XRISM view of giant stellar flares: Plasma diagnostics with high-resolution X-ray spectroscopy

Mar. 6, 2026 | GATEWAY to Academic Articles

KURIHARA Miki / Dept. of Astronomy, The University of Tokyo &
Dept. of Space Astronomy and Astrophysics, ISAS

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.

Research Summary

High-temperature plasma exists in the outer atmospheres of stars, forming what is known as the corona. In active stars, the coronal plasma can reach tens of millions of degrees and emits X-rays. In this coronal environment, giant flares take place, which includes explosive release of magnetic energy. Since extremely energetic flares are rare or have never been observed on the Sun, stellar observations play an irreplaceable role here. The rarity increases with flare energy, making giant flares particularly difficult to capture by relying on the Sun alone. The impact of solar flares on modern technological civilization has recently drawn significant attention, and the study of giant stellar flares has become an important motivation in this context. However, stellar flare observations present unique challenges. Because of their large distances, the imaging techniques commonly used in solar physics cannot be applied. As a result, spectroscopy becomes the primary observational approach.

The XRISM satellite (Figure 1) carries a spectrometer suited for studying high-temperature plasma in giant stellar flares. Resolve, the high-resolution spectrometer onboard XRISM, achieves unprecedented spectral resolution in the iron K-shell transition band. These lines can probe plasma with temperatures of roughly 10 to 100 million Kelvin, corresponding to the hottest plasma components produced in giant stellar flares. High-resolution spectroscopy of iron K-shell lines was pioneered by past solar missions, including Hinotori, nearly half a century ago. With XRISM, these techniques can now be applied and extended to giant stellar flares for the first time. We have demonstrated this for the observations of GT Mus and HR 1099 (both RS CVn-type binaries) during the performance verification phase of XRISM and published the results in separate papers for each target.

The spectra obtained during the quiescent phase and during the flaring phase, as presented in the paper, look similar at first glance. However, by applying appropriate spectral modeling, we can demonstrate that the intensities of weak and fine-structure lines change between the two states. Some of the spectral lines used were resolved for the first time with XRISM. Using multiple emission line ratios, we diagnosed plasma properties at the sites of giant stellar flares, including electron temperature, iron ion temperature, departures from collisional ionization equilibrium^*7 and possible non-thermal electrons, as well as elemental abundances. We confirmed the validity of equilibrium assumptions during quiescent periods, whereas during the flare, we found the enhancement of high-temperature plasma components and elemental abundance variations consistent with the “standard flare scenario”^*8 established from solar observations. These results are consistent with previous study, demonstrating the capability of plasma diagnostics of giant stellar flares using lines in the XRISM era.

Although XRISM successfully captured the entire flare, the temporal evolution of the plasma properties could not be traced in this dataset because of the limited photon statistics. Non-equilibrium effects are expected to become most prominent during the early stages of flares, and time-dependent variations in temperature structure and elemental abundances enable detailed comparisons with theoretical models. Future XRISM observations of brighter flares are highly anticipated to bring breakthroughs by tracing such temporal evolution.

Terminologies

• ^*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.
• ^*7 Collisional ionization equilibrium (CIE): In hot plasmas, bound electrons in ions and atoms can be removed through collisions with energetic free electrons (“collisional ionization”), while ions recombine by capturing electrons from their surroundings (“recombination”). CIE refers to the state where the rates of ionization and recombination balance with each other.
• ^*8 Standard scenario (of solar/stellar flares): A scenario in which magnetic field lines in the corona reconnect and release energy, heating the plasma, which is then ablated from the lower atmospheric layers into loop structures and emits soft X-rays.

Information

Links

• Personal website
• Ebisawa/Tsujimoto Lab site

Author

KURIHARA Miki
March 2021 B.S. in Physics, Frontier Science Program, Chiba University
March 2023 M.S. in Astronomy, Graduate School of Science, The University of Tokyo
April 2021-Present Ebisawa Laboratory, Institute of Space and Astronautical Science
April 2023-Present PhD candidate, Department of Astronomy, Graduate School of Science, The University of Tokyo