Active stars, such as RS CVn-type binaries*1, are known to produce flares*2 that are orders of magnitude larger than those observed on the Sun. These stellar flares are important targets for studying their energy release mechanisms and the impact on the surrounding environment. In this study, we report the first detection of stellar flares with the X-Ray Imaging and Spectroscopy Mission (XRISM)*3, based on observations of two RS CVn-type systems.
XRISM is an international X-ray astronomy mission led by Japan in collaboration with the US and Europe and launched in September 2023. Its high-resolution X-ray spectrometer, Resolve, can separate iron K-shell transition lines*4 with unprecedented spectral precision, enabling detailed diagnostics of high-temperature plasma*5. Building on the iron K-shell spectroscopy techniques developed in earlier solar missions such as Hinotori*6, we extended these methods to giant stellar flares and diagnosed their plasma properties, including temperature, ionization state, non-thermal electrons, and elemental abundances.
Future observations of brighter stellar flares with XRISM will allow us to track the temporal evolution of plasma conditions during such energetic events, providing new insights into the flare physics.
Click here for more information: https://www.isas.jaxa.jp/home/research-portal/en/gateway/2026/0306/
Glossary
*1 RS CVn-type binaries: Binary systems consist of two stars gravitationally bound to each other. RS CVn-type binaries are a class of magnetically active close binaries, represented by RS Canum Venaticorum, that often exhibit strong stellar flares. They typically have short orbital periods and are detached systems.
*2 Stellar flare: An eruptive event in the outer stellar atmosphere, caused by the sudden release of magnetic energy accumulated in the corona.
*3 XRISM (X-Ray Imaging and Spectroscopy Mission): An international X-ray astronomy mission led by Japan, launched from the Tanegashima Space Center in September 2023. XRISM carries two instruments: Xtend, a wide-field X-ray imager, and Resolve, a high-resolution X-ray spectrometer.
*4 K-shell transition lines: X-ray emission lines produced when electrons transition to the innermost atomic shell (K-shell).
*5 Plasma: A collection of charged particles (ions and electrons), often referred to as the fourth state of matter.
*6 Hinotori (ASTRO-A): A Japanese solar X-ray mission operated from February 1981 to July 1991. It carried the soft X-ray spectrometer SOX and performed high-resolution spectroscopy using K-shell emission lines of some iron ions in solar flares.
