7.1.2. Low-Energy Polarimeters

Apart from adding significant sensitivity in the energy range of ∼10–1000 keV, missions are also proposed to perform the first GRB polarization measurements at keV energies.

As previously mentioned, the first polarization measurements at these energies, albeit of point sources, were recently performed by the small scale PolarLite mission [195]. The IXPE mission [197], which uses a similar measurement technology as PolarLite, is planned to be launched in 2021. However, as it is optimized for point sources, it has a narrow FoV. This in combination with a long slewing time makes it unlikely to measure any GRBs. The larger-scale eXTP mission, however, will still be optimized for point sources but is designed to also observe targets of opportunity such as GRBs using a shorter slewing time. As such, eXTP will be capable of measuring the polarization of the afterglow of GRBs in the 2–10 keV energy range as well as any potential X-ray flares occurring in the afterglow [198]. eXTP is a joint Chinese–European mission and is currently foreseen to be launched in 2028.

The above-mentioned instruments are optimized for point sources and therefore have a small field of view to optimize the signal to noise. In order to measure the polarization of the prompt emission from GRBs, which appears at random positions in the sky, a relatively large field of view is required. A mission under consideration with this capability at keV energies is the Low-energy Polarimetry Detector (LPD) under development at the GuangXi University (Private communication with Prof. Hongbang Liu). The instrument is foreseen to have a sensitivity to polarization in the energy range of 2–30 keV and a maximum effective area of <sup>∼</sup><sup>300</sup> cm<sup>2</sup> around 10 keV by using a similar technology as that used by Polarlight with an optimization of the gas for higher energies. The instrument is under consideration to be placed alongside POLAR-2 on the CSS, allowing to perform combined measurements of the prompt emission from 2 keV to 800 keV.
