We observed two full orbital phase curves of the transiting brown dwarf KELT-1b , at 3.6 \mu m and 4.5 \mu m , using the Spitzer Space Telescope .
Combined with previous eclipse data from [ Beatty et al .
( 2014 ) ] , we strongly detect KELT-1b ’ s phase variation as a single sinusoid in both bands , with amplitudes of 964 \pm 36 ppm at 3.6 \mu m and 979 \pm 54 ppm at 4.5 \mu m , and confirm the secondary eclipse depths measured by [ Beatty et al .
( 2014 ) ] .
We also measure noticeable Eastward hotspot offsets of 28.4 \pm 3.5 degrees at 3.6 \mu m and 18.6 \pm 5.2 degrees at 4.5 \mu m. Both the day-night temperature contrasts and the hotspot offsets we measure are in line with the trends seen in hot Jupiters \citep [ e.g .
] [ ] crossfield2015 , though we disagree with the recent suggestion of an offset trend by [ Zhang et al .
( 2018 ) ] .
Using an ensemble analysis of Spitzer phase curves , we argue that nightside clouds are playing a noticeable role in modulating the thermal emission from these objects , based on : 1 ) the lack of a clear trend in phase offsets with equilibrium temperature , 2 ) the sharp day-night transitions required to have non-negative intensity maps , which also resolves the inversion issues raised by [ Keating & Cowan ( 2017 ) ] , 3 ) the fact that all the nightsides of these objects appear to be at roughly the same temperature of 1000 K , while the dayside temperatures increase linearly with equilibrium temperature , and 4 ) the trajectories of these objects on a Spitzer color-magnitude diagram , which suggest colors only explainable via nightside clouds .