2012A&A...545A..93S

We observe occultations of WASP-24b to measure brightness temperatures and to determine whether or not its atmosphere exhibits a thermal inversion (stratosphere). We observed occultations of WASP-24b at 3.6 and 4.5µm using the Spitzer Space Telescope. It has been suggested that there is a correlation between stellar activity and the presence of inversions, so we analysed existing HARPS spectra in order to calculate logR'_HK for WASP-24 and thus determine whether or not the star is chromospherically active. We also observed a transit of WASP-24b in the Stroemgren u and y bands, with the CAHA 2.2-m telescope. We measure occultation depths of 0.159±0.013 per cent at 3.6µm and 0.202±0.018 per cent at 4.5µm. The corresponding planetary brightness temperatures are 1974±71K and 1944±85K respectively. Atmosphere models with and without a thermal inversion fit the data equally well; we are unable to constrain the presence of an inversion without additional occultation measurements in the near-IR. We find logR'_HK=-4.98±0.12, indicating that WASP-24 is not a chromospherically active star. Our global analysis of new and previously-published data has refined the system parameters, and we find no evidence that the orbit of WASP-24b is non-circular. These results emphasise the importance of complementing Spitzer measurements with observations at shorter wavelengths to gain a full understanding of hot Jupiter atmospheres.