  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 17x17 arcmin 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 seven 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.

Astrophysics