Confirmation of water emission in the dayside spectrum of the ultrahot Jupiter WASP-121b

We present four new secondary eclipse observations for the ultrahot Jupiter WASP-121b acquired using the Hubble Space Telescope Wide Field Camera 3. The eclipse depth is measured to a median precision of 60 ppm across 28 spectroscopic channels spanning the 1.12- $1.64\, \mu \rm {m}$ wavelength range. This is a considerable improvement to the 90 ppm precision we achieved previously for a single eclipse observation using the same observing setup. Combining these data with those reported at other wavelengths, a blackbody spectrum for WASP-121b is ruled out at >6σ confidence and we confirm the interpretation of previous retrieval analyses that found the data is best explained by a dayside thermal inversion. The updated spectrum clearly resolves the water emission band at 1.3- $1.6\, \mu \rm {m}$ , with higher signal-to-noise than before. It also fails to reproduce a bump in the spectrum at $1.25\, \mu \rm {m}$ derived from the first eclipse observation, which had tentatively been attributed to VO emission. We conclude the latter was either a statistical fluctuation or a systematic artefact specific to the first eclipse dataset.