We report on the results of our simultaneous observations of three large stellar flares with soft X-rays (SXRs) and an H$\mathrmα$ emission line from two binary systems of RS CVn type. The energies released in the X-ray and H$\mathrmα$ emissions during the flares were $10^{36}$--$10^{38}$ and $10^{35}$--$10^{37}$ erg, respectively. It renders the set of the observations as the first successful simultaneous X-ray/H$\mathrmα$ observations of the stellar flares with energies above $10^{35}$ erg; although the coverage of the H$\mathrmα$ observations of the stellar flares with energies above $10^{35}$ erg; although the coverage of the H$\mathrmα$ observations was limited, with $\sim$10\% of the $e$-folding time in the decay phase of the flares, that of the SXR ones was complete. Combining the obtained physical parameters and those in literature for solar and stellar flares, we obtained a good proportional relation between the emitted energies of X-ray and H$\mathrmα$ emissions for a flare energy range of $10^{29}$--$10^{38}$ erg. The ratio of the H$\mathrmα$-line to bolometric X-ray emissions was $\sim$0.1, where the latter was estimated by converting the observed SXR emission to that in the 0.1--100 keV band according to the best-fitting thin thermal model. We also found that the $e$-folding times of the SXR and H$\mathrmα$ light curves in the decaying phase of a flare are in agreement for a time range of $1$--$10^4$~s. Even very large stellar flares with energies of six orders of magnitude larger than the most energetic solar flares follow the same scaling relationships with solar and much less energetic stellar flares. This fact suggests that their physical parameters can be estimated on the basis of the known physics of solar and stellar flares.
Eclipsing Stellar Flare on the Demon Star Algol Binary System Observed during the MAXI-NICER Follow-up Campaign in 2018
Kyoto University（京都大学）；Chiba University（千叶大学）；RIKEN Center for Advanced Photonics (RAP)（理化学研究所先进光子学中心）；University of Colorado Boulder（科罗拉多大学博尔德分校）；National Solar Observatory（国家太阳观测台）；NASA’s Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）
Multi-Observatory Study of Young Stellar Energetic Flares (MORYSEF): No Evidence For Abnormally Strong Stellar Magnetic Fields After Powerful X-ray Flares
Stellar flares cannot explain the Galactic 511 keV emission
Julius-Maximilians-Universität Würzburg（维尔茨堡大学）；Laboratoire d’Annecy de Physique Théorique (LAPTh), CNRS, USMB（安娜西理论物理实验室，法国国家科学研究中心，尚贝里大学）；University of Western Australia（西澳大利亚大学）；Kyoto University（京都大学）；RIKEN Nishina Center（理化学研究所仁科中心）；The University of Tokyo（东京大学）；Rheinische Friedrich-Wilhelms-Universität Bonn（波恩大学）
