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

TL;DR

This study analyzes the time evolution of a white-light flare on the M-dwarf EV Lacertae, revealing details about flare temperature, energy, and postflare loops, which are crucial for understanding stellar flare mechanisms and planetary habitability.

Contribution

First simultaneous optical photometry and spectroscopy of a stellar flare with high temporal resolution, providing new insights into flare temperature evolution and postflare loop phenomena.

Findings

Flare continuum fits a blackbody spectrum at ~8122 K.

Optical radiative energy of the flare is approximately 4.4 x 10^32 erg.

Detection of delayed Hα flux increase indicating postflare loops.

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.