  (Note: this is a shortened version of the original A&A-style structured
abstract). The physical nature of the strong photometric variability of T Tau
Sa, the more massive member of the Southern "infrared companion" to T Tau, has
long been debated. Intrinsic luminosity variations due to variable accretion
were originally proposed but later challenged in favor of apparent fluctuations
due to time-variable foreground extinction. In this paper we use the timescale
of the variability as a diagnostic for the underlying physical mechanism.
Because the IR emission emerging from Sa is dominantly thermal emission from
circumstellar dust at <=1500K, we can derive a minimum size of the region
responsible for the time-variable emission. In the context of the variable
foreground extinction scenario, this region must be (un-) covered within the
variability timescale, which implies a minimum velocity for the obscuring
foreground material. If this velocity supercedes the local Kepler velocity we
can reject foreground extinction as a valid variability mechanism. The variable
accretion scenario allows for shorter variability timescales since the
variations in luminosity occur on much smaller scales, essentially at the
surface of the star, and the disk surface can react almost instantly on the
changing irradiation with a higher or lower dust temperature and according
brightness. We have detected substantial variations at long wavelengths in T
Tau S: +26% within four days at 12.8 micron. We show that this short-term
variability cannot be due to variable extinction and instead must be due to
variable accretion. Using a radiative transfer model of the Sa disk we show
that variable accretion can in principle also account for the much larger
(several magnitude) variations observed on timescales of several years. For the
long-term variability, however, also variable foreground extinction is a viable
mechanism.
