Astronomical X-ray polarimetry: a new window about to open

X-ray polarimetry allows one to measure the orientation of magnetic fields or constrain the geometry of high energy objects. It is predicted to be a powerful means of measuring the spin of black holes, probing the inner structure of relativistic jets, distinguishing pulsar models, and testing quantum electrodynamics (QED) effects in the presence of strong magnetic fields. However, no experiment with X-ray polarimetry has been conducted for almost 40 years due to technical difficulties. In the past 10 years, substantial progresses have been made and sensitive X-ray polarimetry becomes possible for future X-ray astronomy, especially in China. I will briefly review the science with X-ray polarimetry and introduce two space mission concepts in China, the X-ray Timing and Polarization (XTP) and the Lightweight Asymmetry and Magnetism Probe (LAMP). XTP is an observatory-class mission concept, designed with unprecedented capability of X-ray polarimetry, while LAMP is a dedicated soft X-ray polarimeter based on micro-satellite technology. Both polarimeters are being developed at Tsinghua and approaching a high technology readiness, enabling us to open this new window in the near future.