Investigating Brown Dwarf Variability at 3.4 & 4.6 Microns with AllWISE Multi-Epoch Photometry

TL;DR

This study uses AllWISE multi-epoch photometry to analyze variability in brown dwarfs at 3.4 and 4.6 microns, finding it useful mainly for detecting large amplitude variability or bright objects.

Contribution

It provides a quantitative assessment of brown dwarf variability using AllWISE data and discusses the limitations due to photometric uncertainties.

Findings

Average photometric uncertainty is ~0.2 magnitudes.

Variability detection is limited to flux changes larger than ~20%.

Multi-epoch data is valuable for bright or known variable objects.

Abstract

Multi-epoch photometry from AllWISE provides the opportunity to investigate variability at 3.4 and 4.6 microns for most known brown dwarfs. WISE observed the same patch of sky repeatedly and within a day's time, roughly 12 observations were obtained on a given patch of sky; then, another 12 were obtained roughly six months later when that patch of sky was again in view. For most of the sky, AllWISE contains two separate epochs of about a dozen observations each, although ~30% of the sky has three such epochs available in AllWISE. With the AllWISE multi-epoch photometry of ~1500 known M, L, T, and Y dwarfs, I computed the Stetson J Index and quantified variability as a function of spectral type. I found that the average single-exposure photometric uncertainty in AllWISE (~0.2 magnitudes) is too large to robustly identify flux variability smaller than ~20%. However, multi-epoch photometry from AllWISE remains a useful resource in cases where flux variability is known to be present with large amplitudes, or for bright nearby objects with lower photometric uncertainties.