A precursor in a large x-ray burst and the expanding envelope of the neutron star

A very long X-ray burst with precursor from XB 1715-321 was observed on July 20 1982 by the HAKUCHO satellite. From the analysis of this event, we found the evidence for the luminosity saturation and the very soft X-ray spectra for both the fall of the precursor and the rise of the main burst, which leads us to the possible interpretation of the precursor in terms of the expanding envelope as follows: i) the real onset of the burst is the precursor, ii) the temperature begins to decrease due to a rapid increase of the radius, so that the X-ray intensity in the observable energy bands diminishes, iii) the temperature decreases, and the X-rays become observable again. This change in temperature is caused by the expansion and the contraction of the envelope photosphere by the radiation pressure.