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We present new, contemporaneous HST STIS and XMM observations of the O7III(n)((f)) star xi Per. We supplement the new data with archival IUE spectra,to analyze the variability of the wind lines and X-ray flux of xi Pper. Thevariable wind of this star is known to have a 2.086 day periodicity. We use asimple, heuristic spot model which fits the low velocity (near surface) IUEwind line variability very well, to demonstrate that the low velocityabsorption in the new STIS spectra of N IV 1718 and Si IV 1402 vary with thesame 2.086 day period. It is remarkable that the period and amplitude of theSTIS data agree with those of the IUE spectra obtained 22 years earlier. Wealso show that the time variability of the new XMM fluxes are also consistentwith the 2.086 day period. Thus, our new, multi-wavelength coordinatedobservations demonstrate that the mechanism which causes the UV wind linevariability is also responsible for a significant fraction of the X-rays insingle O stars. The sequence of events for the multi-wavelength light curveminima is: Si IV 1402, N IV 1718, and X-ray flux, each separated by a phase ofabout 0.06 relative to the 2.086 day period. Analysis of the X-ray fluxes showsthat they become softer as they weaken. This is contrary to expectations if thevariability is caused by periodic excess absorption. Further, the highresolution X-ray spectra suggest that the individual emission lines at maximumare more strongly blue shifted.  If we interpret the low velocity wind line light curves in terms of ourmodel, it implies that there are two bright regions, i.e., regions with lessabsorption, separated by 180 deg, on the surface of the star. We note that thepresence and persistent of two spots separated by 180 deg suggests that a weakdipole magnetic field is responsible for the variability of the UV wind lineabsorption and X-ray flux in xi Per.

Coordinated UV and X-ray spectroscopic observations of the O-type giant xi Per: the connection between X-rays and large-scale wind structure