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?MJ 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 %, with a related brightness temperature in KS of TB?=?1995?±?40?K, centered at ?HJD. We could set a limit on the eccentricity of ecosω?=?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.
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