Coherent oscillations and the evolution of the emission area in the decaying phase of radius-expansion bursts from 4U 1636–53

We analyzed all archival data of the low-mass X-ray binary system 4U 1636– 53 with the Rossi X-ray Timing Explorer (1490 observations). We found a total of 336 type-I X-ray bursts from this source. From fits to the time-resolved spectra, we classified 69 of these bursts as photospheric radius-expansion (PRE) bursts. PRE bursts show a characteristic time profile in which the fitted blackbody radius increases rapidly at the beginning of the burst, and then drops abruptly close to the peak of the burst. The lowest value of the radius after the expansion phase defines the so-called touchdown point. We found that in 17 of the PRE bursts, after the touchdown point, the blackbody radius increases again quickly after about 1 second, and from then on the radius decreases slightly or it remains more or less constant. In the other 52 PRE bursts, after touchdown, the radius of the blackbody stays more or less constant for 2 − 8 seconds, and after that it increases slowly. Interestingly, those PRE bursts in which the blackbody radius remains more or less constant for > 2 seconds show coherent oscillations in the tail of the burst, whereas those PRE in which the blackbody radius changes rapidly after touchdown show no coherent oscillations in the tail of the burst. From a Kolmogorov-Smirnov test we find that the difference between the two groups of PRE bursts is significant at a 5-σ level. 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