A very long X-ray burst with a precursor from XB 1715-321

Abstract

The Hakucho satellite observed a very long X-ray burst with a precursor from XB 1715-321 on 1982 July 20. The precursor precedes the onset of the main burst in the 1-9 keV range by about 7 s and lasts a few seconds. The main burst lasts about 300 s; in the first half of this period, the spectral hardness increases gradually up to that corresponding to the blackbody temperature of 3 keV, while the bolometric luminosity for the assumed blackbody spectrum stays nearly constant for about 100 s. The event is interpreted as a single burst with the expansion and contraction of the envelope.