Solar and stellar flares are explosive phenomena in which magnetic energy stored around starspots is suddenly released through magnetic reconnection. The radiation emitted during flares covers a broad range of wavelengths from radio to X-rays, each tracing different aspects of the flare process. In X-rays, the emission arises from hot thermal plasma heated by nonthermal electrons that travel upward from the chromosphere into the corona.
RS CVn-type binaries and protostars exhibit giant flares that are several orders of magnitude more energetic than those on the Sun, and it remains unclear whether their underlying physical processes are fundamentally the same as in solar flares. Furthermore, the impact of high-energy radiation from flares on exoplanetary environments has attracted increasing attention. In particular, X-ray emission from protostars has recently drawn significant interest from the star and planet formation community in the context of X-ray–driven chemistry, as it may strongly affect the surrounding protoplanetary disks.
In X-ray observations of stellar flares, NICER — with its combination of large effective area and high observational agility — has played a key role. Furthermore, XRISM, which has an order of magnitude higher energy resolution than NICER, was launched in 2023. In this talk, I focus on the Fe K-shell emission lines in X-ray, which are covered by both of these satellites, and introduce the physics of stellar flares and its effect on exoplanetary environments that can be inferred from their line intensities and structures.