Ground-based detection of the near-infrared emission from the dayside of WASP-5b

Context. Observations of secondary eclipses of hot Jupiters allow one to measure the dayside thermal emission from the planets' atmospheres. The combination of ground-based near-infrared observations and space-based observations at longer wavelengths constrains the atmospheric temperature structure and chemical composition. Aims. This work aims at detecting the thermal emission of WASP-5b, a highly irradiated dense hot Jupiter orbiting a G4V star every 1.6 days, in the J, H and K near-infrared photometric bands. The spectral energy distribution is used to constrain the temperature-pressure profile and to study the energy budget of WASP-5b. Methods. We observed two secondary-eclipse events of WASP-5b in the J, H, K bands simultaneously using the GROND instrument on the MPG/ESO 2.2 m telescope. The telescope was in nodding mode for the first observation and in staring mode for the second observation. The occultation light curves were modeled to obtain the flux ratios in each band, which were then compared with atmospheric models. Results. Thermal emission of WASP-5b is detected in the J and K bands in staring mode. The retrieved planet-to-star flux ratios are 0.168(-0.052)(+0.050)% in the J band and 0.269 +/- 0.062% in the K band, corresponding to brightness temperatures of 2996(-261)(+212) K and 2890(-269)(+246) K, respectively. No thermal emission is detected in the H band, with a 3 sigma upper limit of 0.166% on the planet-to-star flux ratio, corresponding to a maximum temperature of 2779 K. On the whole, our J, H, K results can be explained by a roughly isothermal temperature profile of similar to 2700 K in the deep layers of the planetary dayside atmosphere that are probed at these wavelengths. Together with Spitzer observations, which probe higher layers that are found to be at similar to 1900 K, a temperature inversion is ruled out in the range of pressures probed by the combined data set. While an oxygen-rich model is unable to explain all the data, a carbon-rich model provides a reasonable fit but violates energy balance. The nodding-mode observation was not used for the analysis because of unremovable systematics. Our experience reconfirms that of previous authors: staring-mode observations are better suited for exoplanet observations than nodding-mode observations.