On the periodic X-ray emission from theta(1) Orionis C

The O7 V star theta(1) Orionis C was very recently found to present a strong periodic variation of the X-ray emission (Gagne et al. 1997) that is in agreement with the period determined from the variability of spectral lines (Stahl et al. 1996). The authors suggested that the variations are caused by an oblique magnetic rotator with a large-scale magnetic field modulating the wind. We present here the hypothesis of a dipolar magnetic field embedded in the radiation-driven wind of theta(1) Orionis C and analyze the X-ray data in the framework of the magnetically confined wind shock (MCWS) model (Babel Br Montmerle 1997). We find that both the ROSAT HRI count rate and the periodic variability of theta(1) Orionis C are quantitatively reproduced by the model, provided that the held has an intensity at the stellar surface of B* similar or equal to 300 G. The variability is caused in large part by the circumstellar cooling disk predicted by the MCWS model. These results give strong support to a magnetic origin for the variability of theta(1) Orionis C and render this star the best candidate for a high-mass analog to magnetic Bp stars.