Neutron Stars

9 Black holes and neutron stars are fascinating objects with great potential for testing physical theories in extreme conditions. The gravitational fields near these objects provide an opportunity for tests of General Relativity in the strong-field limit and the properties of matter at the remarkably high densities that exist within neutron stars are unknown. While many of these objects are in binary systems where accreting matter from the stellar companion provides a probe of the compact object, a main difficulty in making measurements that lead to definitive tests has been uncertainty about basic information such as distances to the sources, orientation of their binary orbits, and masses of the compact objects. Through astrometry, SIM Lite will, for the first time, be able to obtain precise distances and proper motions for dozens of these systems and will also be capable of mapping out orbital motion, leading to direct compact object mass measurements. Obtaining such information is critical for a wide variety of investigations that range from probing the space-time around the compact object to constraining the origin and evolution of the systems themselves.