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

We conducted four-night multiwavelength observations of an active M-dwarf star EV Lacertae on 2022 October 24-27 with simultaneous coverage of soft X-rays (NICER; 0.2-12 keV, Swift XRT; 0.2-10 keV), near-ultraviolet (Swift UVOT/UVW2; 1600-3500 Å), optical photometry (TESS; 6000-10000 Å), and optical spectroscopy (Nayuta/MALLS; 6350-6800 Å). During the campaign, we detected a flare starting at 12:28 UTC on October 25 with a white-light bolometric energy of 3.4 × 10³² erg. At about 1 h after this flare peak, our Hα spectrum showed a blueshifted excess component at a corresponding velocity of ~100 km s^-1. This may indicate that the prominence erupted with a 1 h delay of the flare peak. Furthermore, the simultaneous 20 s cadence near-ultraviolet (NUV) 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 ~0.49, which may suggest that the temperature of the blackbody is low (<9000 K) or the maximum energy flux of a non-thermal electron beam is less than 5 × 10¹¹ erg cm^-2 s^-1. Our simultaneous observations of the NUV and white-light flare raise the issue of a simple estimation of UV flux from optical continuum data by using a blackbody model.