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The soft X-ray emission from high density plasma in CTTS is associated withthe accretion process. It is still unclear whether this high density coolplasma is heated in the accretion shock, or if it is coronal plasmafed/modified by the accretion process. We conducted a coordinatedquasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph(Chandra/HETGS data to constrain the X-ray emitting plasma components, andoptical observations to constrain the characteristics of accretion and magneticfield). We analyze a 200 ks Chandra/HETGS observation of V2129 Oph, subdividedinto two 100 ks segments, corresponding to two different phases within onestellar rotation. The X-ray emitting plasma covers a wide range oftemperatures: 2-34 MK. The cool plasma component of V2129 Oph varies betweenthe two segments of the Chandra observation: high density plasma (log Ne ~12.1) with high EM at ~ 3-4 MK is present during the 1st segment; during the2nd segment this plasma component has lower EM and lower density (log Ne <11.5), although the statistical significance of these differences is marginal.Hotter plasma components, T > 10 MK, show variability on short time scales (~10 ks), typical of coronal plasma. A clear flare, detected in the 1st segment,could be located in a large coronal loop (> 3 Rstar). Our observation providesfurther confirmation that the dense cool plasma at a few MK in CTTS is materialheated in the accretion shock. The variability of this cool plasma component onV2129 Oph may be explained in terms of X-rays emitted in the accretion shockand seen with different viewing angles at the two rotational phases probed byour observation. During the 1st time interval direct view of the shock regionis possible, while, during the 2nd, the accretion funnel itself intersects theline of sight to the shock region, preventing us from observingaccretion-driven X-rays.

Variable X-ray emission from the accretion shock in the Classical T Tauri Star V2129 Oph