Time evolution of white-light flare accompanied by probable postflare loops on M-type dwarf EV Lacertae

Abstract

White-light flares are explosive phenomena accompanied by brightening of continuum from near-ultraviolet to optical, which occur on the Sun and stars. In order to investigate the mechanism of white-light flares, we carried out simultaneous optical photometry (TESS : 6000-10000 \r{A}) and spectroscopy (Seimei Telescope : 4100-8900 \r{A}) of a M-dwarf EV Lac on 2019 September 14. We detected a flare with high-time-cadence ($\sim 50$ sec) spectroscopic observation. At the peak, the continuum of the flare component is well fitted by a blackbody spectrum with temperature of $T = 8122 \pm 273$ K, which is comparable with the results of previous studies that reported the spectral energy distribution of near-ultraviolet to optical during the flare could be approximated by single-temperature blackbody radiation at $T \sim 10^{4}$ K. We also estimated the time evolution of the flare temperature during the decay phase. The radiative energy of this flare within the optical range is $4.4 \times 10^{32}$ erg, taking into account the time-dependent variation in the decreasing flare temperature and expanding flare area. Furthermore, we detected a delayed increase in the flux of H$\alpha$ after the photometric flare peak, secondary increase, and gradual increase even after the white-light flare ended. Comparison of our results with light curves obtained by the Sun-as-a-star analysis of solar flares indicates that these signals may be due to postflare loops near the stellar limb. Our result about time evolution of white-light continuum will help to gain more insight into the mechanism of white-light flares both on the Sun and stars. Additionally, since extreme ultraviolet radiation from flare loops plays a key role in planetary atmospheric escape, the existence of postflare loops on stellar flares and its time evolution will help future studies about habitability of close-in planets.