Background Studies for the Balloon-Borne Hard X-ray Polarimeter PoGOLite

Sammanfattning: The polarisation degree and angle of the X-ray flux emitted by astrophysical objects holds valuable information on the responsible emission mechanisms and on the emission environments. PoGOLite is a balloon-borne hard X-ray polarimeter designed to measure polarisation using a segmented plastic scintillator array. The instrument was launched for its first scientific, near-circumpolar, flight in July 2013 from the Esrange Space Centre in Northern Sweden. The primary observation target for this flight, the Crab, was observed during the first 2 days of flight. One of the main challenges for PoGOLite is the relatively high measurement background, predicted to be induced by atmospheric neutrons. No measurement data on the neutron environment for the flight conditions of PoGOLite is however available, making exact predictions impossible. This environment was therefore studied in detail. A Monte Carlo based model of the atmospheric neutron flux was developed. This model is capable of providing differential neutron energy spectra for all altitudes, latitudes and solar activities. The predictions of this model were found to be in good agreement both with measurement data, measured by high altitude aircraft, and with predictions by location and time specific models. The results from the model were verified with data recorded by a purpose-build balloon-borne neutron detector, PoGOLino. The PoGOLino instrument uses novel neutron sensitive LiCAF scintillators sandwiched between BGO crystals which serve as an anti-coincidence system. PoGOLino was launched from the Esrange Space Centre to an altitude of 31 km on March 20th 2013 and performed the first successful measurements of the neutron flux for the PoGOLite flight conditions. Using the developed model the background as measured by the PoGOLite mission in 2013 was studied. Monte Carlo simulations were used to confirm that the PoGOLite background during flight is dominated by neutrons. The simulated neutron induced signal rate and its variations with time were furthermore found to be in good agreement with measurements. Based on these results the implications of the background on the polarisation measurements of the Crab were studied. Lastly, based on the acquired knowledge of the background, changes to the instrument geometry for potential future flight of PoGOLite were studied along with the expected achievable improvement in performance for such flights.