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We report multi-wavelength monitoring observations of an M-dwarf flare starAD Leonis with Seimei Telescope (6150--7930 {\AA}), SCAT (SpectroscopicChuo-university Astronomical Telescope; 3700--7500 {\AA}), NICER (Neutron StarInterior Composition Explorer; 0.2--12.0 keV), and collaborations of OISTER(Optical and Infrared Synergetic Telescopes for Education and Research)program. Twelve flares are detected in total which include ten H$\alpha$, fourX-ray, and four optical-continuum flares; one of them is a superflare with thetotal energy of $\sim$ 2.0$\times$10$^{33}$ erg. We found that (1) during thesuperflare, the H$\alpha$ emission line full width at 1/8 maximum dramaticallyincreases to 14 {\AA} from 8 {\AA} in the low-resolution spectra (R$\sim$ 2000)accompanied with the large white-light flares, (2) some weak H$\alpha$/X-rayflares are not accompanied with white-light emissions, and (3) the non-flaringemissions show clear rotational modulations in X-ray and H$\alpha$ intensity inthe same phase. To understand these observational features, one-dimensionalhydrodynamic flare simulations are performed by using the RADYN code. As aresult of simulations, we found the simulated H$\alpha$ line profiles with hardand high-energy non-thermal electron beams are consistent with that of theinitial phase line profiles of the superflares, while those with more soft-and/or weak-energy beam are consistent with those in decay phases, indicatingthe changes in the energy fluxes injected to the lower atmosphere. Also, wefound that the relation between optical continuum and H$\alpha$ intensity isnonlinear, which can be one cause of the non-white-light flares. The flareenergy budget exhibits diversity in the observations and models, and moreobservations of stellar flares are necessary for constraining the occurrence ofvarious emission line phenomena in stellar flares.

Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER and OISTER