  We report the first detection of CO emission at high spectral resolution in
the day-side infrared thermal spectrum of an exoplanet. These emission lines,
found in the atmosphere of the transiting ultra hot Jupiter (UHJ) WASP-33 b,
provide unambiguous evidence of its thermal inversion. Using spectra from the
MMT Exoplanet Atmosphere Survey (MEASURE, $R\sim15,000$), covering pre- and
post-eclipse phases, we cross-correlate with 1D PHOENIX spectral templates to
detect CO at S/N = 7.9 ($v_{sys}=0.15^{+0.64}_{-0.65}$ km/s,
$K_{p}=229.5^{+1.1}_{-1.0}$ km/s). Moreover, using
cross-correlation-to-log-likelihood mapping, we find that the scaling parameter
which controls the spectral line contrast changes with phase. We thus use the
general circulation model SPARC/MITgcm post-processed by the 3D gCMCRT
radiative transfer code to interpret this variation, finding it consistent with
an eastward-shifted hot spot. Pre-eclipse, when the hot spot faces Earth, the
thermal profiles are shallower leading to smaller line contrast despite greater
overall flux. Post-eclipse, the western part of the day-side faces Earth and
has much steeper thermal profiles, leading to larger line contrast despite less
overall flux. This demonstrates that within the log-likelihood framework, even
relatively moderate resolution spectra can be used to understand the 3D nature
of close-in exoplanets, and that resolution can be traded for photon-collecting
power when the induced Doppler-shift is sufficiently large. We highlight CO as
a good probe of UHJ thermal structure and dynamics that does not suffer from
stellar activity, unlike species that are also present in the host star e.g.
iron lines.
