Structure of plasma surrounding compact objects
Black holes, neutron stars, or white dwarfs --- these remnant objects which remain after the death of stars are called "compact objects". These are often associated with plasma flows attracted by their strong gravity. These swirling plasma flows are called accretion disks or accretion flows. They are natural probes for us to observe the spacetime structure under strong gravity, and X-ray astronomy has investigated them significantly. One of the most important results is the iron emission line profile observed by ASCA satellite. The iron line profile is distorted not only by the high speed rotation of accretion disk, but also by the gravitational redshift caused by the central black hole. Follow-up investigations showed that the accreting flow is not simple, but consists of various structures of plasma of different density or temperature, varying in time. In order to distinguish these structures and understand the nature of accretion flows and distorted spacetime, we need the precision X-ray spectroscopy provided by XRISM.