The Polarized Gamma-ray Explorer (PoGOLite) is a balloon-borne instrument designed to measure polarization in the energy range 25–80 keV from many classes of astronomical objects, including pulsars, accretion discs and astrophysical jets. Using coincident detection of Compton scattering and photoelectric absorption in an array of 217 detector cells, the modulation in scattering angles can be determined. By this technique, the instrument will be able to measure as low as 10% polarization from a 200 mCrab source in a 6-h flight at an altitude of 40 km.

The maiden flight of a 61-unit “pathfinder” instrument is scheduled to take place from the Esrange ballooning facility in northern Sweden in mid-2011. This flight will focus on measuring polarization from the Crab nebula and possibly Cygnus X-1, as well as to study the in-flight background caused by cosmic ray particles, X-ray and gamma-ray photons, and albedo neutrons. In order to reject such background, the instrument features a combination of active and passive shielding, as well as both active and passive collimation of the incident photons.

Here, the design and status of the PoGOLite pathfinder instrument will be reviewed. Pre-flight calibration and performance tests will also be presented.