SIMBAD references

X-ray emission in early-type OB stars is thought to be generated by shock-heated gas in the radiatively driven wind of these stars. Calculations of the X-ray production for such a scenario depend on the underlying shock structures, especially the occurrence rate of shocks, cooling length and cooling time which directly influence the source location of the X-rays in the stellar wind. We present a detailed variability analysis of the available ROSAT data for our two program stars σ Ori and ζ Ori. The long-term analysis of both stars covers a time range of 3 years and includes seven pointed PSPC observations, an additional pointed HRI observation for σ Ori and the ROSAT all-sky survey data of both stars. In the case of σ Ori we find no evidence for variability on all analysed time scales. Over a long time range the timing analysis of the X-ray light curve of ζ Ori provides also no evidence for variability. Only during a period of 2 days (September 23-25 1992) did we detect a moderate increase in X-ray count rate by ∼ 15%.