Li Hancheng, University of Geneva — July 24, 2023

Gamma-Ray Burst (GRB) prompt polarisation has been measured in more than thirty cases. However, as they suffered from huge systematical/statistical uncertainties, they showed a wide range distribution of polarization degrees (PDs). The theoretical community has recently paid more attention to the POLAR mission (developed by Uni. Geneva), which reported PDs of 14 GRBs at mostly a level of ∼10% and a hint of polarisation angle (PA) evolution over time. If the prompt gamma-rays are produced by photospheric emission, multiple scattering will significantly reduce the PD; synchrotron radiation would also allow a low PD if the magnetic field is dissipated. In another non-uniform jet scenario, if stochastic variations (patchy shells or mini-jets at scales ≪ 1/Γ) indeed endure with intrinsically independent magnetic field orientation and evolution, the integrated PD would be suppressed and PA evolution would occur. More realistic theoretical models of both time- and energy-dependent polarisation based on advanced numerical simulations are needed to better interpret the results. Meanwhile, the next-generation polarimeter POLAR-2 is required to improve the measurement accuracy. POLAR-2 will be launched in 2025 to the China Space Station and consists of three detectors: a High-energy polarisation Detector (Uni. Geneva), a Low-energy polarisation Detector (Guangxi Uni.) and a Broad-band Spectroscopy Detector (IHEP), sharing most of their mission time to monitor jointly the sky with overlapped fields of view. The synergies of the three detectors will allow POLAR-2 to significantly improve the accuracy (~10 times better) of GRB polarimetry, and shed new light on the jet physics of GRBs.