Simultaneous optical and X-ray observations of a flare on BY Draconis.

The authors present a first report on a campaign for simultaneous observations in the visual, radio and X-ray ranges of stellar flares on the binary BY Dra. During two nights of observations they observed one significant flare, simultaneously in soft X-rays and visible wavelengths, and one or two smaller bursts. The main flare impulsively heated an area of ⪉2×10⁷km² of the stellar photosphere to a brightness temperature ⪆25000K, during about 5 min, and generated hot plasma, emitting soft X-rays. This emission reached maximum after the impulsive burst and lasted for about an hour; it covered a much larger area on the star than the optical emission. There was no significant hard X-ray component. The authors ascribe the optical continuous emission to the flare's impulsive phase, and the soft X-ray emission to the gradual phase.