Evaluation of polarization characteristics of multilayer mirror for hard X-ray observation of astrophysical objects

Polarization measurements in the hard X-ray band can provide crucial information about high energy emission at astrophysical objects. Such measurements require an instrument with high signal-to-noise ratio and high photon throughput for which the multilayer hard X-ray mirror is ideally suited. Future hard X-ray missions equipped with the multilayer mirror including ASTRO-H (Formerly NeXT) and NuStar are expected to detect hard X-ray sources in the flux range of 10μCrab. We expect a polarimeter mission using such mirrors to follow. Reflection off the mirror surface can introduce small but finite artificial polarization at the focal plane. We have conducted an experiment to set an upper limit for such polarization using a polarized hard X-ray beam at SPring-8 in Japan. In the experiment, reflectance of a multilayer surface has been measured as a function of the angle between the scattering plane and the hard X-ray polarization plane. The angular dependence of the measured reflectance can be translated to the artificial polarization. We have derived from the experimental results that the artificial polarization is no more than ±0.8% at the scattering angle 0.51°. This upper limit ensures that we can measure polarization down to ∼1% using the multilayer X-ray mirror.