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We present a new primary transit observation of the hot-jupiter HD189733b,obtained at 3.6 microns with the Infrared Array Camera (IRAC) onboard theSpitzer Space Telescope. Previous measurements at 3.6 microns suffered fromstrong systematics and conclusions could hardly be obtained with confidence onthe water detection by comparison of the 3.6 and 5.8 microns observations. Weuse a high S/N Spitzer photometric transit light curve to improve the precisionof the near infrared radius of the planet at 3.6 microns. The observation hasbeen performed using high-cadence time series integrated in the subarray mode.We are able to derive accurate system parameters, including planet-to-starradius ratio, impact parameter, scale of the system, and central time of thetransit from the fits of the transit light curve. We compare the results withtransmission spectroscopic models and with results from previous observationsat the same wavelength. We obtained the following system parameters:R_p/R_\star=0.15566+0.00011-0.00024, b=0.661+0.0053-0.0050, anda/R_\star=8.925+0.0490-0.0523 at 3.6 microns. These measurements are threetimes more accurate than previous studies at this wavelength because theybenefit from greater observational efficiency and less statistic and systematicerrors. Nonetheless, we find that the radius ratio has to be corrected forstellar activity and present a method to do so using ground-based long-durationphotometric follow-up in the V-band. The resulting planet-to-star radius ratiocorrected for the stellar variability is in agreement with the previousmeasurement obtained in the same bandpass (Desert et al. 2009). We also discussthat water vapour could not be evidenced by comparison of the planetary radiusmeasured at 3.6 and 5.8 microns, because the radius measured at 3.6 microns isaffected by absorption by other species, possibly Rayleigh scattering by haze.

Transit spectrophotometry of the exoplanet HD189733b. II. New Spitzer observations at 3.6 microns