We report differential I -band and J -band photometry of S Ori 45 , a cool ( spectral type M8.5 ) , young ( 1–8 Myr ) brown dwarf of the \sigma Orionis cluster with a likely mass estimated at around 20 times the mass of Jupiter .
We detect variability ( amplitudes ranging from 34 to 81 mmag ) and observe a modulation at a period of 2.5–3.6 h in both optical and near-infrared light curves .
The most recent optical data set , however , presents a modulation at the very short period of 46.4 \pm 1.5 min , which remains a mystery .
The origin of the 2.5–3.6 h modulation is analized in terms of various scenarios : inhomogeneous features ( dust clouds or magnetically induced dark spots ) co-rotating with the object ’ s surface , and presence of an unseen very low-mass companion that is steadily transferring mass to the primary .
Because of the very young age of the object and its persistent strong H \alpha emission , the possible presence of an accreting disk is also discussed .
If the period of a few hours is related to rotation , our results suggest that \sigma Orionis low-mass brown dwarfs are rotating faster than more massive cluster brown dwarfs at a rate consistent with their theoretically inferred masses and radii , implying that all of these objects have undergone similar angular momentum evolution .