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

TL;DR

This paper details the optimization of the PoGOLite balloon-borne X-ray polarimeter, enhancing its sensitivity and performance for future observations of cosmic sources like the Crab nebula.

Contribution

It introduces design improvements based on Monte Carlo simulations that significantly increase the polarimeter's detection capabilities for hard X-ray sources.

Findings

Improved the Minimum Detectable Polarisation from 19.8% to 11.1%.

Enhanced the instrument's ability to detect and analyze cosmic X-ray polarization.

Validated the optimization through simulations and preliminary performance estimates.

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 5{\sigma} 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.