We monitor the star HD 149026 and its Saturn-mass planet at 8.0 µm over slightly more than half an orbit using the Infrared Array Camera ( IRAC ) on the Spitzer Space Telescope .
We find an increase of 0.0227 \% \pm 0.0066 \% ( 3.4 \sigma significance ) in the combined planet-star flux during this interval .
The minimum flux from the planet is 45 \% \pm 19 \% of the maximum planet flux , corresponding to a difference in brightness temperature of 480 \pm 140 K between the two hemispheres .
We derive a new secondary eclipse depth of 0.0411 \% \pm 0.0076 \% in this band , corresponding to a dayside brightness temperature of 1440 \pm 150 K. Our new secondary eclipse depth is half that of a previous measurement ( 3.0 \sigma difference ) in this same bandpass by Harrington et al .
( 37 ) .
We re-fit the Harrington et al .
( 37 ) data and obtain a comparably good fit with a smaller eclipse depth that is consistent with our new value .
In contrast to earlier claims , our new eclipse depth suggests that this planet ’ s dayside emission spectrum is relatively cool , with an 8 µm brightness temperature that is less than the maximum planet-wide equilibrium temperature .
We measure the interval between the transit and secondary eclipse and find that that the secondary eclipse occurs 20.9 ^ { +7.2 } _ { -6.5 } minutes earlier ( 2.9 \sigma ) than predicted for a circular orbit , a marginally significant result .
This corresponds to e \cos { ( \omega ) } = -0.0079 ^ { +0.0027 } _ { -0.0025 } where e is the planet ’ s orbital eccentricity and \omega is the argument of pericenter .