2011A&A...530A...5C

Secondary eclipses are a powerful tool to measure directly the thermal emission from extrasolar planets, and to constrain their type and physical parameters. We started a project to obtain reliable broad-band measurements of the thermal emission of transiting exoplanets. 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 1.12M_J hot Jupiter on a 1.34day orbit around G7V star. The data show a clear ≥10σ detection of the planet's thermal emission at 2.2µm. 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±40K, centered at T_C=2455102.61162^+0.00071_–0.00077HJD. We could set a limit on the eccentricity of ecos ω=0.0027±0.0018, compatible with a near-circular orbit. 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.