Aurora Y. Kesseli, Siddharth Gandhi, Lisa Dang, Alexis Lavail, Alejandro Sanchez-Lopez, Ying Shu, Emily Rauscher, Hayley Beltz, Romain Allart, Stefan Pelletier, Vatsal Panwar

Hot Jupiters are expected to be tidally locked and synchronously rotating due to their short orbital periods. These conditions create large day-night temperature contrasts and are thought to drive eastward super-rotating jets. Indeed, the majority of hot Jupiters are observed to have the hottest region of the planet either at the substellar point or offset in the eastern direction. However, the full phase curve of CoRoT-2b, observed with the Spitzer Space Telescope, exhibits robust evidence of a western hotspot offset. To determine the origin of this peculiar hotspot offset, we present phase-resolved high-resolution observations of CoRoT-2b from the CRIRES+ spectrograph on the Very Large Telescope (VLT) and the IGRINS spectrograph on Gemini South, covering both pre- and post-eclipse phases (0.34--0.63). We detect the signal from the planet (S/N$>$4) in both pre- and post-eclipse phases separately, and therefore perform separate cross-correlation and retrieval analyses at the two epochs. The phase-resolved retrievals show highly consistent abundances and C/Os, but prefer a hotter and more isothermal temperature-pressure profile at post-eclipse phases, consistent with the phase curve observations that indicated a western hotpsot offset. By testing multiple hypotheses invoked to drive a western hotspot offset, we find the most likely explanation to be sub-synchronous planetary rotation. We measure the planet's rotational broadening to be $2.24\substack{+0.81\\-0.77}$ km s$^{-1}$, whereas the expectation from tidally locked rotation is $4.37\pm0.13$ km s$^{-1}$ (2.6-$σ$ discrepant). Other observations, such as high precision phase curves or eclipse mapping, would help to further confirm the western hotspot offset and sub-synchronous rotation.