A Gamma-Ray Polarimeter on a Deep-Space Platform Measuring Polarisation Rotation Accumulated Over Cosmological Paths

This paper presents a space mission design, computed from stated assumptions and not measured, built to measure one quantity: the polarisation rotation accumulated over a gigaparsec path attributed to a torsion-coupled birefringence, for which the design case adopts 0.9 degrees of rotation. The instrument is a Compton polarimeter of layered silicon and cadmium-zinc-telluride detectors with a wide field of view, mounted on a deep-space platform away from the radiation belts, recording the azimuthal scattering distribution of photons from gamma-ray bursts and bright active galaxies event by event. It operates from a high Earth orbit with apogee near 200,000 kilometres, outside the trapped-radiation environment for most of each period for 4 years. The one-day measurement uncertainty is 4.5 degrees; averaged over the full mission of 1461 days it falls to 0.118 degrees, giving a signal-to-noise ratio of 7.64 for the design case and a five-sigma detection limit of 0.589 degrees. The spacecraft has a dry mass of 900 kilograms, a wet mass of 1040 kilograms including propellant, and a total electrical load of 1362 watts supplied by a solar array of 3.93 square metres at the operating distance. Every performance figure in this paper comes from the deposited computation; the design is simulated, not measured, and no hardware described here has been built or flown.