Design of a celestial Thomson-scattering x-ray polarimeter

A general discussion of the scientific importance and status of stellar x-ray polarimetry is presented. A stellar x-ray polarimeter designed to fit into the bottom half of the NASA Orbiting Solar Observatory-I (OSO-I) wheel compartment or other similar spacecraft is described. In this design, the linear polarization is obtained as a function of energy. The sensitivity of the polarimeter in the 4 - 24 keV energy range was optimized with the aid of a Monte Carlo simulation computer program and is given for several important celestial x-ray sources. Estimates of sensitivity thresholds for a much larger polarimeter, suitable for flight in the NASA High-Energy Astronomy Observatory (HEAO), are also given. The minimum detectable polarization for several x-ray sources is given. For example, the minimum detectable polarization at the 99% confidence level for the Crab nebula in a 24-h observation time is 1.4% for an OSO polarimeter in a compartment (50 × 30 × 30) cm and 0.27% for a HEAO polarimeter in a compartment (100 × 100 × 30) cm. These satellite experiments are feasible and will yield significant polarization results which will have an important bearing on our understanding of x-ray source mechanisms.