Detection of $J$-band photometric periodicity in the T8 dwarfs 2MASS J09393548-2448279 and EQ J1959-3338

Abstract

Aims: We aim to study the near-infrared variability of the T8 dwarfs 2MASS J09393548-2448279 and EQ J1959-3338 by analyzing their $J$-band photometric signal, which can provide new insights into the atmospheric dynamics of cold brown dwarfs. Methods: We used FLAMINGOS-2 on the Gemini South telescope to perform $J$-band differential photometry continuously over 4 h for each target. The resulting light curves have a cadence of 20 s and a photometric uncertainty of 2-4 mmag. Results: We detect periodic variability in both T8 dwarfs, with an amplitude of $16.6\pm0.9$ mmag and a period of $1.364\pm0.012$ h for EQ J1959-3338, which we attribute to rotational modulation. For 2MASS J09393548-2448279, we observe an amplitude of $4.6\pm0.4$ mmag and a period of $1.733\pm0.040$ h, though this periodicity could represent a fraction of a longer period. Conclusions: With the detection of variability in 2MASS J09393548-2448279 and EQ J1959-3338, the number of known variable T8 dwarfs has doubled, making them prime candidates for infrared space-based monitoring and radio observations to investigate atmospheric dynamics and the influence of the magnetic field in very cool atmospheres.