Using the SofI instrument on the 3.5 m New Technology Telescope , we have conducted an extensive near-infrared monitoring survey of an unbiased sample of 69 brown dwarfs spanning the L0 to T8 spectral range , with at least one example of each spectral type .
Each target was observed for a 2 – 4 hour period in the J _ { s } -band , and the median photometric precision of the data is \sim 0.7 % .
A total of 14 brown dwarfs were identified as variables with min-to-max amplitudes ranging from 1.7 % to 10.8 % over the observed duration .
All variables satisfy a statistical significance threshold with a p -value \leq 5 % based on comparison with a median reference star light curve .
Approximately half of the variables show pure sinusoidal amplitude variations similar to 2MASSJ2139+0220 , and the remainder show multi-component variability in their light curves similar to SIMPJ0136+0933 .
It has been suggested that the L/T transition should be a region of a higher degree of variability if patchy clouds are present , and this survey was designed to test the patchy cloud model with photometric monitoring of both the L/T transition and non-transition brown dwarfs .
The measured frequency of variables is 13 ^ { +10 } _ { -4 } % across the L7 – T4 spectral range , indistinguishable from the frequency of variables of the earlier spectral types ( 30 ^ { +11 } _ { -8 } % ) , the later spectral types ( 13 ^ { +10 } _ { -4 } % ) , or the combination of all non-transition region brown dwarfs ( 22 ^ { +7 } _ { -5 } % ) .
The variables are not concentrated in the transition , in a specific colour , or in binary systems .
Of the brown dwarfs previously monitored for variability , only \sim 60 % maintained the state of variability ( variable or constant ) , with the remaining switching states .
The 14 variables include nine newly identified variables that will provide important systems for follow-up multi-wavelength monitoring to further investigate brown dwarf atmosphere physics .