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

We report on multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with the Seimei Telescope (6150-7930 angstrom), SCAT (Spectroscopic Chuo-university Astronomical Telescope; 3700-7500 angstrom), and NICER (Neutron Star Interior Composition Explorer; 0.2-12.0 keV), with the collaboration of the OISTER (Optical and Infrared Synergetic Telescopes for Education and Research) program. Twelve flares are detected in total, including ten H alpha, four X-ray, and four optical-continuum flares; one of them is a superflare with a total energy of similar to 2.0 x 10(33) erg. We found that: (1) during the superflare, the H alpha emission line full width at 1/8 maximum dramatically increases to 14 angstrom from 8 angstrom in the low-resolution spectra (R - 2000) accompanied by large white-light flares, (2) some weak H alpha/X-ray flares are not accompanied by white-light emissions, and (3) the non-flaring emissions show clear rotational modulations in X-ray and H alpha intensity in the same phase. To understand these observational features, one-dimensional hydrodynamic flare simulations are performed using the RADYN code. We find the simulated H alpha line profiles with hard and high-energy non-thermal electron beams to be consistent with the initial phase line profiles of the superflares, while those with a softer and/or weak-energy beam are consistent with those in decay phases, indicating the changes in the energy fluxes injected to the lower atmosphere. Also, we find that the relation between the optical continuum and H alpha intensity is nonlinear, which can be one cause of the non-white-light flares. The flare energy budget exhibits diversity in the observations and models, and more observations of stellar flares are necessary for constraining the occurrence of various emission line phenomena in stellar flares.