POLAR-2: The First Large Scale Gamma-ray Polarimeter

The mechanisms responsible for the prompt emission of Gamma-Ray Bursts (GRBs) remain poorly understood despite a wealth of measurements performed over the last 5 decades. After detailed measurements of the photon direction, time and energy, recent years have seen the first measurements of the polarization of the prompt emission by two dedicated polarimeters, POLAR and GAP. The typical low levels of polarization observed by POLAR and the temporal evolution of the polarization angle observed by both instruments indicates that time resolved studies are required to constrain theoretical emission models. Additionally a larger sample of GRB polarization measurements is required to answer the remaining open questions on gamma-ray emission in GRBs. The POLAR-2 instrument aims to provide such measurements by having an effective area of one order of magnitude larger than that of POLAR. The detector, proposed by an international collaboration, was recently selected to be launched in 2024. It will then be placed on the China Space Station and operate for at least 2 years. By improving the design of POLAR and scaling up the instrument, an increase in effective area of one order of magnitude compared to POLAR can be achieved. Using the improved design, which includes the replacement of photomultiplier tubes with silicon photomultipliers, as well as other technological upgrades, the instrument is foreseen to perform detailed polarization measurements of 50 GRBs per year. The instrument design will be discussed in detail together with the scientific potential expected from the mission.