The Imaging X-ray Polarimetry Explorer (IXPE): Prospects for Spatially-Resolved X-ray Polarimetry of Extended Sources and In-Orbit Calibrations

TL;DR

The IXPE mission enhances X-ray polarimetry by enabling spatially-resolved observations of extended celestial sources and establishing in-orbit calibration methods, addressing background challenges and confirming instrument performance.

Contribution

This work demonstrates the feasibility of in-orbit calibration and observing faint extended sources with IXPE, advancing X-ray polarimetry capabilities and methodologies.

Findings

Calibration sources function as expected in orbit

Background mitigation is essential for faint sources

Imaging capabilities meet mission requirements

Abstract

This thesis focuses on X-ray polarimetry and its recent resurgence due to the NASA/ASI Imaging X-ray Polarimetry Explorer (IXPE) mission launched in December 2021. It aims to address two critical tasks: in-orbit calibrations and observing faint extended celestial sources. For in-orbit calibrations, IXPE carries polarized and unpolarized calibration sources because known celestial sources, like the Crab Nebula, aren't suitable due to variability. Tests were performed on Flight Models of these calibration sources, ensuring their functionality in thermal vacuum when combined with IXPE detectors. Concerning observing faint extended sources, instrumental and diffuse backgrounds pose challenges. The impact of various background sources, such as instrumental, diffuse Galactic plane emission, and cosmic X-ray background, on X-ray polarimetry of faint, extended sources was examined. The feasibility study also covered observing two extended sources: the Tycho supernova remnant and the molecular clouds of the Sgr A complex. Monte Carlo simulations were employed to evaluate the detectability of polarization. Results show that the on-board calibration system will assess and verify the GPD's functionality in orbit. Background mitigation techniques will be required for faintest extended sources. For sources like Cas A, Tycho, and PSW MSH 15-52, background effects are negligible. In conclusion, the IXPE mission opens up spatially resolved X-ray polarimetry, enabling the determination of polarization angles and degrees. Preliminary data after launch indicates that on-board calibration sources are performing as expected, and imaging capabilities meet requirements. This research is essential for advancing X-ray polarimetry and understanding high-energy celestial sources.