Multiwavelength observation of an active M-dwarf star EV Lacertae and its stellar flare accompanied by a delayed prominence eruption
Shun Inoue, Teruaki Enoto, Kosuke Namekata, Yuta Notsu, Satoshi Honda, Hiroyuki Maehara, Jiale Zhang, Hongpeng Lu, Hiroyuki Uchida, Takeshi Go Tsuru, Daisaku Nogami, Kazunari Shibata
Abstract
Abstract 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.