A detailed X-ray investigation of ζ Puppis IV. Further characterization of the variability

Context. One of the optically brightest and closest massive stars, zeta Pup, is also a bright X-ray source. Previously, its X-ray emission was found to be variable with light curves harbouring "trends" with a typical timescale longer than the exposure length, i.e. > 1 d. The origin of these changes was proposed to be linked to large-scale structures in the wind of zeta Pup, but further characterization of the variability at high energies was needed to investigate this scenario. Aims. Since the previous papers of this series, a number of new X-ray observations have become available. Furthermore, a cyclic behaviour with a 1.78 d period was identified in long optical photometric runs, which is thought to be associated with the launching mechanism of large-scale wind structures. Methods. We analysed these new X-ray data, revisited the old data, and compared the X-ray light curves with the optical data, notably those taken simultaneously. Results. The behaviour of zeta Pup in X-rays cannot be explained in terms of a perfect clock because the amplitude and shape of its variations change with time. For example, zeta Pup was much more strongly variable between 2007 and 2011 than before and after this interval. Comparing the X-ray spectra of the star at maximum and minimum brightness yields no compelling difference beyond the overall flux change: the temperatures, absorptions, and line shapes seem to remain constant, well within errors. The only common feature between X-ray datasets is that the variation amplitudes appear maximum in the medium (0.6-1.2 keV) energy band. Finally, no clear and coherent correlation can be found between simultaneous X-ray and optical data. Only a subgroup of observations may be combined coherently with the optical period of 1.78 d, although the simultaneous optical behaviour is unknown. Conclusions. The currently available data do not reveal any obvious, permanent, and direct correlation between X-ray and optical variations. The origin of the X-ray variability therefore still needs to be ascertained, highlighting the need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.