As avid readers of this blog, you no doubt remember PoGOLite – a balloon-borne hard X-ray polarisation mission which is part of the Swedish National Space Board national programme for balloon and sounding rocket research at the Esrange Space Centre. After a number of frustrating set-backs (broken balloons, bad weather, …), the Crab was successfully observed in July 2013 – providing the first measurement of the polarisation of emissions in the 20 – 120 keV energy band. Technical difficulties encountered during the flight meant that the polarisation parameters could only be determined with modest precision. The relatively high polarisation fraction observed suggests an ordered environment at the emission site, and the polarisation angle is consistent with the inferred projected direction of the pulsar spin axis reported in the optical regime. This alignment suggests that polarised hard X-ray emission originates close to the pulsar. The results also demonstrated that the instrument concept was sound and indicated how the design could be modified to achieve better performance for future flights. The improved polarimeter, called PoGO+, and the associated mission paraphernalia has been taking shape during the last couple of years at KTH and at our industrial partners, SSC and DST Control.
The PoGO+ launch campaign started at Esrange in mid-May 2016. Once the polarimeter was integrated with the attitude control system and the balloon systems, integrated system tests were conducted, including pointing tests and the final scientific calibration of the polarimeter. The launch window opened on July 1st (once the Crab was sufficient displaced from the sun). Weather conditions permitted launch attempts on 9th, 10th and 11th July with the final attempt resulting in a launch on July 12th 2016 at 05:17. Launch conditions were perfect with low winds and clear skies. We were pleased to hear from OKC colleague Tanja Nymark who spotted the 150 m diameter balloon flying at 40 km altitude over her holiday home in Vesterålen, close to the Lofoten Islands in northern Norway! Once at operational altitude, observations of the Crab and Cygnus X-1 started and continued without problem during the flight. Conditions were excellent and each source was observed daily, for the duration of the almost week long flight from Esrange to Victoria Island, Canada. Science data storage units were returned to Sweden soon after the flight ended and data reduction and analysis is now in progress.
In X-ray polarimetry, performance of is often expressed in terms of the MDP – Minimum Detectable Polarisation. If a measurement yields a polarisation fraction equal to the MDP, there is a 1% probability that the measured value arises from a statistical fluctuation of an unpolarised flux. For PoGOLite in 2013, the achieved MDP for the Crab was 28%. For PoGO+, the MDP is expected to be <10% which permits a 5 sigma determination of the polarisation parameters and should also allow pulsar and nebula contributions to be separated. In contrast to the 2013 flight, Cygnus X-1 was in the hard spectral state during the flight and there-by observable by PoGO+. In the hard state, X-rays from Compton up-scattering thermal X-rays may be reflected off the accretion disk and become polarised. The polarisation parameters observed can elucidate the inclination of the system – a characteristic which is challenging to acquire by other means.
Links:
PoGOLite mission design (Experimental Astronomy)
PoGOLite calibration (Astroparticle Physics)
PoGOLite results on the Crab (MNRAS Letters)
PoGO+ design (Astroparticle Physics)
-Mark Pearce (pearce@kth.se)