We present the complete optical transmission spectrum of the hot Jupiter WASP-4b from 440-940 nm at R \sim 400-1500 obtained with the Gemini Multi-Object Spectrometers ( GMOS ) ; this is the first result from a comparative exoplanetology survey program of close-in gas giants conducted with GMOS .
WASP-4b has an equilibrium temperature of 1700 K and is favorable to study in transmission due to a large scale height ( 370 km ) .
We derive the transmission spectrum of WASP-4b using 4 transits observed with the MOS technique .
We demonstrate repeatable results across multiple epochs with GMOS , and derive a combined transmission spectrum at a precision about twice above photon noise , which is roughly equal to to one atmospheric scale height .
The transmission spectrum is well fitted with a uniform opacity as a function of wavelength .
The uniform opacity and absence of a Rayleigh slope from molecular hydrogen suggest that the atmosphere is dominated by clouds with condensate grain size of \sim 1 \mu m. This result is consistent with previous observations of hot Jupiters since clouds have been seen in planets with similar equilibrium temperatures to WASP-4b .
We describe a custom pipeline that we have written to reduce GMOS time-series data of exoplanet transits , and present a thorough analysis of the dominant noise sources in GMOS , which primarily consist of wavelength- and time- dependent displacements of the spectra on the detector , mainly due to a lack of atmospheric dispersion correction .