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We analyse emission spectra of WASP-12b from a partial phase curve observedover three epochs with the Hubble Space Telescope, covering eclipse,quadrature, and transit, respectively. As the 1.1-day period phase curve wasonly partially covered over three epochs, traditional methods to extract theplanet flux and instrument systematic errors cannot recover the thermalemission away from the secondary eclipse. To analyse this partial phase curve,we introduce a new method, which corrects for the wavelength-independentcomponent of the systematic errors. Our new method removes the achromaticinstrument and stellar variability, and uses the measured stellar spectrum ineclipse to then retrieve a relative planetary spectrum in wavelength at eachphase. We are able to extract the emission spectrum of an exoplanet atquadrature outside of a phase curve for the first time; we recover thequadrature spectrum of WASP-12b up to an additive constant. The daysideemission spectrum is extracted in a similar manner, and in both cases we areable to estimate the brightness temperature, albeit at a greatly reducedprecision. We estimate the brightness temperature from the dayside(Tday=3186+-677 K) and from the quadrature spectrum (Tquad=2124+-417 K) andcombine them to constrain the energy budget of the planet. We compare ourextracted relative spectra to global circulation models of this planet, whichare generally found to be a good match. However, we do see tentative evidenceof a steeper spectral slope in the measured dayside spectrum compared to ourmodels. We find that we cannot match this increased slope by increasing opticalopacities in our models. We also find that this spectral slope is unlikely tobe explained by a non-equilibrium water abundance, as water advected from thenightside is quickly dissociated on the dayside.

A new approach to spectroscopic phase curves: The emission spectrum of WASP-12b observed in quadrature with HST/WFC3