*7.1. Future Instruments* 7.1.1. POLAR-2 and LEAP

In the Compton energy range of ∼10–1000 keV, four instruments are proposed. Both the LEAP [311] and SPHiNX [312] instruments have been proposed for launches in the coming decade, while the POLAR-2 project has already been accepted for launch in 2024 [313]. Additionally, the Daksha mission, a larger-scale full sky monitor follow-up mission based on the Astrosat CZTI is proposed to be launched in the coming decade as well [240]. Out of these four, the POLAR-2 and LEAP projects aim to make the next step in this field by producing instruments with an effective area an order of magnitude larger than the POLAR instrument. The SPHiNX project instead has an effective area similar to that of POLAR and will therefore have to make gains over currently existing measurements by aiming for a longer mission life time. For Daksha, the effective area is planned to be an order of magnitude larger than that of Astrosat CZTI. As the experiment will consist of two satellites, each observing half the sky, this increase in effective area is evenly distributed over the full sky. The design allows for a significant increase in the number of GRBs for which polarization measurements are possible, while also increasing the precision of each such measurement, although not by one full order of magnitude.

The POLAR-2 instrument is similar in design to POLAR with, apart from several minor design improvements, a focus on an improvement in three parts. The first is the size, which is four times larger than POLAR, resulting in a total geometrical area of approximately 2500 cm<sup>2</sup> . Secondly, the scintillator readout technology is improved to decrease the low-energy threshold of the instrument from 50 keV to 20 keV, giving a total energy range of 20–800 keV for polarization measurements. Finally, POLAR-2 will be equipped with spectrometers making it independent of other instruments for spectral and location parameters of GRBs, which reduces the systematic error on many GRB measurements. The instrument was approved for launch in early 2024 towards the Chinese Space Station (CSS).

The LEAP instrument is similar to POLAR-2 both in size and in the detection mechanism that uses plastic scintillators. Contrary to POLAR-2, the LEAP instrument will also use high Z scintillators, which increase the absorption cross section. Therefore, the instrument will have a larger sensitivity to polarization and a better spectral response but a reduction in its effective area and field of view. Whereas the total effective area for LEAP that is useful for spectrometry is <sup>∼</sup><sup>3500</sup> cm<sup>2</sup> at <sup>250</sup> keV, for polarization it is around <sup>∼</sup><sup>1000</sup> cm<sup>2</sup> [314]. For POLAR-2, the effective area of the polarimeter usable for spectrometry is <sup>∼</sup><sup>2000</sup> cm<sup>2</sup> , and therefore significantly smaller than LEAP. For polarization, however, it is <sup>∼</sup><sup>1400</sup> cm<sup>2</sup> and therefore larger than LEAP. The reduction in effective area of the polarimeter for spectrometry in POLAR-2 is compensated by separate spectrometers, which will increase this by at least 50%.

The two instruments therefore have different strengths. With a proposed launch in 2025 for LEAP towards the International Space Station (ISS), the combination of both of these instruments in orbit would allow for detailed polarization measurements of the majority of GRBs with fluences (as measured in the 10–1000 keV energy range) above 10−<sup>6</sup> erg cm−<sup>2</sup> .
