Results of the search (pn data, P = 0.1468−100 s). The frequency range is Δν ~ 6.8 Hz, the energy bands are 0.2–1.7 keV (top), 0.2−0.5 keV (centre) and 0.5–1.7 keV (bottom). The size of the extraction region is 9′′ amounting to ~119 000, ~82 500 and ~36 800 counts, respectively, in those bands. Dashed horizontal lines show confidence levels of 1σ to 4σ for the detection of a periodic signal, given the frequency range, duration of the observation and the number of source photons in the search. A periodic signal at P_⋆ ~ 3.39 s is detected at >4σ when the energy band is restricted to the harder source photons.