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-UT 24:04) 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 maximum (L_f,U = 4.6 · 10³¹ ergs s^-1) represents an increase of 735 times the flux of the quiescent star.  This is twice as large as a flare reported by Roizman and Shevchenko (1982, Sov. Astron. Lett. 8, 85-86) and is 88% of the bolometric luminosity of EV Lac.  The eruptive phase of the rise took 72 seconds, and the flare returned to half its maximum value in t_0.5 = 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 ergs.  It accounts for 93% of the energy released during the entire flare event (E_U = 7.8 · 10³³ ergs). 

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 1991.