*3.3. GRB Polarimeters*

In this section, we aim to provide a summary of the different instruments that have performed GRB polarization to date. For a detailed overview of each individual measurement (up to 2016), the reader is referred to [186].

As mentioned earlier, all polarization measurements of the prompt GRB emission have been performed by making use of Compton scattering. While in the majority of cases the Compton scattering takes place in the detector, there is one exception. The attempts at performing polarization measurements with data from the BATSE detector made use of Compton scattering from the Earth's atmosphere [207,208]. The BATSE detector consisted of several scintillator-based detectors and by itself had no capability to directly perform polarimetry [209]. Instead, it used several detectors pointing towards the Earth, each at different relative angles, to measure the relative intensity of photons scattering off different parts of the Earth's atmosphere. As the probability for photons to scatter off the atmosphere towards different detectors depends on their polarization properties, such a measurement is possible for any detector with an Earth-facing sensitive surface. It does however require a highly detailed modelling of the Earth's atmosphere, software capable of simulating the scattering effects properly, and detailed understanding of the detector response as well as the location and spectra of the GRB. The large number of sources for systematic errors resulted in inconclusive measurements of GRB 930131 [208]. Despite the initial lack of success, improvements have been made since then regarding Compton-scattering models in software such as Geant4 [210]. Furthermore, instruments such as *Fermi*-GBM have measured 1000 s of GRBs over the last decade, and similar studies using this data could prove to be successful in the future.

Systematic errors are a major issue not only for the creative polarization measurement solution used in BATSE but in all GRB polarization results published thus far from different instruments. It is especially important for measurements performed using detectors not originally designed to perform polarimetry such as RHESSI [211] and the SPI and IBIS detectors on board INTEGRAL. Both RHESSI and SPI make use of a segmented detector consisting of germanium detectors and thereby allow to study Compton scattering events by looking for coincident events between different detectors. The IBIS instrument [212] uses two separate sub-detectors instead, namely, the ISGRI detector consisting of 16384 CdTe detectors and the Pixellated Imaging CsI Telescope (PICsIT), an array of 4096 CsI scintillator detectors. Since, similar to RHESSI and SPI, IBIS was not originally designed

to perform polarization measurements, the trigger logic in the instrument was not setup to keep coincidence events in the PICsIT or ISGRI alone. Rather, only coincidence events between the PICsIT and the ISGRI are kept, which although lowering the statistics for polarization measurements, still allows for such measurements [213].

Since all three instruments were not designed as polarimeters, one immediate downside of using them as such is the lack of sensitivity. A clear example of this is the nonoptimized trigger logic of IBIS. In the case of RHESSI, different analyses of the same GRB [16] resulted in vastly different results, in part due to the difficulty in selecting valid coincident events between different germanium detector channels, again a result of a nonoptimized online event selection. The relatively imprecise time measurement of each event prompted a large coincidence window to be set in one of the analyses, which resulted in chance coincidence events induced by different photons or background particles instead of the Compton scattering event [17,18]. If the instrument had been designed and tested on the ground as a polarimeter, the coincidence trigger logic and time measurement would likely have been optimized and event selection methods tested during the calibration phase.

The lack of on-ground calibration for polarization additionally makes verification of the detector response models difficult and prone to errors—for example, dead material around the detector can affect the polarization of the incoming flux when it interacts with it. While such issues are important in spectrometers as well, it can be argued that it is more important in a polarimeter. Imperfect modelling of certain detector channels for a spectral measurement can cause issues. However, if on average the channels are modelled correctly, having a few badly modelled channels will not greatly affect the final flux or spectral result, as over- and under-performing channels can cancel each other out. In a polarimeter it is the difference in the number of events between the detector channels that provides the final measurement and not, as in a simple spectrometer, the average of all the channels. For a polarimeter, however, one single over-performing detector channel would see a larger number of scattering events than expected, causing certain scattering angles to be favoured and thereby faking a polarization signal.

Similarly, dead material in front of the detector channels can easily obscure certain channels more than others causing a similar effect. Understanding all these issues during on-ground calibration is therefore crucial to reduce systematic errors. As a result of such difficulties, the polarization results published by the SPI collaboration clearly mention the possibility of significant systematic errors not taken into account in the analysis, which can affect the results [214,215].

In order to overcome such issues, more recent instruments, such as GAP [191] and POLAR [192], employ small coincidence windows and trigger logics optimized for polarization measurements. Most importantly, such detectors were calibrated prior to launch with polarized photons in different configurations, such as different photon energies and incoming angles [191,216,217]. GAP was the first dedicated GRB polarimeter. It made use of plastic scintillators used to detect Compton scattering photons together with 12 CsI scintillators used to detect the photon after scattering. The instrument flew for several years on the IKAROS solar sail mission during which it detected a few GRBs for which polarization measurements were possible. The POLAR detector also uses plastic scintillators, 1600 in total, to detect the Compton scattering interaction but uses the same scintillators to detect the secondary interaction. As a result, the instrument is less efficient for detecting the secondary interaction and has a poorer energy resolution. However, it allows for a larger scalable effective area as well as a larger field of view, which in the case of GAP is restricted by the CsI detectors that shield the plastic scintillators from a far off-axis source. The POLAR detector took data for six months on board the Tiangong-2 space laboratory, which resulted in the publication of 14 GRB polarization measurements [187].

Two other detectors, which although not fully optimized for polarimetry, were calibrated on the ground for such measurements. They are COSI [218] and the CZTI on Astrosat [194]. The balloon-borne COSI detector uses two layers of germanium doublesided strip detectors allowing for precise measurements of the interaction locations in the

instrument. During its long-duration balloon flight in 2017, COSI saw one bright GRB for which a polarization measurement could be performed [219]. The CZT Imager on board the AstroSAT satellite uses, as the name suggests, a CZT semiconducter detector. As this detector is segmented, it allows to look for Compton scattering events. The detector was calibrated with polarized beams prior to launch to study the instrument response to on-axis sources [194]. AstroSAT CZTI has detected a large number of GRBs since its launch and has published polarization measurements of 13 of these to date while it continues to be operational.
