Future X-ray Missions to Study Relativistic Astrophysics

In order to revolutionize the current understanding of the high energy universe, a number of new X-ray missions are being developed and planned. Among them, e-ROSITA/SRG, NuSTAR, ASTROSAT, GEMS and ASTRO-H will be realized in the next decade. And then, much larger missions, such as IXO, have been proposed for the 2020's. NuSTAR and ASTRO-H will open up completely new field of spatial studies of non-thermal emission above 10 keV by hard X-ray telescopes. They will also uniquely allow mapping of the spatial extent of the hard X-ray emission in diffuse sources, thus tracing the sites of particle acceleration in structures ranging in size from clusters of galaxies down to supernova remnants. Multi-wavelength spectra by ASTROSAT and ASTRO-H are indispensable to understand physical processes in high energy phenomena, such as particle acceleration in the Universe. Imaging spectroscopy with an energy resolution <5-7 eV brought by the micro-calorimeter onboard ASTRO-H can reveal line broadening and Doppler shifts due to turbulent or bulk velocities in extended sources. GEMS will perform the first sensitive X-ray polarization survey of several classes of X-ray emitting sources characterized by strong gravitational or magnetic fields. Here we present the key science goals for future X-ray missions designed to address a number of fundamental questions in contemporary astrophysics.