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tokens: 70–600 keV). Therefore, the observation covers a wide energy band simultaneously and with high sensitivity making it possible to do detailed spectroscopic studies of the source. The data were reduced and processed with the standard procedures and using the latest caldb version ’20140825’. The XIS was operated in the standard data mode with the window mode that provided a timing resolution of 2 s. The nominal pointing was set to XIS. The XIS data was examined for the presence of pile-up. A central region corresponding to a pile-up fraction >4 per cent was discarded from all further analysis. 3 TIMING ANALYSIS: PULSATION SEARCH AND LIGHT CURVES We applied barycentric corrections to the event files for the timing analysis, using the FTOOLS task ‘aebarycen’. Orbital correction was not required as the observation duration is much less than the decoherence time-scale due to the orbital motion of the system (Chakrabarty et al. 1997 , decoherence time-scale is ∼54 d, equation A9). Light curves were extracted with a time resolution of 2 s and 1 s, respectively, from the XISs (0.2–12 keV) and HXD/PIN (10–70 keV). To search for pulsations in the data, pulse folding and χ 2 maximization technique was applied. Pulsations were found in the light curve at 835.29 ± 0.29 s, which was subsequently used to bin the light curves and also to create corresponding pulse profiles. Fig. 1 shows the light curves in the low (XIS: 0.2–12 keV) and high (PIN: 10–70 keV) energy bands along with the hardness ratio (HR) curve, all binned at the spin period of the source. The ‘HR’ is defined as the ratio of the count rate in the XIS band (with the three XISs added together) to the PIN band. It is clear from the figure that the count rates in both the energy bands vary by a factor up to 5 during the observation. Moreover the HR shows a variation by a similar factor with the PIN count rate (see right of Fig. 1 ) that indicates spectral variability in the source. In order to investigate the true energy dependence of the pulse profiles in this source and to construct an average model of the representative X-ray spectrum of the source, it is important to choose a time interval free from intensity or spectral variations. Therefore, in order to construct the pulse profiles and the energy spectrum of X Persei, we choose a stretch of the observation where 0.02 HR 0.03 and 0.8 PINc/s 1.4, as not much spectral variation is expected in this narrow stretch. Figure 1. Left figure shows the light curves in low- and high-energy bands with the lowest panel showing the HR. The light curves are binned at the spin period of 835.29 s. Right figure shows the HR plotted against the PIN count rate to highlight the variation in HR with the count rate in the high-energy band. 3.1 Energy dependence of the pulse profiles To investigate any possible energy dependence of the pulse profiles, we created the energy resolved pulse profiles by folding the data in different energy bands at the obtained pulse