Photospheric radius expansion X-ray bursts as standard candles

We present a phenomenological model to explain a wide variety of stellar flares. We assume that a flare consists of a fast and relatively strong rise in brightness followed by a slow and fainter component. The latter is a result of re-radiation of the part of the energy of the prime flare by the photosphere of the star. The model is based primarily on geometrical consideration in which the position of the flare with respect to the stellar observable disk gives rise to different flare morphologies. In general, flares near the center of the disk consist of a very small pre-flare dimming. When the flare occurs towards the edge of the stellar disk the pre-flare dimming becomes fainter and eventually unobservable, while the re-radiated component also becomes fainter and redder. When the flare takes place on the backside of the stellar surface as seen by us, but near the limb, we may see only the re-radiated component.