Correlated optical and X-ray variability in CTTS Indications of absorption-modulated emission

Aims. 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(star) 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. Methods. 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. Results. 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).