Scientific prospects for hard X-ray polarimetry

X-ray polarimetry promises to give qualitatively new information about high-energy sources. Examples of interesting source classes are binary black hole systems, rotation and accretion powered neutron stars, microquasars, active galactic nuclei and gamma-ray bursts. Furthermore, X-ray polarimetry affords the possibility for testing fundamental physics, e.g. to observe signatures of light bending in the strong gravitational field of a black hole, to detect third order quantum electrodynamic effects in the magnetosphere of magnetars, and to perform sensitive tests of Lorentz invariance. In this paper we discuss scientific drivers of hard (>10 keV) X-ray polarimetry emphasizing how observations in the hard band can complement observations at lower energies (0.1-10 keV). Subsequently, we describe four different technical realizations of hard X-ray polarimeters suitable for small to medium sized space borne missions, and study their performance in the signal-dominated case based on Monte Carlo simulations. We end with confronting the instrument requirements for accomplishing the science goals with the capabilities of the four polarimeters.