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, kTbb, as measured at the moment that the flux has decayed to 10 per cent of the Eddington flux. We find no correlation between the persistent flux near the burst and this temperature nor with the duration of the burst. Since the burst duration probably depends on the chemical composition of the bursting layers, these results suggest that: (i) burst spectra show deviations from a Planckian spectrum, which depend on the chemical composition of the bursting layer, and (ii) this composition is not a simple function of the persistent flux and consequently of the accretion rate. In view of this, it is not possible to convert observed colour temperatures into effective temperatures, which makes it difficult to determine reliable estimates of the neutron star radius.