On-ground characterization of the IXPE polarization angle knowledge

Abstract

The Imaging X-ray Polarimetry Explorer (IXPE) has been recently selected for development as part of NASA's Small Explorer program (SMEX), with a launch date in 2021. Developed in a collaboration between NASA and the Italian Space Agency (ASI), IXPE will perform groundbreaking measurements of imaging polarization in X-rays for a number of different classes of sources including isolated and accreting neutron stars, pulsar wind nebulae, stellar and supermassive black holes. Combining 30 arcsec (HPD) grazing-incidence X-ray optics with the polarization-sensitive Gas Pixel Detectors (GPDs), IXPE will provide two-orders of magnitude improvement in sensitivity over the past flown instruments. The IXPE requested precision on the measurement of the polarization angle (better than 0.2 degrees at instrument level) poses strict constraints on the detector unit (DU) mechanical design and requires the implementation of a specific alignment and measurement strategy to meet the scientific requirements. In this paper we describe the design solutions that will be implemented in the DU flight models as well as a step-by-step metrology procedure that will ensure the fulfillment of the scientific requirement.