Polarization properties of X-ray tubes used for Imaging X-ray Polarimetry Explorer calibration

TL;DR

This study measures the polarization of X-ray emissions from calibration tubes used in the IXPE mission, revealing how electron geometry influences bremsstrahlung polarization, with results aligning with theoretical models.

Contribution

It provides the first detailed characterization of polarized bremsstrahlung X-rays from calibration sources for space-based X-ray polarimetry.

Findings

Bremsstrahlung polarization depends on electron-photon geometry.

Unpolarized when electrons are parallel to emitted photons.

Polarization increases with energy when electrons are perpendicular.

Abstract

In this work, we measured the polarization properties of the X-rays emitted from the X-ray tubes, which were used during the calibration of the instrument onboard Imaging X-ray Polarimetry Explorer (IXPE). X-ray tubes are used as a source of unpolarized X-rays to calibrate the response of the gas pixel detectors to unpolarized radiation. However, even though the characteristic fluorescent emission lines are unpolarized, continuum bremsstrahlung emission can be polarized based on the geometry of the accelerated electrons and emitted photons. Hence, characterizing the contribution of polarized X-rays from bremsstrahlung emission is of interest, also for future measurements. We find that when accelerated electrons are parallel to the emitted photons, the bremsstrahlung emission is unpolarized, and when they are perpendicular, the polarization increases with energy, as expected from the theoretical predictions. A comparison with the theoretical predictions is also shown.