A large amplitude flaring dMe star in the 1978 October 6B $\gamma$-ray burst error box

Abstract

Spectroscopic observations provide evidence that the optical transient object observed in 1966 on archival plates of Sonneberg Observatory is a B=18.5 dMe star. For an assumed flare duration of 40 min. (or longer) the flare amplitude was $\bigtriangleup$m$_{B}$ $\approx$ 5.2 mag. If the recorded event was belonging to the class of 'fast' flares its amplitude could have reached 7-9 mag.. We estimate a lower limit for the optical flare energy release of 10$^{35}$ (d/100 pc)$^2$ erg/s. We discuss the positional coincidence of this large amplitude flaring star with the error box of the $\gamma$-ray burst 1978 October 6B and conclude that the flare star is probably not related to the $\gamma$-ray burst. The fact that no other optical flare was found from this object restricts their frequency to be less than 7$\times$10$^{-4}$ h$^{-1}$ for amplitudes (in B) greater than 4.5 mag. Nevertheless, large amplitude flares constitute a serious background problem for the wide field search for possible optical counterparts of $\gamma$-ray bursters.