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

From a statistical study of X-ray bursts observed with EXOSAT from 4U/MXB 1636–53 and EXO 0748–67, we find a strong correlation between the burst duration and the blackbody temperature of the burst source, as measured at 10% of the Eddington flux. In view of the evidence that the burst duration depends on the chemical composition of the bursting layers, this correlation suggests that deviations of burst spectra from a Planck curve depend on the chemical composition of the bursting layers.

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