Calibration of the IXPE focal plane X-ray polarimeters to polarized radiation

TL;DR

This paper details the ground calibration process of IXPE's focal plane detectors, focusing on their response to polarized X-ray radiation and the measurement of the modulation factor to assess sensitivity.

Contribution

It presents the calibration results of IXPE detector units, specifically analyzing their response to polarized radiation and the modulation factor measurement methodology.

Findings

Calibration achieved accurate measurement of detector response

Modulation factor determined for sensitivity assessment

Ground calibration improves in-flight polarization measurements

Abstract

IXPE (Imaging X-ray Polarimetry Explorer) is a NASA Small Explorer mission -- in partnership with the Italian Space Agency (ASI) -- dedicated to X-ray polarimetry in the 2--8 keV energy band. The IXPE telescope comprises three grazing incidence mirror modules coupled to three detector units hosting each one a Gas Pixel Detector (GPD), a gas detector that allows measuring the polarization degree by using the photoelectric effect. A wide and accurate ground calibration was carried out on the IXPE Detector Units (DUs) at INAF-IAPS, in Italy, where a dedicated facility was set-up at this aim. In this paper, we present the results obtained from this calibration campaign to study the IXPE focal plane detector response to polarized radiation. In particular, we report on the modulation factor, which is the main parameter to estimate the sensitivity of a polarimeter.