent years, data from GAP, POLAR, and Astrosat CZTI have significantly increased the number of measurements; however, the measured PD shows a large range between the different instruments. POLAR finds results that are mostly compatible with a low or unpolarized flux, whereas Astrosat CZTI reports high levels of polarization in [188], with best fitting PD for 10 out of the 11 GRBs exceeding 50%. Although in numerical form only an upper limit is provided for some of these GRBs by Astrosat CZTI (which are the numbers reported in Table 1) mthe contour plots for these GRBs in Figure 13 of [188] indicate that high levels of PD are favoured for all. In most cases, the best fitting PD is close to the upper limit. The only exception is 160623A where a best fitting PD of approximately 30% is found. It should be noted though that for GRB 160821A, two separate analyses provided different results for the main emission period. The first from [188] indicates a rather high level of polarization, whereas [231] found a time-integrated PD compatible with a lowly or unpolarized flux. The analysis methods used for both analyses were different, while additionally the selected time intervals differed (a period with low fluence was added in [231]). Although the interval selection is not discussed in detail, in [240] it is mentioned that the intervals used in [188] were optimized to maximize the significance of the PD detection, giving a possible explanation. The same analysis as applied in [188] was applied in [241] for GRB 171010A where an upper limit of 42% was reported.

The overall impression given by the Astrosat CZTI results is that GRBs are rather highly polarized. From the POLAR results this is not the case as no significant PD was detected, and all results are compatible with an unpolarized flux within the 99% confidence interval. The POLAR results favors low polarization degrees, with PD values exceeding 50% excluded by five of the brightest GRBs with a 99% confidence level. The results from GAP show both GRBs with a high level of polarization, as well as those with a low level, while COSI, the last of the four detectors, which was well calibrated on ground, additionally excludes high values of PD.

Despite the significant increase in available measurements, no clear conclusion on the PD of GRBs has emerged. It therefore appears that simply continuing to push for more measurements with the current generation of instruments might not be the best way forward. Rather, detailed studies scrutinizing the different results found by different instruments are an easier and more promising way forward. One way to achieve this, which is discussed later on, is the use of more standardized analyses methods as well as by making the polarization data public for an independent analysis by different groups.
