Polarimetric observations of celestial sources in the hard X-ray band stand to provide new information on emission mechanisms and source geometries .
PoGO+ is a Compton scattering polarimeter ( 20–150 keV ) optimised for the observation of the Crab ( pulsar and wind nebula ) and Cygnus X-1 ( black hole binary ) , from a stratospheric balloon-borne platform launched from the Esrange Space Centre in summer 2016 .
Prior to flight , the response of the polarimeter has been studied with polarised and unpolarised X-rays allowing a Geant4-based simulation model to be validated .
The expected modulation factor for Crab observations is found to be M _ { \mathrm { Crab } } = ( 41.75 \pm 0.85 ) \% , resulting in an expected Minimum Detectable Polarisation ( MDP ) of 7.3 \% for a 7 day flight .
This will allow a measurement of the Crab polarisation parameters with at least 5 \sigma statistical significance assuming a polarisation fraction \sim 20 \% – a significant improvement over the PoGOLite Pathfinder mission which flew in 2013 and from which the PoGO+ design is developed .