Hard X-ray polarimeter for small satellite missions

Abstract

We present here the design and estimated sensitivity of a hard X-ray polarimeter suitable for a small-satellite mission. Polarization fraction and direction of the X-rays from astronomical objects will be measured by recording the azimuthal anisotropy in the intensity of X-rays subsequent to their Thomson scattering on a circular disk of a light element like Li or Be. Extreme care is taken to make the instrument as azimuthally symmetric as possible and thus avoid spurious anisotropies from being seen. After describing the design of the present instrument, we present a semi-analytic calculation of the angular distribution of the scattered Xrays, taking explicitly into account the competing effects of photoelectric absorption. Thereafter, we derive the sensitivity of the instrument to measure the polarization fraction and the direction for astronomical sources with two possible spectral characteristics - a power law and thermal bremsstrahlung.