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Using SofI on the 3.5m New Technology Telescope, we have conducted anextensive near-infrared monitoring survey of an unbiased sample of 69 browndwarfs spanning the L0 and T8 spectral range, with at least one example of eachspectral type. Each target was observed for a 2-4 hour period in the Js-band,and the median photometric precision of the data is ~0.7%. A total of 14 browndwarfs were identified as variables with min-to-max amplitudes ranging from1.7% to 10.8% over the observed duration. All variables satisfy a statisticalsignificance threshold with a p-value <5% based on comparison with the medianreference star light curve. Approximately half of the variables show sinusoidalamplitude variations similar to 2M2139, and the remainder shows short timescaleevolving light curves similar to SIMP0136. The L/T transition has beensuggested to be a region of a higher degree of variability if patchy clouds arepresent and this survey was designed to test the patchy cloud model withphotometric monitoring of both the L/T transition and non-transition browndwarfs. Considering the targets identified as variable with the samestatistical threshold, the measured variability frequency of 13^{+10}_{-4}% forthe L7 -- T4 transition region is indistinguishable from that of the earlierspectral types (32^{+11}_{-8}%), the later spectral types (13^{+10}_{-4}%), orthe combination of all non-transition region brown dwarfs (21^{+7}_{-5}%). Thevariables are not concentrated at the transition, in a specific colour, or withbinaries. We note that of the systems previously monitored for variability only~60% maintained the state of variability (variable or constant), with theremaining switching states. The 14 variables include nine newly identifiedvariables that will provide important systems for follow-up multi-wavelengthmonitoring to further investigate brown dwarf atmosphere physics.

The Brown dwarf Atmosphere Monitoring (BAM) Project I: The largest near-IR monitoring survey of L- & T-dwarfs