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

Abstract

We analysed all archival data of the low-mass X-ray binary 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. In the time-resolved spectra of 69 of these bursts, close to the peak of the burst, the best-fitting blackbody radius shows the sharp increase and decrease that is typical of photospheric radius-expansion (PRE) bursts. We found that in 17 of these 69 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 $\sim 2-8$ seconds, and after that it increases slowly. Interestingly, those PRE bursts in which the blackbody radius remains more or less constant for $\simmore 2$ seconds show coherent oscillations in the tail of the burst, whereas those PRE bursts in which the blackbody radius changes rapidly after touchdown show no coherent oscillations in the tail of the burst. We found that the distribution of durations of the post touchdown phase between these two groups of PRE bursts is significantly different; the Kolmogorov-Smirnov probability that the two groups of PRE bursts come from the same parent populations is only $3.5\times 10^{-7}$. 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.

11 pages, 18 figures, Accepted for publication in MNRAS

References in corpus (4)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Thermonuclear burst oscillations
• Surface Emission from Neutron Stars and Implications for the Physics of their Interiors
• The Latitude of Type I X-Ray Burst Ignition on Rapidly Rotating Neutron Stars

Cited by in corpus (12)

• Thermonuclear X-ray bursts
• Observatory science with eXTP
• Characterizing the Evolving X-Ray Spectral Features During a Superburst from 4U 1636-536
• NICER Detection of Strong Photospheric Expansion during a Thermonuclear X-Ray Burst from 4U 1820-30
• The accretion rate dependence of burst oscillation amplitude
• Neutron star cooling and the rp process in thermonuclear X-ray bursts
• Thermonuclear X-ray bursts from LMXB 4U 1636$-$536 observed with AstroSat
• Nuclear-powered X-ray millisecond pulsars
• Phase lags of quasi-periodic oscillations across source states in the low-mass X-ray binary 4U 1636-53
• The study of multi-peaked type-I X-ray bursts in the neutron-star low mass X-ray binary 4U 1636$-$536 with RXTE
• Broad iron emission line and kilohertz quasi-periodic oscillations in the neutron star system 4U 1636-53
• Time and energy dependent characteristics of Thermonuclear burst oscillations