  AA Tau was observed for about 5h per XMM orbit (2 days) over 8 successive
orbits, which covers two optical eclipse periods (8.2 days). The XMM optical/UV
monitor simultaneously provided UV photometry with a ~15 min sampling rate.
Some V-band photometry was also obtained from the ground during this period in
order to determine the dates of the eclipses. Two X-ray and UV measurements
were secured close to the center of the eclipse. The UV flux is the highest
just before the eclipse starts and the lowest towards the end of it. We model
the UV flux variations with a weekly modulation (inner disk eclipse), plus a
daily modulation, which suggests a non-steady accretion. No eclipses are
detected in X-rays. For one measurement, the X-ray count rate was nearly 50
times stronger than the minimum observed level, and the plasma temperature
reached 60 MK, i.e., a factor of 2-3 higher than in the other observations.
This X-ray event, observed close to the center of the optical eclipse, is
interpreted as an X-ray flare. We identify the variable column density with the
low-density accretion funnel flows blanketing the magnetosphere. The lack of
X-ray eclipses indicates that X-ray emitting regions are located at high
latitudes. Furthermore, the occurrence of a strong X-ray flare near the center
of the optical eclipse suggests that the magnetically active areas are closely
associated with the base of the high-density accretion funnel flow. We
speculate that the impact of this free falling accretion flow onto the strong
magnetic field of the stellar corona may boost the X-ray emission (abridged).
