The POLAR-2 project is a follow-up of the POLAR mission that was launched in 2016, mounted on the exterior of the Chinese Spacelab Tiangong-2. Following its application to the Recognized Experiment Committee earlier this year, it has been recommended to be granted CERN Recognized Experiment status.

The POLAR instrument took data successfully for 6 months during 2016 and 2017 during which it studied the emission from astrophysical events called Gamma-Ray Bursts. The main scientific goal of POLAR was to perform the most detailed polarization measurements of the gamma-ray emission from these events. GRBs are the brightest electromagnetic explosions in the Universe since the Big Bang and as such are some of the most studied astrophysical sources. Despite all this research GRBs remain however poorly understood and specifically many open questions remain regarding the origin of their high energy emission. Although polarization measurements have the potential to answer many of these open questions few have been performed successfully in the past due to their complexity. Two important difficulties with the measurements are the low efficiency of event detection for gamma-ray polarization measurements and the requirement to perform such measurements in space. Furthermore, as the polarization is deduced from small asymmetries in the angular distribution of events detected by the instrument, the measurements are very susceptible to systematic errors. Despite all the difficulties involved the POLAR collaboration recently published the first catalogue of detailed polarization measurements of GRBs. These results show an overall low polarization as well as an unexpected complexity in the time evolution of the polarization during a GRB. They therefore indicate that measurements with a significantly improved precision are required in order to gain a full picture of the emission mechanism.

Figure 1: The Chinese Space Station, the design of the full POLAR-2 instrument and the details of a single detector module.

With the recent discovery of gravitational waves and their connection to GRBs, a new era in multi-messenger astrophysics has started. This major advance together with the discoveries made by POLAR warrant a GRB polarimeter with large acceptance capable of both providing high precision polarization measurements as well as detecting very weak GRBs. An international collaboration, led by the University of Geneva and consisting of leading members of the POLAR collaboration and new members from the Max Planck Institute for Extraterrestrial Physics, proposed the POLAR-2 instrument. The instrument will be an order of magnitude more sensitive to the polarization signal from GRBs using recent advances in the field of detector technology. In summer of 2019, the instrument was accepted for launch towards the China Space Station in 2024, from where it will perform polarization measurements for at least 2 years. Furthermore, as the most sensitive instrument in its energy range, POLAR-2 will be capable of detecting weak GRBs invisible to the current generation of gamma-ray detectors. One important feature is that POLAR-2 will be set up to issue alerts with position information for transient events to other instruments, thereby increasing the scientific potential of both POLAR-2 and other instruments.

Figure 2: The effective area of POLAR-2 compared to that of POLAR and that of POLAR multiplied by 4. It indicates nicely what we gain by increasing the size of the instrument by a factor 4 illustrating the gain (low energy) through technological R&D improvements.

The instrument design of POLAR-2 is largely based on that of its predecessor POLAR. In the detector the polarization of the incoming gamma-rays is measured using a segmented scintillator array. The design is optimized for gamma-rays which enter the array of 6400 scintillator bars to Compton scatter in one bar and subsequently get photo-absorbed in a second one. Using the relative position of the two bars the Compton scattering angle can be deduced, which in turn can be used to infer the polarization angle of the incoming gamma-ray. Whereas the measurement principle is similar between POLAR and POLAR-2 several improvements to the detector design are made. Firstly the size of the scintillator array is increased by a factor 4, allowing it to detect more gamma-rays per GRB. Secondly, while POLAR used multi-anode photomultipliers to read out the plastic scintillators POLAR-2 will use silicon photomultipliers (SiPMs) instead. The advantages of the SiPMs are not only the higher photo-detection efficiency of these devices but also their larger mechanical resilience. This, as well as their lower operating voltages compared to PMTs, are significant advantages for space based detectors which have to sustain large vibrations and shocks during launch. Many lessons learned from the POLAR mission are used to further improve the design. Examples are optimizations of the scintillator shape and shock dampers, simplifications of the electronics, data compression and improvement of in-orbit to ground communication protocols. Finally, the POLAR-2 instrument will not only consist of the most sensitive space gamma-ray polarimeter, it will also contain spectrometers which can be used to measure both the spectrum and the location of high energy transient events such as GRBs associated with gravitational waves.

The POLAR-2 mission is currently in the final stages of the design. For this purpose studies of different components to be used in the design are being tested for performance, radiation tolerance, shock and vibration resilience and performance in thermal vacuum. Recently seemingly simple components such as reflective foils used to optimize the light yield of the scintillators, were tested. These tests were performed at the CERN EP department with experts who used such materials previously in, for example, LHCb. While the standard material used for this purpose throughout physics experiments around the world, Vikuiti by 3M, was found to be the best reflector, a material produced by Toray in France was found to have a similar performance for POLAR-2, but with the advantage of being significantly thinner. On the other hand the SiPMs are currently undergoing tests at the National Centre for Nuclear Physics in Poland for radiation damage at different temperatures. The first results indicate that while the performance indeed significantly deteriorates with high dose of radiation, at the operating temperatures planned for POLAR-2 no significant issues are expected after operation of 2 years in space. In the near future additional tests are foreseen such as potential irradiation and charge particle response tests at CERN, and detailed instrument calibration tests using synchrotron radiation at the European Synchrotron Radiation Facility (ESRF).

Figure 3: Current measurement setup at CERN showing a single detector module with our first version of the read out electronics.

The POLAR-2 instrument design will continue to be developed during 2020 and early 2021 followed by the instrument construction phase lasting three years. The launch towards the China Space Station is planned for early 2024 after which the instrument will take data for at least 2 years. During these 2 years POLAR-2 will not only be the most sensitive polarimeter ever launched but also one of the most sensitive gamma-ray detectors in space. As such, POLAR-2 will help to unravel many of the mysteries surrounding the most violent explosions in the Universe and play a leading role in the new era of multi-messenger astrophysics.