The physical processes postulated to explain the high-energy emission mechanisms of compact astrophysical sources are in many cases predicted to result in polarized soft gamma-rays. The polarisation arises naturally for synchrotron radiation in large-scale ordered magnetic fields and for photons propagating through a strong magnetic field. Polarization can also result from anisotropic Compton scattering. In all cases, the orientation of the polarization plane is a powerful probe of the physical environment around compact astrophysical sources. Observations with PoGOLite will help resolve the source geometry for many classes of astrophysical objects. PoGOLite applies well-type phoswich technology to polarization measurements in the 25–80 keV energy range. The instrument uses Compton scattering and photoelectric absorption in an array of detector cells made of plastic and BGO scintillators, surrounded by a BGO side anticoincidence shield. A pathfinder balloon flight is scheduled for 2010 from the Esrange facility in the north of Sweden with the Crab and Cygnus X-1 as the main observational targets.
Despite the wealth of sources accessible to polarization measurements and the importance of these measurements, there has been a paucity of missions with dedicated instrumentation. The most recent was a measurement of the Crab at 2.6 keV and 5.2 keV by an experiment on the OSO-8 satellite in 1976. Measurements using instruments on-board the INTEGRAL satellite have reinvigorated the field of late. At soft gamma-ray energies, nonthermal processes are likely to produce high degrees of polarization.