Time resolved spectroscopic observations of an M-dwarf flare star EV Lac during a flare.

Flares are explosions on the surface of the Sun and stars. We can find stellar flares frequently on M-dwarf stars. EV Lac, one of the M-dwarf flare stars, is a good target for investigation of stellar flares. We have carried out continuous spectroscopic observations of the Halpha line with medium wavelength resolution (R~10,000) by using 2m Nayuta telescope at Nishi-Harima Astronomical Observatory in Japan from August to December, 2015. EV Lac always has Halpha emission line and it was stronger on 15 August than on the other nights. In this night, we observed a rapid enhancement (~20min) and following slow decrease (~1.5h) of the emission-line intensity of Halpha, which was probably caused by a flare. We also found an asymmetrical change in the blue region of the Halpha line. In many cases, red asymmetry has been observed in the Halpha line profile during solar flares, but blue asymmetry has been sometimes observed during stellar flares.