A ground-based KS-band detection of the thermal emission from the transiting exoplanet WASP-4b

Context. Secondary eclipses are a powerful tool to measure directly the thermal emission from extrasolar planets, and to constrain their type and physical parameters. Aims. We started a project to obtain reliable broad-band measurements of the thermal emission of transiting exoplanets. Methods. Ground-based high-cadence near-infrared relative photometry was used to obtain a sub-millimagnitude precision light curve of a secondary eclipse of WASP-4b - a 1.12 M-J hot Jupiter on a 1.34 day orbit around G7V star. Results. The data show a clear >= 10 sigma detection of the planet's thermal emission at 2.2 mu m. The calculated thermal emission corresponds to a fractional eclipse depth of 0.185(-0.013)(+0.014)% , with a related brightness temperature in K-S of T-B = 1995 +/- 40 K, centered at T-C = 2455102.61162(-0.00077)(+ 0.00071) HJD. We could set a limit on the eccentricity of e cos omega = 0.0027 +/- 0.0018, compatible with a near-circular orbit. Conclusions. The calculated brightness temperature and the specific models suggest a highly inefficient redistribution of heat from the day-side to the night-side of the planet, and a consequent emission mainly from the day-side. The high-cadence ground-based technique is capable of detecting the faint signal of the secondary eclipse of extrasolar planets, which makes it a valuable complement to space-based mid-IR observations.