Non-Planckian behaviour of burst spectra - Dependence of the blackbody radius on the duration of bursts

Exosat observations indicate a strong correlation between X-ray burst duration and the blackbody temperature measured at the moment when the flux has decayed to 10 percent of the Eddington flux. No correlation is observed between the persistent flux near the burst and this temperature. Burst spectra reveal deviations from a Planckian spectrum which depend upon the chemical composition of the bursting layer. It is noted that this composition is not a simple function of the accretion rate.