  Optical and X-ray emission from classical T Tauri stars (CTTSs) has long been
known to be highly variable. Our long, uninterrupted optical observation of the
NGC2264 region with CoRoT allows the optical variability in CTTS to be studied
with unprecedented accuracy and time coverage. Two short Chandra observations
obtained during the CoRoT pointing with a separation of 16 days allow us to
study whether there is a correlation between optical and X-ray variability on
this timescale, thus probing the physical mechanisms driving the variability in
both bands. We have computed the optical and X-ray fractional variability
between the two 30 ks duration windows covered by both the Chandra and CoRoT
observations, for a sample of classical and weak line T Tauri stars (WTTSs) in
NGC2264. A scatter plot clearly shows that the variability of CTTSs in the
optical and soft X-ray (0.5-1.5 keV) bands is correlated, while no correlation
is apparent in the hard (1.5-8.0 keV) band. Also, no correlation in either band
is present for WTTSs. We show that the correlation between soft X-ray and
optical variability of CTTSs can be naturally explained in terms of
time-variable shading (absorption) from circumstellar material orbiting the
star, in a scenario rather similar to the one invoked to explain the observed
phenomenology in the CTTS AA Tau. The slope of the observed correlation implies
(in the hypothesis of homogeneous shading) a significant dust depletion in the
circumstellar material (with a gas-to-dust ratio approximately 5 times lower
than the standard value for interstellar material).
