Context. The southern ``infrared companion'' of TTauis known to show strong photometric variations of several magnitudes ontimescales of years, as well as more modest 1magvariations on timescales as short as one week. The physical mechanismdriving these variations is debated, intrinsic luminosity variationsdue to a variable accretion rate were initially proposed, but laterchallenged in favor of apparent fluctuations due to time-variableforeground extinction. Aims. We seek to investigate the nature of the observedphotometric variability. Based on simple geometric arguments and basicphysics laws, aminimum variability timescale can be derived forwhich variable extinction is a viable mechanism. Because this timescaleincreases rapidly with wavelength, observations at long wavelengthsprovide the strongest constraints. Methods. We used VISIR at the VLT to image the TTau systemat two epochs in February2008, separated by 3.94days.Inaddition we compiled an extensive set of near- and mid-infraredphotometric data from the literature, supplemented by a number ofpreviously unpublished measurements, and constructed light curves forthe various system components. We constructed a2Dradiativetransfer model for the disk of TTauSa, consisting of apassively irradiated dusty outer part and a central, actively accretingcomponent. Results. Our VISIR data reveal a +262% change in the TTauS/TTauN flux ratio at 12.8mwithin four days, which can be attributed to a brightening ofTTauSa. Variable extinction can be excluded as a viablemechanism for the observed flux variation based on the short timescaleand the long observing wavelength. We show that also the high long-termphotometric variability and its associated color-magnitude behavior canbe plausibly explained with variable accretion. However, variableextinction is also a viable mechanism for the long-term variability,and a combination of both mechanisms may be required to explain thecollective photometric variability ofSa. Conclusions. We conclude that the observed short-termvariability is caused by a variable accretion luminosity inTTauSa, which leads to substantial fluctuations in theirradiation of the disk surface and thus induces rapid variations inthe disk surface temperature and IRbrightness. Both variableaccretion and variable foreground extinction can plausibly explain thelong-term color and brightness variations. We suggest that the periodsof high and variable brightness of Sa that we witnessed in the earlyand late 1990s were due to enhanced accretion induced by the periastronpassage of Sb, which gravitationally perturbed the Sadisk. Key words: stars: individual: TTau - stars: variables:TTauri, Herbig Ae/Be - accretion, accretion disks - circumstellarmatter - infrared: stars - stars: pre-main sequence
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