Due to many-site multicolor synchronous monitoring of the flare star EV Lac and the careful treatment of the data obtained, we have confirmed the reality of rapid optical brightness oscillations during a significant part of a flare lifetime, as was discovered by Rodonó (1974).
Our findings, based on three flare events in EV Lac, allow us to conclude that high frequency optical oscillations are coherent, with periods of 13 and 26 s and amplitudes of 0.025 mag in the B band and larger by a factor of 5 in the U band. These findings raise a question: what oscillate in a flare?
It is known that such oscillations of a quasi-period of 5-10 s were detected at 1.4 GHz in the very active dMe flare star AD Leonis in a dozen bursts with the 305 m diameter Arecibo radio telescope (Abada-Simon et al. 1995). A closeness of the frequencies mentioned suggests a physical connection; however, it is clear that radio and optical emissions cannot originate from the same region. Therefore, one should think only about heterogeneous dynamical structures. Such structures - closed magnetic loops - are already considered for X-ray flare models in red dwarf stars - see for instance Reale et al. (1997).
For an oscillation period of 20 s and the Alfven velocity for (Baranovsky et al. 2000) and G, an oscillating body's size is . According to Alekseev & Gershberg (1997), a typical area of an EV Lac flare with an amplitude mag is several units of . Only 1 or 2 loops with estimated l can be located on such area. In such a case, one can observe coherent oscillations. This should be regarded as an argument in favour of selfconsistence in our scheme. However, the cause of oscillations remains unknown.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001