High-resolution X-ray spectroscopy of astrophysical plasmas with X-ray microcalorimeters

High spectral resolution with a resolving power, $$E/\Delta E \gtrsim 1000$$ at 6 keV, is now available in X-ray astronomy. X-ray observations are particularly effective for plasma studies since major atomic transitions appear as spectral features in the X-ray band. High-resolution spectroscopy enables us to probe a wide variety of astrophysical plasmas, which are not obtainable from ground experiments, regarding their temperature, density, magnetic field, gravity, and velocity. In this review, we describe what are the X-ray emitting plasmas in the Universe, along with basic plasma diagnostics, and depict historical development of the techniques used for the X-ray spectroscopy. We outline the X-ray microcalorimeter instrument, soft X-ray spectrometer (SXS), onboard the ASTRO-H satellite. Despite the short lifetime of the satellite in orbit for about a month, observations with the SXS have shown the remarkable power of high-resolution spectroscopy in X-ray astronomy. Observed spectrum of the hot plasma in the core region of the Perseus cluster showed He-like Fe K-line to be clearly resolved into resonance, forbidden and intercombination lines for the first time. The line width indicates that the turbulent pressure amounts to only 4% of the thermal pressure of the plasma. We also describe new findings and constraints obtained from the superb spectrum of the Perseus cluster, which all indicate a great potential of X-ray spectroscopy. The recovery of the spectroscopy science of ASTRO-H is aimed at with XRISM, a Japanese mission planned for launch in early 2020s. In further future, Athena will expand the rich science with its high sensitivity and spectral resolution in early 2030s.