The data-driven flare temperature and energy estimate based on the simultaneous spectroscopic and photometric observations of Wolf~359's flare activity

Chia-Lung Lin^1*, Li-Ching Huang², Wing-Huen Ip^1,3,4, Wie-Jei Hou¹

¹Graduate Institute of Astronomy, National Central University, Taoyuan City, Taiwan
²Department of Physics, National Taiwan Normal University, Taipei City, Taiwan
³Institute of Space Science, National Central University, Taoyuan City, Taiwan
⁴Department of Physics, Macau University of Science and Technology, Macau, Macau

* Presenter:Chia-Lung Lin, email:m1059006@gm.astro.ncu.edu.tw

We present a data-driven flare temperature estimate and its impact on the flare energy calculation with the spectrophotometric time series observation of the flare activity of an M dwarf, Wolf 359. The observation was carried out with the low resolution (R~800) LISA spectrograph and U band photometry simultaneously at the Lulin observatory on 2021 April 3. Two flares are detected in the U band light curve. The first flare lasting about 55 minutes has a peak amplitude of 0.45 and an energy of 6.4×〖10〗^29 erg in the U band. The second flare has a shorter duration of about 26 minutes and a higher peak amplitude of 0.61. The energy of the second flare estimated from the U band light curve is about 3.3×〖10〗^29 erg. The Balmer lines emissions increase with the flares, and their amplitude variations follow that of the U band light curve. Convoluting the V band transmission with the observed spectra, the V band light curve is produced. Only the second U band flare is detectable in the V band curve. Because of the cadences of spectroscopic and photometric observations are different, we fitted the flare profile models of U and V band curves to estimate the U-V color evolution of the flare. The amplitude of the best-fit U band and V band profile are 1.18 and 0.06. respectively. The temperature evolution of the second flare is derived from the U-V color, reaching 9700 K at peak then gradually cooling. The difference between the U-V color temperature estimated flare bolometric energy (~1.07×〖10〗^32 erg) and the bolometric energy (~2.23×〖10〗^31 erg) that derived from the empirical value of 9000 K is discussed. We found that the 9000 K flare temperature assumption underestimates the flare energy by about 20%.

Keywords: Flare activtiy, Low-mass star, Multi-wavelength observation