1999A&A...344..154S

We present evidence for rotational modulation of X-ray flares by an analysis of four outbursts on late-type stars. Two of the flares we discuss are found in ROSAT observations of T Tauri Stars and were obtained between September and October 1991. A flare on the T Tauri Star V773 Tau was observed by ASCA in September 1995. To this sample we add a Ginga observation of a flare on Algol observed in January 1989. The structure of the X-ray lightcurves observed in this selection of flare events is untypical in that the maximum emission extends over several hours producing a round hump in the lightcurve instead of a sharp peak. We explain this deviation from the standard shape of a flare lightcurve as the result of a flare erupting on the back side of the star and gradually moving into the line of sight due to the star's rotation. Making use of the known rotational periods of the stars our model allows to determine the decay timescale of the flares and the size of the X-ray emitting volume according to the standard magnetic loop model. Spectral information, which is available in sufficient quality for the Algol observation only, supports our proposition that changes of the visible volume are responsible for the observed time development of these flares.