We present results from a concept study of a soft X-ray polarimeter that follows from our design of a sounding rocket instrument, the Rocket Experiment Demonstration of a Soft X-ray Polarimeter (the REDSoX Polarimeter, aka REDSoX). The proposed mission, the Globe Orbiting Soft X-ray Polarimeter (GOSoX), is an orbital version of REDSoX that will operate in the 0.2-0.4 keV band. The goals of the mission are to 1) test models of vacuum birefringence and proton cyclotron lines in neutron star spectra, 2) test inverse Compton and synchrotron emission models of blazar spectra, and 3) provide a broader view of polarized emission across the electromagnetic spectrum via complementary measurements by the Imaging X-ray Polarization Explorer (IXPE, 2-8 keV), optical instruments, or radio telescopes.
The science instrument leverages previous funding for lab development to make focusing optics, multilayers, gratings, and CCDs. Together, these high TRL technologies enable us to build GOSoX. The science payload consists of Wolter type I X-ray optics and Critical Angle Transmission gratings on an optical bench and a focal plane with four X-ray detectors and three multi-layer mirrors. The instrument senses polarization by matching the dispersion of the gratings to the lateral grading of multilayer (ML) coated mirrors set at about 45 degrees to the incoming light and viewed by a CCD. By orienting 3 ML coated mirrors at position angles 120 degrees apart, we can measure the three Stokes parameters I, Q, and U. A smaller version was also studied that has only one detector and ML mirror, requiring rotation to measure the Stokes parameters. Based on our goals and our instrument design, we have selected a representative sample of sources to observe and goals for minimum detectable polarizations. The baseline objectives can be satisfied in one year in orbit.