7.1.3. High-Energy Polarimeters

In the MeV energy range, one possible mission to be launched in the coming decade is AMEGO [315]. The AMEGO mission makes use of many layers of silicon placed on top of a calorimeter. This makes it ideal to perform polarization measurements using Compton scattering in the ∼100 keV to 5 MeV energy range. AMEGO will yield polarization measurements for the brightest 1% of GRBs that it will observe.

A second instrument under development is a satellite version of the COSI balloon mission [316]. This instrument will make use of germanium strip detectors capable of measuring the three-dimensional interaction position of incoming photons. The energy range is similar to that of AMEGO (200 keV to 5 MeV). Thanks to its large field of view, it will observe around <sup>∼</sup>40 GRBs per year with a fluence exceeding <sup>4</sup> <sup>×</sup> <sup>10</sup>−<sup>6</sup> erg cm−<sup>2</sup> for which it can perform measurements with an MDP of around 50%.

A highly promising instrument concept for polarimetry at MeV energies is the Advanced Particle-astrophysics Telescope [317]. The instrument is designed to maximize the effective area for photons in the MeV to TeV energies without using passive materials for photon conversion. The detector aims to use high Z scintillator crystals for the conversion in combination with scintillating fibres. This allows for a large-scale detector with precise measurements of both electron positron pairs and Compton-scattered photons. The current mission concept would be an order of magnitude more sensitive as a gamma-ray detector than Fermi-LAT and would be capable of performing polarization measurements at MeV energies for GRBs as weak as 170817A, for which an MDP of ∼40% was simulated. The project is in its early stages, and currently a path finder mission is planned for a balloon flight.

Apart from these two instruments the earlier-mentioned HARPO detector [200] will be capable of performing polarization measurements in the MeV energy range using pair production in a gas TPC. Unlike AMEGO and COSI, which are both under consideration for a launch in the coming decade, the HARPO instrument, of which a prototype has been successfully calibrated on ground [201], is currently not under consideration for a launch.
