  We aim to characterize the U-band variability of young brown dwarfs in the
Taurus Molecular Cloud and discuss its origin. We used the XMM-Newton Extended
Survey of the Taurus Molecular Cloud, where a sample of 11 young bona fide
brown dwarfs (spectral type later than M6) were observed simultaneously in
X-rays with XMM-Newton and in the U-band with the XMM-Newton Optical/UV Monitor
(OM). We obtained upper limits to the U-band emission of 10 brown dwarfs
(U>19.6-20.6 mag), whereas 2MASSJ04141188+2811535 was detected in the U-band.
Remarkably, the magnitude of this brown dwarf increased regularly from U~19.5
mag at the beginning of the observation, peaked 6h later at U~18.4 mag, and
then decreased to U~18.65 mag in the next 2h. The first OM U-band measurement
is consistent with the quiescent level observed about one year later thanks to
ground follow-up observations. This brown dwarf was not detected in X-rays by
XMM-Newton during the OM observation. We discuss the possible sources of U-band
variability for this young brown dwarf, namely a magnetic flare, non-steady
accretion onto the substellar surface, and rotational modulation of a hot spot.
We conclude that this event is related to accretion from a circumsubstellar
disk, where the mass accretion rate was about a factor of 3 higher than during
the quiescent level.
