Balloon-borne hard X-ray polarimetry with PoGO+
Professor Mark Pearce (KTH Royal Institute of Technology, Stockholm)

Polarimetry has long been a routine method to probe sources within radio, optical and infra-red astronomy. The application to the X-ray regime has not evolved as rapidly and advances in the field are instead driven by spectroscopy, imaging and timing studies. Many astrophysical X-ray sources are dominated by non-thermal emission with radiation transferred in highly asymmetric systems. A measurement of the linear polarisation of the emitted radiation therefore constitutes a key observable and diagnostic for sources which cannot be spatially resolved. PoGO+ is a balloon-borne hard X-ray polarimeter operating in the 20 - ~200 keV energy band. A one-week long flight was conducted during the Summer of 2016, launching from the Esrange Space Center, Sweden, and landing on Victoria Island, Canada. The design and polarimetric calibration of the PoGO+ instrument will be described and observational results on two bright X-ray sources, the Crab and Cygnus X-1, discussed.