Optimising a balloon-borne polarimeter in the hard X-ray domain: From the PoGOLite Pathfinder to PoGO+

Abstract

PoGOLite is a balloon-borne hard X-ray polarimeter dedicated to the study of point sources. Compton scattered events are registered using an array of plastic scintillator units to determine the polarisation of incident X-rays in the energy range 20 -240 keV. In 2013, a near circumpolar balloon flight of 14 days duration was completed after launch from Esrange, Sweden, resulting in a measurement of the linear polarisation of the Crab emission. Building on the experience gained from this Pathfinder flight, the polarimeter is being modified to improve performance for a second flight in 2016. Such optimisations, based on Geant4 Monte Carlo simulations, take into account the source characteristics, the instrument response and the background environment which is dominated by atmospheric neutrons. This paper describes the optimisation of the polarimeter and details the associated increase in performance. The resulting design, PoGO+, is expected to improve the Minimum Detectable Polarisation (MDP) for the Crab from 19.8% to 11.1% for a 5 day flight. Assuming the same Crab polarisation fraction as measured during the 2013 flight, this improvement in MDP will allow a 5σ constrained result. It will also allow the study of the nebula emission only (Crab off-pulse) and Cygnus X-1 if in the hard state.

Optical/Infrared Polarised Emission in X-ray Binaries

“…In addition, because FLP is detected from the optically thin synchrotron emission in the steady jets, it is worth considering the extent of the optically thin power law to higher frequencies. If the power law extends to X-ray or γ-ray energies, polarisation could be detected at these energies [33,46] with new and upcoming experiments such as PoGOLite/PoGO+ [47], X-Calibur [48], XIPE [49] and IXPE [50].…”

Instrumentation and Future Missions in the Upcoming Era of X-ray Polarimetry

“…Moreover, an additional passive neutron shield made of polyethylene surrounds the instrument. This experience allowed to optimize the detector to design the PoGO+ polarimeter, which flew in the summer 2016 [100][101][102][103]130].…”

The PoGO+ Balloon-Borne Hard X-ray Polarimetry Mission

“…PoGO+ benefited tremendously from data collected during the 2013 flight of PoGOLite. The polarimeter was extensively redesigned and optimized [1], including (i) new detector units and improved reflective wrapping yielding higher sensitivity; (ii) improved front-end electronics with a wider dynamic range and a higher waveform sampling rate providing improved efficiency for the pulse shape discrimination and event selection; (iii) additional polyethylene neutron shielding for suppressing the flight background; and (iv) higher sensitivity for the side anticoincidence shield, yielding a lower energy threshold for the background rejection. On the gondola side, an Iridium Pilot antenna had been added, allowing for better monitoring, operations, and data download rates during over-the-horizon communications.…”