Rotationally modulated X-ray emission from the single O star ζOphiuchi

Archived measurements by the X-ray Telescope on board the ASCAsatellite of the single runaway O9.5V star, ζOphiuchi, are analysed. The data set is unique as it covers just more than one full rotational period of the star. We report a clearly detected periodic X-ray flux variability with amplitude ~20% in the ASCApassband (0.5-10 keV). The detected period ${\sim} 0\fd77$possibly indicates a connection with the recurrence time ($0\fd875\pm 0\fd167$) of the discrete absorption components (DACs) in UV spectra of the star, thought to be due to the presence of large scale structures in the stellar wind modulated by rotation. We attribute the X-ray fluctuation with an uneven distribution of X-ray absorbing material. We also report that an analysis of similar ASCAobservations of ζPuppis failed to confirm earlier reported variability of X-rays from this star based on Rosatobservations.