The complete sample of type-I X-ray bursts in the low-mass X-ray binary 4U 1636-53 with RXTE

Type-I X-ray bursts are due to unstable burning of H and He on the surface of accreting neutron stars in low-mass X-ray binaries (LMXBs). One of the best studied sources of X-ray bursts is the LMXB 4U 1636-53. Here we present a complete analysis of all X-ray bursts of this source observed to date ( 270 at the time of writing) with the Rossi X-ray Timing Explorer (RXTE). From the bolometric flux light curves of individual bursts we find that about 97% of the bursts show single-peaked profiles, whereas about 3% show a double-peaked profile. We also discovered a triple-peaked X-ray burst in this source. This is the first burst of this kind reported from any LMXB using RXTE. This triple-peaked burst, a similar one observed with EXOSAT in the 1980's, and the double-peak bursts in this source, all took place when 4U 1636-53 occupied a relatively narrow region in the colour-colour diagram, corresponding to a relatively high (inferred) mass-accretion rate. From fits to time-resolved spectra, we find that about 20% of the single-peaked bursts are photospheric radius-expansion (PRE) bursts, whereas no multi-peaked burst is a PRE burst. From the timing analysis we find oscillations both in PRE and no-PRE bursts. We also study the cooling phase of all the X-ray bursts in 4U 1636-53. We find that in the tail of the bursts, at a given flux level, PRE and multipeaked bursts are cooler than non-PRE single-peaked bursts, and that the difference in temperature increases as the flux in the tail of the burst decreases. We finally discuss some scenarios that could explain our findings in terms of other properties of the source.