XRISM view of a stellar flare: High-resolution Fe K spectra of HR 1099, an RS CVn-type star

Abstract A high-resolution X-ray spectroscopic observation was made of the RS CVn-type binary star HR 1099 using the Resolve instrument onboard XRISM for its calibration purposes. During the $\sim$400 ks telescope time covering 1.5 binary orbit, a flare lasting for $\sim$100 ks was observed with a released X-ray radiation energy of ${\sim }10^{34}$ erg, making it the first stellar flare ever observed with an X-ray microcalorimeter spectrometer. The flare peak count rate is 6.4 times higher than that in quiescence and is distinguished clearly in time thanks to the long telescope time. Many emission lines were detected in the 1.7–10 keV range both in the flare and quiescent phases. Using the high spectral resolution of Resolve in the Fe K band (6.5–7.0 keV), we resolved the inner-shell lines of Fe xix–xxiv as well as the outer-shell lines of Fe xxv–xxvi. These lines have peaks in the contribution functions at different temperatures over a wide range, allowing us to construct the differential emission measure (DEM) distribution over the electron temperature of 1–10 keV (roughly 10–100 MK) based only on Fe lines, thus without an assumption of the elemental abundance. The reconstructed DEM has a bimodal distribution, and only the hotter component increased during the flare. The elemental abundance was derived based on the DEM distribution thus constructed. A significant abundance increase was observed during the flare for Ca and Fe, which are some of the elements with the lowest first ionization potential among those analyzed, but not for Si, S, and Ar. This behavior is seen in some giant solar flares and the present result is a clear example in stellar flares.

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Journal
Publications of the Astronomical Society of Japan
Published
2025-11-25
DOI
https://doi.org/10.1093/pasj/psaf124
Primary Topic
Stellar, planetary, and galactic studies
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article
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article

XRISM view of a stellar flare: High-resolution Fe K spectra of HR 1099, an RS CVn-type star

Ehud Behar, M. Audard, V. Kashyap, Eric D. Miller et al.
Publications of the Astronomical Society of Japan
Stellar, planetary, and galactic studies
article

XRISM view of a stellar flare: High-resolution Fe K spectra of HR 1099, an RS CVn-type star

Ehud Behar, M. Audard, V. Kashyap, Eric D. Miller, Kenji Hamaguchi, Megan E. Eckart, Yukiko Ishihara, Adam Foster, Masahiro Tsujimoto, Yoshitomo Maeda, Liyi Gu, Miki Kurihara, Michael Loewenstein, M. Guainazzi, Satoru Katsuda, Maurice A. Leutenegger, F. S. Porter, Natalie Hell, Shun Inoue, Makoto Sawada, David H. Brooks, Nagisa Nagashima, Caroline A. Kilbourne, Yohko Tsuboi
article en

Abstract

Abstract A high-resolution X-ray spectroscopic observation was made of the RS CVn-type binary star HR 1099 using the Resolve instrument onboard XRISM for its calibration purposes. During the $\sim$400 ks telescope time covering 1.5 binary orbit, a flare lasting for $\sim$100 ks was observed with a released X-ray radiation energy of ${\sim }10^{34}$ erg, making it the first stellar flare ever observed with an X-ray microcalorimeter spectrometer. The flare peak count rate is 6.4 times higher than that in quiescence and is distinguished clearly in time thanks to the long telescope time. Many emission lines were detected in the 1.7–10 keV range both in the flare and quiescent phases. Using the high spectral resolution of Resolve in the Fe K band (6.5–7.0 keV), we resolved the inner-shell lines of Fe xix–xxiv as well as the outer-shell lines of Fe xxv–xxvi. These lines have peaks in the contribution functions at different temperatures over a wide range, allowing us to construct the differential emission measure (DEM) distribution over the electron temperature of 1–10 keV (roughly 10–100 MK) based only on Fe lines, thus without an assumption of the elemental abundance. The reconstructed DEM has a bimodal distribution, and only the hotter component increased during the flare. The elemental abundance was derived based on the DEM distribution thus constructed. A significant abundance increase was observed during the flare for Ca and Fe, which are some of the elements with the lowest first ionization potential among those analyzed, but not for Si, S, and Ar. This behavior is seen in some giant solar flares and the present result is a clear example in stellar flares.

Publications of the Astronomical Society of JapanVol. 78(1)
University of Geneva (CH), Lawrence Livermore National Laboratory (US), Goddard Space Flight Center (US), Rikkyo University (JP), George Mason University (US), Technion – Israel Institute of Technology (IL), Kyoto University (JP), Japan Aerospace Exploration Agency (JP), European Space Agency (FR), Department of Space (IN), Center for Astrophysics Harvard & Smithsonian (US), European Space Research and Technology Centre (NL), University College London (GB), Space Research Organisation Netherlands (NL), Massachusetts Institute of Technology (US), Saitama University (JP), The University of Tokyo (JP), University of Maryland, Baltimore County (US), Chuo University (JP)
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Stellar, planetary, and galactic studies
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