Rotationally modulated X-ray emission from the single O star zeta Ophiuchi

Archived measurements by the X-ray Telescope on board the ASCA satellite of the single runaway O9.5V star, zeta Ophiuchi, are analysed. The data set i s 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 amplitu de similar to 20% in the ASCA passband (0.5-10 keV). The detected period si milar to0.(4)77 possibly indicates a connection with the recurrence time (0 .(d) 875 +/- 0.(d) 167) of the discrete absorption components (DACs) in UV spectra of the star, thought to be due to the presence of large scale struc tures in the stellar wind modulated by rotation. We attribute the X-ray flu ctuation with an uneven distribution of X-ray absorbing material. We also r eport that an analysis of similar ASCA observations of zeta Puppis failed t o confirm earlier reported variability of X-rays from this star based on Ro sat observations.