We developed an instrument design capable of measuring linear X-ray polarization over a broad band using conventional spectroscopic optics. A set of multilayer-coated flats reflects the dispersed X-rays to the instrument detectors. The intensity variation with position angle is measured to determine three Stokes parameters: I, Q, and U – all as a function of energy. By laterally grading the multilayer optics and matching the dispersion of the gratings, one may take advantage of high multilayer reflectivities and achieve modulation factors >50% over the entire 0.2–0.8 keV band. This instrument could be used in a small orbiting mission or scaled up for the International X-ray Observatory. Laboratory work has begun that would demonstrate the capabilities of key components.
The soft X-ray band (0.1–1.0 keV) should prove to be a fruitful region to explore for polarized emission. One concept, the Polarimeter for Low Energy X-ray Astrophysical Sources (PLEXAS), proposed the use of multilayer-coated mirrors tuned to 0.25 keV as Bragg reflectors. As in similar Bragg reflection systems, the PLEXAS design had a narrow bandpass, reducing its attractiveness for astrophysical observations because one expects polarization to be energy dependent, so a wide bandpass is desired.
Marshall described a method to overcome this limitation by using transmission gratings to disperse in the incoming X-rays. Following up on this approach, Marshall suggested an arrangement that can be used in missions ranging from a small explorer to the International X-ray Observatory (IXO).