Calibration and performance studies of the balloon-borne hard X-ray polarimeter PoGO+

Abstract

Polarimetric observations of celestial sources in the hard X-ray band stand to provide new information on emission mechanisms and source geometries. PoGO+ is a Compton scattering polarimeter (20-150 keV) optimised for the observation of the Crab (pulsar and wind nebula) and Cygnus X-1 (black hole binary), from a stratospheric balloonborne platform launched from the Esrange Space Centre in summer 2016. Prior to flight, the response of the polarimeter has been studied with polarised and unpolarised X-rays allowing a Geant4-based simulation model to be validated. The expected modulation factor for Crab observations is found to be M Crab = (41.75 ± 0.85)%, resulting in an expected Minimum Detectable Polarisation (MDP) of 7.3% for a 7 day flight. This will allow a measurement of the Crab polarisation parameters with at least 5σ statistical significance assuming a polarisation fraction ∼ 20% -a significant improvement over the PoGOLite Pathfinder mission which flew in 2013 and from which the PoGO+ design is developed.

Gamma-ray burst localization strategies for the SPHiNX hard x-ray polarimeter

“…2 for SPHiNX. Once the GRB location is known relative to SPHiNX, the value of M 100 is determined from computer simulations validated at discrete energies by laboratory measurements (Chauvin et al, 2017). Any uncertainty in the GRB localisation will propagate to an uncertainty in PF,PA and increase the MDP (reduce sensitivity).…”

“…These systematic uncertainties caused by imperfections in detector calibration, will be estimated post detector assembly by benchmarking simulations against calibration measurements as done in Chauvin et al (2017); Yonetoku et al (2011).…”

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

“…Many of the results suffer, however, from systematic effects, leaving open questions about the emission mechanisms at work in the observed astrophysical sources. The goal of the PoGO mission is to provide statistically constrained polarimetric observations (a minimum detectable polarization of ∼10% before background subtraction), in a previously unexplored energy range, with well-understood systematic effects, using an instrument that has been extensively calibrated with both polarized and unpolarized beams [5].…”

“…Many of the results suffer, however, from systematic effects, leaving open questions about the emission mechanisms at work in the observed astrophysical sources. The goal of the PoGO mission is to provide statistically constrained polarimetric observations (a minimum detectable polarization of ∼10% before background subtraction), in a previously unexplored energy range, with well-understood systematic effects, using an instrument that has been extensively calibrated with both polarized and unpolarized beams [5].Here, developments of the PoGO mission are collected, leading up to the 2016 flight and a summary of observational results for the Crab and Cygnus X-1. Section 2 gives a description of the PoGO+ payload, including the instrument, attitude control system, and gondola structure.…”

The Growth of Interest in Astronomical X-Ray Polarimetry

“…Coupled with state-of-the-art X-ray optics, the field of X-ray polarimetry is now ready to open again after 40 years of silence. The forthcoming launch of space-borne X-ray polarimeters [12] and the many balloon [13] and rocket [14] experiments that are currently planned clearly indicate a growing interest in X-ray polarimetry. This paper is intended to quantitatively show the enthusiasm of the research community for X-ray polarimetry.…”