Licentiate seminar: PoGOLite: 2011 flight results and 2012 pre-flight predictions
by
Merlin Kole(KTH, Dept. of Physics)
→
Europe/Stockholm
A5:1003
A5:1003
Description
PoGOLite is a balloon-borne experiment designed to measure the polarisation of hard X-rays emitted by point sources. The polarimeter makes use of phoswich detector cells which serve both to measure the signal and for background rejection. Further background rejection is achieved by an active side anti-coincidence shield and a passive polyethylene shield. The polarimeter was launched for the first time on the 7th of July 2011 from Kiruna, Sweden and was foreseen to land on Victoria Island, Canada 5 days later. Due to a helium leak in the balloon, however, the payload did not reach its designed float altitude and landed 7 hours after launch. Tests during this short maiden flight showed that the different payload components performed as designed. A higher than anticipated event rate was however measured both with the polarimeter and an on-board neutron scintillator.
In preparation for the 2012 flight, the atmospheric neutron spectrum was sim- ulated specifically for the altitude and latitude of the PoGOLite flight. The event rate induced by such a neutron spectrum was shown through simulations to be approximately 8 times higher than previously assumed. These simulation results have been compared to the data acquired during the 2011 flight and were shown to be in good agreement. Comparisons of the simulated and measured spatial distri- butions of events within the instrument further confirm the neutron origin of the high event rate measured during the 2011 flight. These distributions also indicate a possibility for an improvement of the signal over background ratio by considering the geometry of events during analysis.
Using both the simulated signal and background rates, the minimum detectable polarisation of the different targets of the 2012 mission were calculated. It is shown that a significant polarimetric measurement of both the Crab nebula and Cygnus- X1 in the hard X-ray regime is possible during this campaign. A measurement of the Crab pulsar is shown to be a challenge due to the high background rates.
