PoGO+ observations of hard X-ray polarisation from the Crab and Cygnus X-1

06.12.2017 11:15 – 12:15

Polarimetry has long been a routine probe of sources within radio, optical and infra-red astronomy. The application to X-/gamma-rays has not evolved as rapidly due to the relatively long integration times required for measurements - thereby requiring a dedicated mission - and the particular attention which must be paid to systematic effects due to the positive definite nature of measurements. Advances in the field are instead currently 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. Polarisation is determined from coincident interactions in a segmented array of plastic scintillators surrounded by a BGO anticoincidence system and a polyethylene neutron shield. 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 from the flight discussed.

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