Strong evidence for a link between coherent oscillations and the apparent emission area during the thermonuclear bursts from accreting neutron stars.

Coherent oscillations and the evolution of the X-ray spectrum during the thermonuclear X-ray bursts have been studied intensively but separately. Recently we analysed all archival data of the two burst-rich sources 4U 1636-53 and 4U 1728-34 with the Rossi X-ray Timing Explorer. We found that during the bursting time, especially the cooling phase, the bursts in which the blackbody radius remains more or less constant for ≳2 s show coherent oscillations, whereas those bursts in which the blackbody radius changes rapidly show no coherent oscillations. This is the first time that the presence of burst oscillations in the tail of X-ray bursts is associated with a systematic behaviour of the spectral parameters in that phase of the bursts. This result is consistent with predictions of models that associate the oscillations in the tail of X-ray bursts with the propagation of a cooling wake in the material on the neutron-star surface during the decay of the bursts.