An optical megaflare on EV Lac

We present a U-filter light curve of a very intensive flare on EV Lac, observed at 1 s time resolution with a 2 m telescope at BNAO. The monitoring lasted for 7.4 hours (UT 16:40-UT24:04) on 01 November 1991, and EV Lac was affected by flaring throughout this time. The data have been corrected for atmospheric extinction and the presence of an optical companion in the photometer diaphragm. A ΔU=7.2 magnitude flare (Lf,U = 4.6 · 1031 erg s-1) represents an increase of 735 times the flux of the quiescent star. This is twice as large as the flare reported by Roizman and Shevchenko (1982, Sov. Astron. Lett. 8, 85-86). At maximum, Lf,U/Lbol = 0.88. The rapid phase of the rise took less than 2 minutes, and the flare dimmed to half its maximum value in t0.5 = 39 seconds. The classically peak-shaped flare went into a slow decay phase after about 20 minutes. Several secondary flares were superposed on the > 5 hours decay. The U-filter flare energy released during the major peak was 7.23 · 1033 erg. It accounts for 93% of the energy released during the entire data set (EU = 7.8 · 1033 erg). The flare occurred close to maximum brightness of the 4.4 days rotational modulation of EV Lac, which had a V-amplitude 0.06 mag in November 1991