Science prospects for SPHiNX – A small satellite GRB polarimetry mission

Abstract Gamma-ray bursts (GRBs) are exceptionally bright electromagnetic events occurring daily on the sky. The prompt emission is dominated by X-/ γ -rays. Since their discovery over 50 years ago, GRBs are primarily studied through spectral and temporal measurements. The properties of the emission jets and underlying processes are not well understood. A promising way forward is the development of missions capable of characterising the linear polarisation of the high-energy emission. For this reason, the SPHiNX mission has been developed for a small-satellite platform. The polarisation properties of incident high-energy radiation (50–600 keV) are determined by reconstructing Compton scattering interactions in a segmented array of plastic and Gd 3 Al 2 Ga 3 O 12 (Ce) (GAGG(Ce)) scintillators. During a two-year mission,  ∼ 200 GRBs will be observed, with  ∼ 50 yielding measurements where the polarisation fraction is determined with a relative error  ≤ 10%. This is a significant improvement compared to contemporary missions. This performance, combined with the ability to reconstruct GRB localisation and spectral properties, will allow discrimination between leading classes of emission models.