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Phase curve measurements provide a global view of the composition, thermalstructure, and dynamics of exoplanet atmospheres. Although most of the dozensof phase curve measurements made to date are of large, massive hot Jupiters,there is considerable interest in probing the atmospheres of the smallerplanets that are the more typical end product of planet formation. One suchplanet is the ultra-hot Neptune LTT 9779b, a rare denizen of the Neptunedesert. A companion paper presents the planet's secondary eclipses and day-sidethermal emission spectrum; in this work we describe the planet's optical andinfrared phase curves, characterized using Spitzer and TESS photometry. Wedetect LTT 9779b's thermal phase variations at 4.5um, finding a phase amplitudeof 358+/-106 ppm and a longitude of peak emission -10 deg +/- 21 deg east ofthe substellar point. Combined with our secondary eclipse observations, thesephase curve measurements imply a 4.5um day-side brightness temperature of1800+/-120 K, a night-side brightness temperature of 700+/-430 K (<1350 K at 2sigma confidence), and a day-night brightness temperature contrast of1110+/-460 K. We compare our data to the predictions of 3D GCMs and to similarobservations of hot Jupiters experiencing similar levels of stellarirradiation. Though not conclusive, our measurement of its small 4.5um phaseoffset, the relatively large amplitude of the phase variation, and thequalitative differences between our target's day-side emission spectrum andthose of hot Jupiters of similar temperatures all suggest a super-Solaratmospheric metallicity for LTT 9779b, as might be expected given its size andmass. Finally, we provide a refined ephemeris (P=0.79207022+/-0.69 d,T0=2458783.51636+/-0.00027, BJD_TDB) to enable efficient scheduling of futureobservations to further characterize the atmosphere of this intriguing planet.(abstract abridged)

Phase Curves of Hot Neptune LTT 9779b Suggest a High-Metallicity Atmosphere