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Pre-main sequence stars are variable sources. In stars with disks, thisvariability is related to the morphology of the inner circumstellar region(<0.1 AU) and that of the photosphere and corona, all impossible to bespatially resolved with present day techniques. This has been the mainmotivation for the Coordinated Synoptic Investigation of NGC 2264. In thispaper, we focus on the stars with disks. We analyze the X-ray spectralproperties extracted during optical bursts and dips in order to unveil thenature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-Iobservations to search for coherent optical and X-ray flux variability in starswith disks. Then, stars are analyzed in two different samples. In stars withvariable extinction, we look for a simultaneous increase of optical extinctionand X-ray absorption during the optical dips; in stars with accretion bursts,we search for soft X-ray emission and increasing X-ray absorption during thebursts. Results. We find evidence for coherent optical and X-ray fluxvariability among the stars with variable extinction. In 9/24 stars withoptical dips, we observe a simultaneous increase of X-ray absorption andoptical extinction. In seven dips, it is possible to calculate the NH/AV ratioin order to infer the composition of the obscuring material. In 5/20 stars withoptical accretion bursts, we observe increasing soft X-ray emission during thebursts that we associate to the emission of accreting gas. It is not surprisingthat these properties are not observed in all the stars with dips and bursts,since favorable geometric configurations are required. The observed variableabsorption during the dips is mainly due to dust-free material in accretionstreams. In stars with accretion bursts, we observe on average a larger softX-ray spectral component not observed in non accreting stars.

astronomy and astrophysics

CSI 2264: Simultaneous optical and X-ray variability in pre-Main  Sequence stars. I: Time resolved X-ray spectral analysis during optical dips  and accretion bursts in stars with disks