| Abstract
| - Context. Only recently it has become possible to measure the thermal emission from hot-Jupiters at near-Infrared wavelengths using ground-based telescopes, by secondary eclipse observations. This allows the planet flux to be probed around the peak of its spectral energy distribution, which is vital for the understanding of its energy budget. Aims. The aim of the reported work is to measure the eclipse depth of the planet HAT-P-1b at 2.2 μm. This planet is an interesting case, since the amount of stellar irradiation it receives falls in between that of the two best studied systems (HD 209458 and HD 189733), and it has been suggested to have a weak thermal inversion layer. Methods. We have used the LIRIS instrument on the William Herschel Telescope (WHT) to observe the secondary eclipse of HAT-P-1b in the Ks-band, as part of our Ground-based secondary eclipse (GROUSE) project. The observations were done in staring mode, while significantly defocusing the telescope to avoid saturation on the K = 8.4 star. With an average cadence of 2.5 s, we collected 6520 frames during one night. Results. The eclipse is detected at the 4- σ level, the measured depth being 0.109 ± 0.025%. The uncertainties are dominated by residual systematic effects, as estimated from different reduction/analysis procedures. The measured depth corresponds to a brightness temperature of 2136 \hbox{$^{+150}_{-170}$}+150-170 K. This brightness temperature is significantly higher than those derived from longer wavelengths, making it difficult to fit all available data points with a plausible atmospheric model. However, it may be that we underestimate the true uncertainties of our measurements, since it is notoriously difficult to assign precise statistical significance to a result when systematic effects are important.
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