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

## TL;DR

PoGO+ is a balloon-borne hard X-ray polarimeter designed to measure celestial source polarization, with validated response models and expected high significance results for Crab observations, improving upon previous missions.

## Contribution

This study presents the calibration, response validation, and performance assessment of PoGO+, a novel balloon-borne X-ray polarimeter for astrophysical observations.

## Key findings

- Validated Geant4 simulation model of PoGO+ response.
- Expected 41.75% modulation factor for Crab observations.
- Projected 7.3% minimum detectable polarization for a 7-day flight.

## 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 balloon-borne 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 $M_{\mathrm{Crab}}=(41.75\pm0.85)\%$, 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 $5\sigma$ statistical significance assuming a polarisation fraction $\sim20\%$ $-$ a significant improvement over the PoGOLite Pathfinder mission which flew in 2013 and from which the PoGO+ design is developed.