  We argue that measurements of X-ray polarization using the recently launched
Imaging X-ray Polarimetry Explorer will answer many open questions about
magnetars in particular the physical state of their surfaces, whether vacuum
birefringence exists, and the nature of the hard X-ray emission from these
objects. We outline the capabilities of the instrument, specific models and the
results of simulations for the magnetar 4U~0142+61.
