| Summary: | <p><em>Aims. </em>Qatar-1b is a close-orbiting hot Jupiter (<span class="simple-math"><em>R</em><sub>p</sub> ≃ 1.18 <em>R</em><sub>J</sub></span>, <span class="simple-math"><em>M</em><sub>p</sub> ≃ 1.33 <em>M</em><sub>J</sub></span>) around a metal-rich K-dwarf, with orbital separation and period of 0.023 AU and 1.42 days. We have observed the secondary eclipse of this exoplanet in the <em>K</em>s band with the objective of deriving a brightness temperature for the planet and providing further constraints to the orbital configuration of the system.</p>
<p><em>Methods. </em>We obtained near-infrared photometric data from the ground by using the OMEGA2000 instrument at the 3.5 m telescope at Calar Alto (Spain) in staring mode, with the telescope defocused. We have used principal component analysis (PCA) to identify correlated systematic trends in the data. A Markov chain Monte Carlo analysis was performed to model the correlated systematics and fit for the secondary eclipse of Qatar-1b using a previously developed occultation model. We adopted the prayer bead method to assess the effect of red noise on the derived parameters.</p>
<p><em>Results. </em>We measured a secondary eclipse depth of <span class="simple-math">0.196%<sup>+ 0.071%</sup></span><span class="simple-math"><sub>−0.051%</sub></span>, which indicates a brightness temperature in the <em>K</em>s band for the planet of <span class="simple-math">1885<sup>+ 212</sup></span><span class="simple-math"><sub>-168</sub></span> K. We also measured a small deviation in the central phase of the secondary eclipse of <span class="simple-math">−0.0079<sup>+ 0.0162</sup></span><span class="simple-math"><sub>-0.0043</sub></span>, which leads to a value for <span class="simple-math"><em>e</em>cos<em>ω</em></span> of <span class="simple-math">−0.0123<sup>+ 0.0252</sup></span><span class="simple-math"><sub>-0.0067</sub></span>. However, this last result needs to be confirmed with more data.</p>
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