A U-band survey of brown dwarfs in the Taurus Molecular Cloud with the XMM-Newton Optical/UV Monitor

Aims. We aim to characterize the U-band variability of young brown dwarfs in the Taurus Molecular Cloud and discuss its origin. Methods. 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). Results. We obtained upper limits to the U-band emission of 10 brown dwarfs (U≥19.6–20.6 mag), whereas 2MASS J04141188+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 6 h later at U∼18.4 mag, and then decreased to U∼18.65 mag in the next 2 h. 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. Conclusions. 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.