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tokens: 1 bursts, and the blue-filled triangles indicate the Class 2 bursts. We find that most of the multiple-peaked bursts are located close to the vertex of the CCD. Figure 2. Colour–colour diagram (CCD) of all RXTE observations of 4U 1636-53. The colours of 4U 1636 − 53 are normalized to the colours of Crab. The grey points represent all available observations with one point per observation. The black crosses stand for all X-ray burst in this source. The red-filled circles and the blue-filled triangles represent Class 1 and Class 2 bursts, respectively. We extracted the 64-s interval spectrum prior to a burst as persistent emission. We generated the instrument response matrix using the tool pcarsp and the instrumental background using the tool pcabackest in heasoft for each spectrum. In this work, we fitted the spectrum in the 3.0−20 keV band using xspec version 12.10.1 (Arnaud 1996 ). We added a systematic error to the pre-burst spectra because of calibration uncertainties. During the fitting process, we included the effect of interstellar absorption using the cross-sections of Balucinska-Church McCammon ( 1992 ) and solar abundances from Anders Grevesse ( 1989 ), with a fixed hydrogen column density, N H , of (Pandel, Kaaret Corbel 2008 ). We used all available PCUs during the X-ray bursts to produce time-resolved spectrum. We corrected every spectrum for dead time according to the methods supplied by the RXTE team. Since the light curve of bursts decay is quite smooth, to compensate the lower count rates, we extracted spectrum over longer intervals in the tail of the bursts. For each multiple-peaked burst, we generated one instrument response matrix using the pcarsp and the instrumental background using the pcabackest in heasoft . For the burst spectral analysis, we initially used a single-temperature blackbody model, tbabs*bbodyrad , to fit the net burst spectra, which is well established as a standard procedure in X-ray burst analysis (Kuulkers et al. 2002 ; Galloway et al. 2008 ). The model provides the blackbody colour temperature, kT bb , and the normalization, K bb , proportional to the square of the blackbody radius of the burst emission surface, and it allows us to estimate the bolometric luminosity as a function of time assuming a distance of 5.95 kpc (Fiocchi et al. 2006 ). The bursts bolometric flux are calculated as where , R km is the effective radius of the emitting surface in km, and D 10 is distance to the source in units of 10 kpc. We defined r 1,2 as the ratio of the first peak to the second peak flux. We note that, to reduce the effect of the instrument, we used the bolometric flux (not the net burst count rate ratio in Fig. 1 ) from the standard approach to calculate the ratio r 1,2 . After subtracting the instrumental background for the time-resolved spectra, we used another model that allows us to vary the pre-burst spectrum by a free scaling factor (Worpel et al. 2013 ). We used two models to describe the persistent