Long-term 4.6$\mu$m Variability in Brown Dwarfs and a New Technique for Identifying Brown Dwarf Binary Candidates

Abstract

Using a sample of 361 nearby brown dwarfs, we have searched for 4.6$\mu$m variability indicative of large-scale rotational modulations or large-scale long-term changes on timescales of over 10 years. Our findings show no statistically significant variability in \textit{Spitzer} ch2 or \textit{WISE} W2 photometry. For \textit{Spitzer} the ch2 1$\sigma$ limits are $\sim$8 mmag for objects at 11.5 mag and $\sim$22 mmag for objects at 16 mag. This corresponds to no variability above 4.5$\%$ at 11.5 mag and 12.5$\%$ at 16 mag. We conclude that highly variable brown dwarfs, at least two previously published examples of which have been shown to have 4.6$\mu$m variability above 80 mmag, are very rare. While analyzing the data, we also developed a new technique for identifying brown dwarfs binary candidates in \textit{Spitzer} data. We find that known binaries have IRAC ch2 PRF (point response function) flux measurements that are consistently dimmer than aperture flux measurements. We have identified 59 objects that exhibit such PRF versus apertures flux differences and are thus excellent binary brown dwarf candidates.