  This white paper highlights compact object and fundamental physics science
opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray
polarization observations. X-ray polarimetry gives new observables with
geometric information about stellar remnants which are many orders of magnitude
too small for direct imaging. The X-ray polarimetric data also reveal details
about the emission mechanisms and the structure of the magnetic fields in and
around the most extreme objects in the Universe. Whereas the Imaging X-ray
Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first results
for bright objects, a follow-up mission could be one order of magnitude more
sensitive and would be able to use a broader bandpass to perform physics type
experiments for representative samples of sources.
