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We analyzed four Spitzer/IRAC observations at 3.6 and 4.5 {\mu}m of theprimary transit of the exoplanet GJ436b, by using blind source separationtechniques. These observations are important to investigate the atmosphericcomposition of the planet GJ436b. Previous analyses claimed strong inter-epochvariations of the transit parameters due to stellar variability, casting doubtson the possibility to extract conclusively an atmospheric signal; thoseanalyses also reported discrepant results, hence the necessity of thisreanalysis. The method we used has been proposed in Morello et al. (2014) toanalyze 3.6 {\mu}m transit light-curves of the hot Jupiter HD189733b; itperformes an Independent Component Analysis (ICA) on a set ofpixel-light-curves, i.e. time series read by individual pixels, from the samephotometric observation. Our method only assumes the independence ofinstrumental and astrophysical signals, and therefore guarantees a higherdegree of objectivity compared to parametric detrending techniques published inthe literature. The datasets we analyzed in this paper represent a morechallenging test compared to the previous ones. Contrary to previous resultsreported in the literature, our results (1) do not support any detectableinter-epoch variations of orbital and stellar parameters, (2) arephotometrically stable at the level 10e-4 in the IR, and (3) the transit depthmeasurements at the two wavelengths are consistent within 1{\sigma}. We also(4) detect a possible transit duration variation (TDV) of 80 s (2 {\sigma}significance level), that has not been pointed out in the literature, and (5)confirm no transit timing variations (TTVs) >30 s.

Revisiting Spitzer transit observations with Independent Component Analysis: new results for the GJ436 system