Research progress on polarization measurements of gamma-ray burst prompt emissions and the test of pulsar navigation with POLAR

It has been more than half a century since the serendipitous discovery of gamma-ray bursts (GRBs, a kind of extreme violent emission of short-duration flashes of gamma-rays occurring randomly over time and in space at cosmological distances), made by the Vela satellites. So far, the observed samples of GRBs have been greatly improved, and the theoretical studies have also made remarkable achievements. However, there are still many unanswered questions about GRBs, such as their classification and origin(s), the composition of the relativistic jet in which the gamma-rays are generated, the radiation mechanism and the magnetic field configuration, etc. Among the questions, a variety of theoretical models have been proposed for interpreting the radiation mechanism of GRBs, and different models predict different polarization properties of the gamma-rays generated during prompt emissions. Accordingly, the polarization measurements can be used to study the mechanism of GRBs and to understand the physical properties such as the structure of the extreme relativistic jet that generates gamma-rays and the configuration of its magnetic field. However, technically it has been quite challenging to measure the polarizations of GRBs during their prompt emissions. Fortunately, some newly developed polarimeters in recent years have overcome major technical difficulties on polarization measurements in space, openning a new window for space hard X-ray/gamma-ray polarimetry. The gamma-ray burst polarimeter POLAR, which was launched onboard the China's space lab "Tiangong-2" on 15th September 2016, is a space mission dedicated for the polarization measurements of GRB prompt emissions in the 50-500 keV energy range, under the collaboration between Chinese and European scientists. During the about 6 months of space observation, POLAR detected 55 confirmed GRBs jointly with other missions and some possible individual GRBs by itself. The first detailed polarization measurements with high precision for 5 GRBs detected by POLAR have been published in 2019, and finally a catalog of 14 GRBs polarization measurement results in total was published, which is the best GRB polarization measurement results so far thanks to the high sensitivity and large field of view of the instrument, as well as the precisely calibrated systematic errors for polarization measurements. The results show that the detected GRBs are at most modestly polarized. Another new finding of the evolution of the intrapulse polarization angle provides us with a new insight into the GRB physics. POLAR results raised large interests as well as several critical scientific questions regarding GRB physics. Furthermore, the scientific potentials of POLAR have been extended during the flight by optimizing the working parameters which enabled the instrument to detect the Crab pulsar for navigation test studies, as well as several solar flares. The POLAR-2 mission, which is the successor of POLAR, aims to answer some key questions raised by POLAR with the launching date around 2025. In this paper, we first give a brief introduction to POLAR, then present the polarization measurement results of detected GRBs and the progress of the navigation test using the observed pulsar signals. In addition, a brief introduction is given to POLAR-2, and its preliminary scientific capabilities are prospected.