X-RAY ROTATIONAL MODULATION
Rotational modulation is one of the main characteristics of the X-ray emission of the Sun. It is due to the spatial inhomogeneity of emitting structures on solar corona. An example of solar rotational modulation as observed with Yohkoh is given here together with the corresponding full-disk lightcurve.
The detection of rotational modulation in stellar coronal emission is important since it gives information on the size of emitting structures. The determination of the geometry and properties of the coronae of very active stars is crucial to understand their structure and physics and their similarity with the solar corona.
There have been several attempts to observe rotational modulation of stellar coronal emission in fast rotating stars, but in most cases the results have been inconclusive, owing to the superposition of flaring and non-flaring variability and to the lack of observations extending over more than one rotational period (e.g. Collier Cameron et al. 1988, Guedel et al. 1995, Kurster et al. 2001, Audard et al. 2001, Franciosini et al. 2002, Garcia-Alvarez et al. 2003). The best evidence of coronal modulation is the XMM/Newton observation of VXR45 a young supersaturated dG9 star in IC2391 open cluster.
The Orion COUP observation gives us a unique opportunity to study the presence of rotational modulation in pre main sequence stars with rotational periods up to ~10 days, a regime completely unexplored (The Sun is a relatively slow star with a period of 27 days).
In visible light young stars vary since huge star spots are alternately visible and eclipsed causing a modulation. COUP has observed a number of stars with X-ray periods equal to the optical period. The detection of coronal rotational modulation indicates that the corona is highly disomogeneous, as that of the Sun. The emitting structures are smaller than the stellar surface and the high luminosity of these young stars is not due to a complete coverage of the stellar surface with emitting structures.
