  We observed eclipses of the transiting brown dwarf CWW 89Ab at 3.6um and
4.5um using Spitzer/IRAC. The CWW 89 binary system is a member of the
$3.0\pm0.25$ Gyr-old open cluster Ruprecht 147, and is composed of a Sun-like
primary and an early M-dwarf secondary separated by a projected distance of 25
AU. CWW 89Ab has a radius of $0.937\pm0.042$ RJ and a mass of $36.5\pm0.1$ MJ,
and is on a 5.3 day orbit about CWW 89A with a non-zero eccentricity of
$e=0.19$ (Curtis et al. 2016). We strongly detect the eclipses of CWW 89Ab in
both Spitzer channels as $\delta_{3.6}=1147\pm213$ ppm and
$\delta_{4.5}=1097\pm225$ ppm after correcting for the dilution from CWW 89B.
After accounting for the irradiation that CWW 89Ab receives from its host star,
these measurements imply that the brown dwarf has an internal luminosity of
$\log(\mathrm{L_{bol}/\mathrm{L}_\odot})=-4.19\pm0.14$. This is 16 times, or
$9.3\,\sigma$, higher than model predictions given the known mass, radius, and
age of CWW 89Ab. As we discuss, this over-luminosity is not explainable by an
inaccurate age determination, additional stellar heating, nor tidal heating.
Instead, we suggest that the anomalous luminosity of CWW 89Ab is caused by a
dayside temperature inversion -- though a significant error in the evolutionary
models is also a possibility. Importantly, a temperature inversion would
require a super-stellar C/O ratio in CWW 89Ab's atmosphere. If this is indeed
the case, it implies that CWW 89Ab is a 36.5 MJ object that formed via core
accretion processes. Finally, we use our measurement of CWW 89Ab's orbital
eccentricity, improved via these observations, to constrain the tidal quality
factors of the brown dwarf and the host star CWW 89A to be $Q_{BD}>10^{4.15}$
and $Q_*>10^{9}$, respectively.
