2005ApJS..160..450F

We investigate the spatial distribution of X-ray-emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17'x17' field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method. X-ray modulation related to the rotation period is detected in at least 23 stars with periods between 2 and 12 days and relative amplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar rotation period, while in 7 cases it is about half that value, possibly due to the presence of X-ray-emitting structures at opposite stellar longitudes. These results constitute the largest sample of low-mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray-emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (L_X/L_bol∼10^–3) showing that saturation is not due to the filling of the stellar surface with X-ray-emitting regions.