A ground-based K _S-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 ≥10σ detection of the planet's thermal emission at 2.2 μm. The calculated thermal emission corresponds to a fractional eclipse depth of 0.185^+0.014_-0.013%, with a related brightness temperature in K_S of T_B = 1995 ± 40 K, centered at T_C = 2455102.61162^+0.00071 _-0.00077 HJD. We could set a limit on the eccentricity of e cos w = 0.0027 ± 0.0018, compatible with a nearcircular 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.