U-band flares and sympathetic flaring on G1 65

Abstract

We present contemporaneous observations in the U and K bands of some 26 flares, on the active flare star Gl 65. Over a period of 27 yr, the frequency of flaring seems to have changed little. We suggest that this is due to sympathetic flaring, caused by a Moreton-wave type phenomenon, the propagation of fast magnetohydrodynamic (MHD) waves from the flare site out into the corona that are reflected back to the chromosphere. We have also redetermined the time-averaged U-band flare luminosity, |$L_{\text U}^{\ast}=3.26\times10^{26}$| erg s–1, which is a factor of two less than previously reported, but is in closer agreement with the linear |$L_{\text U}^{\ast}-L_{\text X}$| correlation of Doyle & Butler. The flares fall into two main categories, with longer duration flares releasing a smaller (20 per cent) fraction of their energy over the initial rise and decay to half maximum than the shorter-lived flares ( ≥ 50 per cent). We also find that the evolution of some of the flares can be fitted by conduction and radiation from a hot 107 K thermal plasma, as modelled by Mullan. We have also been able to fit Gurzadyan’s model of a flare on the hidden hemisphere of the star, to two broad, low-amplitude continuum enhancements.