Optical and X-ray observations of stellar flares on an active M dwarf AD Leonis with Seimei Telescope, SCAT, NICER and OISTER (2005.04336v2)

Abstract: We report multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with Seimei Telescope (6150--7930 {\AA}), SCAT (Spectroscopic Chuo-university Astronomical Telescope; 3700--7500 {\AA}), NICER (Neutron Star Interior 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$\alpha$, four X-ray, and four optical-continuum flares; one of them is a superflare with the total energy of $\sim$ 2.0$\times$10$^{33}$ erg. We found that (1) during the superflare, the H$\alpha$ emission line full width at 1/8 maximum dramatically increases to 14 {\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-ray flares are not accompanied with 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 by using the RADYN code. As a result of simulations, we found the simulated H$\alpha$ line profiles with hard and high-energy non-thermal electron beams are consistent with that of the initial phase line profiles of the superflares, while those with more soft- 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 found that the relation between 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.

• The paper presents a multiwavelength study that detected 12 flares on AD Leonis, including a superflare with an energy of 2.0×10³³ erg.
• It combines optical spectroscopy, photometry, and X-ray monitoring with RADYN simulations to analyze Hα line broadening and non-white-light flare phenomena.
• The findings enhance understanding of flare energetics and atmospheric dynamics, with implications for modeling stellar activity and assessing exoplanet habitability.

Optical and X-ray Observations of Stellar Flares on an Active M Dwarf AD Leonis

The paper presents comprehensive observations of stellar flares on the M-dwarf star AD Leonis using multiple telescopes and instruments, including the Seimei Telescope, SCAT, NICER, and collaborations from the OISTER program. The paper's primary goal was to gather multi-wavelength data to better understand the nature of flares on magnetically active stars like AD Leonis. The observations detected twelve flares, providing valuable insights into flare characteristics across different wavelengths.

Observations and Key Findings

During an 8.5-night observational campaign, twelve flares were detected in total, consisting of ten Hα, four X-ray, and four optical continuum flares. Notably, one of these was identified as a superflare with total energy approximated at 2.0×10³³ erg. The campaign included spectroscopic and photometric observations combined with X-ray monitoring, offering a holistic view of the flare phenomena.

Several notable phenomena were observed:

1. Line Broadening During Superflare: During the superflare, the Hα emission line's width increased significantly in the low-resolution spectra. This feature, associated with large white-light flares, suggests changes in the atmospheric conditions during such intense events.
2. Hα/X-ray Flares Without White-Light Emissions: Some weak flares showed enhancements in Hα and X-ray emissions without corresponding white-light emissions. This observation could be connected to the variety of flare energies or atmospheric conditions present during different flares.
3. Rotational Modulations: The paper also detected rotational modulations in X-ray and Hα emissions, indicating active regions on the star's surface.

Theoretical and Simulation Insights

To further comprehend the observations, one-dimensional hydrodynamic flare simulations were performed using the RADYN code. This allowed for an analysis of:

• Hα Line Profiles: The simulations indicated that hard and high-energy non-thermal electron beams could reproduce the initial phase Hα line profiles of the superflares, whereas more soft and low-energy beams correlated with the decaying phases. This implies variability in the flare's energy flux to the lower atmosphere during different flare stages.
• Nonlinear Relationship Between Continous and Hα Emissions: A significant finding was the nonlinear relation between optical continuum and Hα intensity. This nonlinearity could potentially explain the presence of non-white-light flares.

Implications and Future Work

The findings from this paper have several important implications:

• Understanding Flare Energetics: The data contributes to understanding the energy distribution in different flare wavelengths. This can aid in predicting flare impacts on stellar atmospheres and potentially on surrounding planets.
• Stellar Atmosphere Modeling: Observational data, combined with simulations, provide a framework for refining models of stellar atmospheres under extreme flare conditions.
• Impact on Habitability: Observations of superflares are crucial for assessing the potential impacts on the habitability of exoplanets around active stars like AD Leonis.

The paper highlights the need for continued multi-wavelength monitoring to further constrain flare models and improve the understanding of the universality and diversity in stellar flare phenomena. Future investigations could build on these findings by leveraging space-based telescopes like TESS to enhance the temporal coverage and resolution of such events. This integration can provide deeper insights into the dynamics of stellar flares and their broader astrophysical implications.