Multiwavelength observation of an active M-dwarf star EV Lac and its stellar flare accompanied by a delayed prominence eruption

Abstract

We conducted 4-night multiwavelength observations of an active M-dwarf star EV Lac on 2022 October 24$-$27 with simultaneous coverage of soft X-rays (NICER; 0.2$-$12 $\mathrm{keV}$, Swift XRT; 0.2$-$10 $\mathrm{keV}$), near-ultraviolet (Swift UVOT/UVW2; 1600$-$3500 Ã ), optical photometry (TESS; 6000$-$10000 Ã ), and optical spectroscopy (Nayuta/MALLS; 6350$-$6800 Ã ). During the campaign, we detected a flare starting at 12:28 UTC on October 25 with its white-light bolometric energy of $3.4\times 10^{32}$ erg. At about 1 hour after this flare peak, our $\mathrm{H}\hat{\mathrm{I}}\pm$ spectrum showed a blue-shifted excess component at its corresponding velocity of $\sim 100\mathrm{km}{\mathrm{s}}^{-1}$. This may indicate that the prominence erupted with a 1-hour delay of the flare peak. Furthermore, the simultaneous 20-second cadence near-ultraviolet and white-light curves show gradual and rapid brightening behaviors during the rising phase at this flare. The ratio of flux in NUV to white light at the gradual brightening was $\sim 0.49$, which may suggest that the temperature of the blackbody is low ($<9000\mathrm{K}$) or the maximum energy flux of a nonthermal electron beam is less than $5\times 10^{11}\mathrm{erg}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$. Our simultaneous observations of NUV and white-light flare raise the issue of a simple estimation of UV flux from optical continuum data by using a blackbody model.

19 pages, 10 figures. Accepted for publication in Publications of the Astronomical Society of Japan